---
title: "Wicking Bed Calculator: The Capillary Rise Physics That Most SIP Guides Never Show You"
canonical: "https://theyieldgrid.com/wicking-bed-calculator/"
model_id: "tyg-710"
model_version: "1.0.0"
last_reviewed: "2026-08-25T01:41:08"
reviewed_by: "Umer Hayiat"
---

# Wicking Bed Calculator: The Capillary Rise Physics That Most SIP Guides Never Show You

> Canonical calculator: [https://theyieldgrid.com/wicking-bed-calculator/](https://theyieldgrid.com/wicking-bed-calculator/)

## What this calculator does

Home - Free Gardening Calculators & Tools - Wicking Bed Calculator: The Capillary Rise Physics That Most SIP Guides Never Show You A sub-irrigated planter works by capillary action, not gravity. Water held at the bottom of a reservoir must be pulled upward through the soil column against the force of gravity, and whether it actually reaches your plant roots depends almost entirely on one variable that almost no garden content mentions: pore radius. The coarser the soil particles, the larger the air pockets between them, and the weaker the upward pull. Fill a tall wicking bed with the wrong growing medium and the physics guarantee failure, regardless of how well you build the reservoir structure.

## Inputs

| Input | ID | Type | Unit | Range or choices | Required |
|---|---|---|---|---|---|
| Soil Texture | `sipcap_soil` | select |  | — Select soil type — = ``; Coarse Sand (pore radius ~0.5mm) = `coarse_sand`; Loam (pore radius ~0.05mm) = `loam`; Peat Mix / Peat-Vermiculite (pore radius ~0.01mm) = `peat_mix`; Silt (pore radius ~0.008mm) = `silt` | No |
| Water Reservoir Depth (inches) | `sipcap_reservoir` | number | inches | 1 to 24 | No |
| Total Planter Bed Height (inches) | `sipcap_bedheight` | number | inches | 4 to 72 | No |

## Outputs

| Output ID | Default state |
|---|---|
| `sipcap_results` | ● Checking… Max Capillary Rise — inches Soil Surface (top) Water Level (bottom) 0% 25% 50% 75% 100% Wicking Bed Assessment Recommended Fix Recommended Products for Your SIP Build Reference: Capillary Rise by Soil Type & Bed Height Soil Type Pore Radius Max Rise Safe Bed Height Status |
| `sipcap_status_badge` | ● Checking… |
| `sipcap_out_primary` | — |
| `sipcap_warnings_box` | Wicking Bed Assessment |
| `sipcap_warnings_title` | Wicking Bed Assessment |
| `sipcap_warnings_list` |  |

## Formula and method

This calculator uses the Jurin's Law constant of 0.15 divided by pore radius to find your wicking ceiling. Show the calculation steps Step 1: Determine pore radius from soil texture category. Each soil category maps to a representative average pore radius in millimetres: Coarse Sand = 0.5 mm, Loam = 0.05 mm, Peat Mix = 0.01 mm, Silt = 0.008 mm. These are converted to centimetres (divide by 10) before entering the formula. Step 2: Apply the Jurin’s Law capillary rise approximation. Max Rise (cm) = 0.15 / PoreRadius (cm) This approximates the full Jurin’s equation (H = 2T cosθ / ρgr) at standard conditions: water at 20 degrees Celsius, zero contact angle, and standard gravity. The constant 0.15 combines the surface tension of water (approximately 0.0728 N/m), the cosine of a zero-degree contact angle (1.0), water density (998 kg/m3), and gravitational acceleration (9.81 m/s2). Step 3: Convert result to inches. Rise (in) = Rise (cm) / 2.54 Rounding is applied to one decimal place for display. Step 4: Calculate distance from reservoir to soil surface. Distance (in) = Bed Height (in) – Reservoir Depth (in) Step 5: Determine whether a dry zone exists. If Distance > Max Rise: Dry Zone (in) = Distance – Max Rise If Distance ≤ Max Rise: No dry zone (status passes to Optimal or Marginal check). Step 6: Classify the result. Optimal: Max Rise ≥ Distance x 1.1 (at least 10% headroom above what is needed) Marginal: Max Rise ≥ Distance but less than 110% of distance Failure: Max Rise < Distance (dry zone exists) Safe bed height recommendation: Safe Bed Height (in) = (Max Rise x 0.8) + Reservoir Depth The 0.8 multiplier applies a 20% safety buffer for compaction, temperature variation, and soil heterogeneity. Assumptions and Limits Uniform pore size: Jurin’s Law assumes a single, consistent pore radius throughout the medium. Real soils have a distribution of pore sizes; actual wicking behaviour will differ from the theoretical ceiling. Zero contact angle: The calculation assumes water wets the soil particles perfectly (contact angle = 0 degrees). Dry, hydrophobic, or waxy growing media (including some commercial peat products that have dried out) may have significantly higher contact angles, reducing actual capillary rise. 20 degrees Celsius water: Surface tension decreases at higher temperatures. At 30 degrees Celsius, surface tension drops by roughly 6 to 7 units, which lowers the effective capillary ceiling slightly. Outdoors in summer, the real-world ceiling may be 5 to 10% below the calculated value. Single-layer medium: The tool models one homogeneous growing medium. Layered profiles (gravel reservoir beneath a soil column, for example) create a perched water table effect that the formula does not capture and which can actually increase effective moisture delivery to the zone just above the transition layer. No evapotranspiration modelling: The tool calculates static capillary rise in a saturated column. Active plant transpiration, which creates an additional upward tension, is not modelled. In practice, transpiration pull can supplement capillary action somewhat. Representative pore radii only: The four soil categories use fixed pore radius values as proxies. Your specific mix, depending on particle size distribution, organic matter content, and compaction state, may fall anywhere within or between categories. No lateral wicking: The model is one-dimensional (vertical). Horizontal wicking from a central reservoir to the edges of a wide planter involves additional resistance not captured here.

## Verified worked examples

### Scenario 1: DIY Raised Bed Filled with Coarse Topsoil (Classic Failure)

Soil Texture: Coarse Sand (pore radius 0.5 mm = 0.05 cm) Water Reservoir Depth: 4 inches Total Planter Bed Height: 18 inches Max capillary rise = 0.15 / 0.05 cm = 3.0 cm = 1.18 inches Distance to surface = 18 – 4 = 14 inches Result: Dry zone = 14 – 1.18 = 12.82 inches. Status: Failure. Water wicks just over one inch above the reservoir. The top 12.8 inches of growing medium receive no moisture from below. Plants rooted in the upper two-thirds of this bed will desiccate, while the bottom zone stays saturated and risks anaerobic decomposition. Reducing bed height would not solve the underlying problem; only switching to a finer medium will.

### Scenario 2: Loam Mix in a Moderately Tall Bed (Borderline Pass)

Soil Texture: Loam (pore radius 0.05 mm = 0.005 cm) Water Reservoir Depth: 4 inches Total Planter Bed Height: 14 inches Max capillary rise = 0.15 / 0.005 cm = 30 cm = 11.81 inches Distance to surface = 14 – 4 = 10 inches Result: Rise (11.81 in) exceeds distance (10 in). No dry zone. Status: Marginal. The capillary ceiling clears the surface by 1.81 inches, which is less than a 20% safety margin. Any compaction from foot traffic near the bed, or a temperature increase above 25 degrees Celsius reducing surface tension, could push this into failure. Reducing bed height to 12 inches or amending with 20 to 30% vermiculite would move it comfortably into Optimal territory.

### Scenario 3: Peat-Vermiculite Mix in a Standard 24-Inch Wicking Bed (Strong Pass)

Soil Texture: Peat Mix / Peat-Vermiculite (pore radius 0.01 mm = 0.001 cm) Water Reservoir Depth: 4 inches Total Planter Bed Height: 24 inches Max capillary rise = 0.15 / 0.001 cm = 150 cm = 59.06 inches Distance to surface = 24 – 4 = 20 inches Result: Rise (59.06 in) vastly exceeds distance (20 in). Reach is at 295% of what is needed. Status: Optimal. A peat-vermiculite mix creates pores fine enough to theoretically wick nearly five feet upward, far exceeding any practical wicking bed depth. This is why experienced SIP builders consistently recommend this combination: it provides an enormous margin of safety for any bed depth below about 51 inches (with a 20% buffer applied).

## Assumptions

Soil Texture: Peat Mix / Peat-Vermiculite (pore radius 0.01 mm = 0.001 cm) Water Reservoir Depth: 4 inches Total Planter Bed Height: 24 inches Max capillary rise = 0.15 / 0.001 cm = 150 cm = 59.06 inches Distance to surface = 24 – 4 = 20 inches Result: Rise (59.06 in) vastly exceeds distance (20 in). Reach is at 295% of what is needed. Status: Optimal. A peat-vermiculite mix creates pores fine enough to theoretically wick nearly five feet upward, far exceeding any practical wicking bed depth. This is why experienced SIP builders consistently recommend this combination: it provides an enormous margin of safety for any bed depth below about 51 inches (with a 20% buffer applied). This calculator uses the Jurin's Law constant of 0.15 divided by pore radius to find your wicking ceiling. Show the calculation steps Step 1: Determine pore radius from soil texture category. Each soil category maps to a representative average pore radius in millimetres: Coarse Sand = 0.5 mm, Loam = 0.05 mm, Peat Mix = 0.01 mm, Silt = 0.008 mm. These are converted to centimetres (divide by 10) before entering the formula. Step 2: Apply the Jurin’s Law capillary rise approximation. Max Rise (cm) = 0.15 / PoreRadius (cm) This approximates the full Jurin’s equation (H = 2T cosθ / ρgr) at standard conditions: water at 20 degrees Celsius, zero contact angle, and standard gravity. The constant 0.15 combines the surface tension of water (approximately 0.0728 N/m), the cosine of a zero-degree contact angle (1.0), water density (998 kg/m3), and gravitational acceleration (9.81 m/s2). Step 3: Convert result to inches. Rise (in) = Rise (cm) / 2.54 Rounding is applied to one decimal place for display. Step 4: Calculate distance from reservoir to soil surface. Distance (in) = Bed Height (in) – Reservoir Depth (in) Step 5: Determine whether a dry zone exists. If Distance > Max Rise: Dry Zone (in) = Distance – Max Rise If Distance ≤ Max Rise: No dry zone (status passes to Optimal or Marginal check). Step 6: Classify the result. Optimal: Max Rise ≥ Distance x 1.1 (at least 10% headroom above what is needed) Marginal: Max Rise ≥ Distance but less than 110% of distance Failure: Max Rise < Distance (dry zone exists) Safe bed height recommendation: Safe Bed Height (in) = (Max Rise x 0.8) + Reservoir Depth The 0.8 multiplier applies a 20% safety buffer for compaction, temperature variation, and soil heterogeneity. Assumptions and Limits Uniform pore size: Jurin’s Law assumes a single, consistent pore radius throughout the medium. Real soils have a distribution of pore sizes; actual wicking behaviour will differ from the theoretical ceiling. Zero contact angle: The calculation assumes water wets the soil particles perfectly (contact angle = 0 degrees). Dry, hydrophobic, or waxy growing media (including some commercial peat products that have dried out) may have significantly higher contact angles, reducing actual capillary rise. 20 degrees Celsius water: Surface tension decreases at higher temperatures. At 30 degrees Celsius, surface tension drops by roughly 6 to 7 units, which lowers the effective capillary ceiling slightly. Outdoors in summer, the real-world ceiling may be 5 to 10% below the calculated value. Single-layer medium: The tool models one homogeneous growing medium. Layered profiles (gravel reservoir beneath a soil column, for example) create a perched water table effect that the formula does not capture and which can actually increase effective moisture delivery to the zone just above the transition layer. No evapotranspiration modelling: The tool calculates static capillary rise in a saturated column. Active plant transpiration, which creates an additional upward tension, is not modelled. In practice, transpiration pull can supplement capillary action somewhat. Representative pore radii only: The four soil categories use fixed pore radius values as proxies. Your specific mix, depending on particle size distribution, organic matter content, and compaction state, may fall anywhere within or between categories. No lateral wicking: The model is one-dimensional (vertical). Horizontal wicking from a central reservoir to the edges of a wide planter involves additional resistance not captured here. Uniform pore size: Jurin’s Law assumes a single, consistent pore radius throughout the medium. Real soils have a distribution of pore sizes; actual wicking behaviour will differ from the theoretical ceiling. Zero contact angle: The calculation assumes water wets the soil particles perfectly (contact angle = 0 degrees). Dry, hydrophobic, or waxy growing media (including some commercial peat products that have dried out) may have significantly higher contact angles, reducing actual capillary rise. 20 degrees Celsius water: Surface tension decreases at higher temperatures. At 30 degrees Celsius, surface tension drops by roughly 6 to 7 units, which lowers the effective capillary ceiling slightly. Outdoors in summer, the real-world ceiling may be 5 to 10% below the calculated value. Single-layer medium: The tool models one homogeneous growing medium. Layered profiles (gravel reservoir beneath a soil column, for example) create a perched water table effect that the formula does not capture and which can actually increase effective moisture delivery to the zone just above the transition layer. No evapotranspiration modelling: The tool calculates static capillary rise in a saturated column. Active plant transpiration, which creates an additional upward tension, is not modelled. In practice, transpiration pull can supplement capillary action somewhat. Representative pore radii only: The four soil categories use fixed pore radius values as proxies. Your specific mix, depending on particle size distribution, organic matter content, and compaction state, may fall anywhere within or between categories. No lateral wicking: The model is one-dimensional (vertical). Horizontal wicking from a central reservoir to the edges of a wide planter involves additional resistance not captured here. Critical Warnings The “Dry Top, Swamp Bottom” failure is the most common SIP build error and it is invisible until plants fail. Coarse or sandy growing media limit capillary rise to roughly 1 to 3 inches in typical topsoil and landscape mix products. A 24-inch bed filled with such material will have its bottom 4 to 5 inches permanently waterlogged (anaerobic, root-damaging) while the top 18 to 20 inches are bone-dry. Neither overhead watering nor adding more reservoir water corrects this; the physics ceiling cannot be overridden by adding volume. Increasing reservoir depth does not increase capillary rise height. A common attempted fix is deepening the reservoir to push more water upward. This is a misunderstanding of how capillary action works. The maximum wicking height is determined entirely by pore radius, not by reservoir volume or depth. Increasing reservoir depth beyond what is needed to maintain consistent water availability is simply waste. Marginal status is fragile over a growing season. A Marginal rating means capillary rise just barely clears the soil surface under ideal conditions. As organic matter decomposes over months, soil settles and pore sizes shift. A bed that starts Marginal in spring may develop a dry zone by midsummer without any change to watering practice. Silt’s theoretical advantage does not translate to practical recommendation. While Silt produces the highest capillary rise of the four categories (over 73 inches theoretically), pure silt compacts under wet-dry cycles, eventually reducing pore connectivity. Pure silt is not a practical growing medium for SIP systems despite the numbers. Minimum Standards for a Reliable SIP Build Growing medium capillary rise must exceed distance-to-surface with at least a 20% safety margin (reflected in the Safe Bed Height recommendation the tool outputs). For beds taller than 12 inches, the growing medium should be at least Loam-class or finer; Coarse Sand is categorically unsuitable for any SIP application regardless of bed height. Beds taller than 18 inches should use Peat Mix or an equivalent medium with comparable pore structure to ensure reliable wicking to the surface. Water temperature in the reservoir affects surface tension. At elevated temperatures, verify your setup remains Optimal rather than drifting to Marginal. The water temperature calculator can help track seasonal reservoir temperatures if your SIP is outdoors or greenhouse-housed. Competitor Trap: Most wicking bed guides instruct builders to “use a good quality potting mix” without specifying what “good” means for capillary physics. Many commercially available raised-bed and potting mixes are blended for outdoor drainage, not for SIP capillary performance, and contain enough coarse bark, perlite, and sand to function essentially as Coarse Sand by this tool’s physics model. Reading a bag label for “fast-draining” or “well-aerated” is a warning sign that the product is engineered for overhead-watered drainage, not upward capillary delivery. When building a SIP, source specifically: a peat-based or coco-based mix with fine vermiculite, not a generic all-purpose potting blend. Pairing this with the Dutch bucket irrigation calculator can help you think through alternative bottom-up delivery methods if your growing medium is coarser than ideal. Growing medium capillary rise must exceed distance-to-surface with at least a 20% safety margin (reflected in the Safe Bed Height recommendation the tool outputs). For beds taller than 12 inches, the growing medium should be at least Loam-class or finer; Coarse Sand is categorically unsuitable for any SIP application regardless of bed height. Beds taller than 18 inches should use Peat Mix or an equivalent medium with comparable pore structure to ensure reliable wicking to the surface. Water temperature in the reservoir affects surface tension. At elevated temperatures, verify your setup remains Optimal rather than drifting to Marginal. The water temperature calculator can help track seasonal reservoir temperatures if your SIP is outdoors or greenhouse-housed. Competitor Trap: Most wicking bed guides instruct builders to “use a good quality potting mix” without specifying what “good” means for capillary physics. Many commercially available raised-bed and potting mixes are blended for outdoor drainage, not for SIP capillary performance, and contain enough coarse bark, perlite, and sand to function essentially as Coarse Sand by this tool’s physics model. Reading a bag label for “fast-draining” or “well-aerated” is a warning sign that the product is engineered for overhead-watered drainage, not upward capillary delivery. When building a SIP, source specifically: a peat-based or coco-based mix with fine vermiculite, not a generic all-purpose potting blend. Pairing this with the Dutch bucket irrigation calculator can help you think through alternative bottom-up delivery methods if your growing medium is coarser than ideal.

## Limitations and safety

Uniform pore size: Jurin’s Law assumes a single, consistent pore radius throughout the medium. Real soils have a distribution of pore sizes; actual wicking behaviour will differ from the theoretical ceiling. Zero contact angle: The calculation assumes water wets the soil particles perfectly (contact angle = 0 degrees). Dry, hydrophobic, or waxy growing media (including some commercial peat products that have dried out) may have significantly higher contact angles, reducing actual capillary rise. 20 degrees Celsius water: Surface tension decreases at higher temperatures. At 30 degrees Celsius, surface tension drops by roughly 6 to 7 units, which lowers the effective capillary ceiling slightly. Outdoors in summer, the real-world ceiling may be 5 to 10% below the calculated value. Single-layer medium: The tool models one homogeneous growing medium. Layered profiles (gravel reservoir beneath a soil column, for example) create a perched water table effect that the formula does not capture and which can actually increase effective moisture delivery to the zone just above the transition layer. No evapotranspiration modelling: The tool calculates static capillary rise in a saturated column. Active plant transpiration, which creates an additional upward tension, is not modelled. In practice, transpiration pull can supplement capillary action somewhat. Representative pore radii only: The four soil categories use fixed pore radius values as proxies. Your specific mix, depending on particle size distribution, organic matter content, and compaction state, may fall anywhere within or between categories. No lateral wicking: The model is one-dimensional (vertical). Horizontal wicking from a central reservoir to the edges of a wide planter involves additional resistance not captured here. Critical Warnings The “Dry Top, Swamp Bottom” failure is the most common SIP build error and it is invisible until plants fail. Coarse or sandy growing media limit capillary rise to roughly 1 to 3 inches in typical topsoil and landscape mix products. A 24-inch bed filled with such material will have its bottom 4 to 5 inches permanently waterlogged (anaerobic, root-damaging) while the top 18 to 20 inches are bone-dry. Neither overhead watering nor adding more reservoir water corrects this; the physics ceiling cannot be overridden by adding volume. Increasing reservoir depth does not increase capillary rise height. A common attempted fix is deepening the reservoir to push more water upward. This is a misunderstanding of how capillary action works. The maximum wicking height is determined entirely by pore radius, not by reservoir volume or depth. Increasing reservoir depth beyond what is needed to maintain consistent water availability is simply waste. Marginal status is fragile over a growing season. A Marginal rating means capillary rise just barely clears the soil surface under ideal conditions. As organic matter decomposes over months, soil settles and pore sizes shift. A bed that starts Marginal in spring may develop a dry zone by midsummer without any change to watering practice. Silt’s theoretical advantage does not translate to practical recommendation. While Silt produces the highest capillary rise of the four categories (over 73 inches theoretically), pure silt compacts under wet-dry cycles, eventually reducing pore connectivity. Pure silt is not a practical growing medium for SIP systems despite the numbers. Minimum Standards for a Reliable SIP Build Growing medium capillary rise must exceed distance-to-surface with at least a 20% safety margin (reflected in the Safe Bed Height recommendation the tool outputs). For beds taller than 12 inches, the growing medium should be at least Loam-class or finer; Coarse Sand is categorically unsuitable for any SIP application regardless of bed height. Beds taller than 18 inches should use Peat Mix or an equivalent medium with comparable pore structure to ensure reliable wicking to the surface. Water temperature in the reservoir affects surface tension. At elevated temperatures, verify your setup remains Optimal rather than drifting to Marginal. The water temperature calculator can help track seasonal reservoir temperatures if your SIP is outdoors or greenhouse-housed. Competitor Trap: Most wicking bed guides instruct builders to “use a good quality potting mix” without specifying what “good” means for capillary physics. Many commercially available raised-bed and potting mixes are blended for outdoor drainage, not for SIP capillary performance, and contain enough coarse bark, perlite, and sand to function essentially as Coarse Sand by this tool’s physics model. Reading a bag label for “fast-draining” or “well-aerated” is a warning sign that the product is engineered for overhead-watered drainage, not upward capillary delivery. When building a SIP, source specifically: a peat-based or coco-based mix with fine vermiculite, not a generic all-purpose potting blend. Pairing this with the Dutch bucket irrigation calculator can help you think through alternative bottom-up delivery methods if your growing medium is coarser than ideal.

## Related calculators

- [Calculators & Tools](https://theyieldgrid.com/category/garden-calculators/)
- [hydroponic EC readings](https://theyieldgrid.com/hydroponic-ec-calculator/)
- [water temperature calculator](https://theyieldgrid.com/water-temperature-calculator/)
- [Dutch bucket irrigation calculator](https://theyieldgrid.com/dutch-bucket-irrigation-calculator/)
- [cal-mag dosing calculator](https://theyieldgrid.com/cal-mag-dosage-calculator/)
- [hydroponic pH-down calculator](https://theyieldgrid.com/hydroponic-ph-down-calculator/)
- [hydroponic nutrient dosing calculator](https://theyieldgrid.com/hydroponic-nutrient-dosing-calculator/)
- [VPD calculator](https://theyieldgrid.com/vpd-calculator/)
- [crop steering calculator](https://theyieldgrid.com/crop-steering-calculator/)
- [Prev Previous](https://theyieldgrid.com/harvest-date-calculator/)

## Provenance

- Model ID: `tyg-710`
- Model version: `1.0.0`
- Reviewed by: Umer Hayiat
- Page modified: 2026-08-25T01:41:08
- Runtime SHA-256: `8380260f3d1f41513bf0f7ee2595574c5faab5964f1247b398ea977bc5112a9e`

This Markdown document is a machine-readable mirror. The canonical interactive calculator is the HTML page linked above.