---
title: "Rain Garden Sizing Calculator: Size by Drain Time, Not Just Area"
canonical: "https://theyieldgrid.com/rain-garden-sizing-calculator/"
model_id: "tyg-2749"
model_version: "1.0.0"
last_reviewed: "2026-08-24T08:37:45"
reviewed_by: "Umer Hayiat"
---

# Rain Garden Sizing Calculator: Size by Drain Time, Not Just Area

> Canonical calculator: [https://theyieldgrid.com/rain-garden-sizing-calculator/](https://theyieldgrid.com/rain-garden-sizing-calculator/)

## What this calculator does

Home - Free Gardening Calculators & Tools - Rain Garden Sizing Calculator: Size by Drain Time, Not Just Area Stormwater sizing is often reduced to a single ratio: make the garden roughly 5–10% of the catchment area. That shortcut ignores the one variable that determines whether a rain garden actually works or turns into a standing-water hazard. Soil percolation rate controls how long water sits in the bowl after a storm, and that number changes everything about whether your garden is safe, functional, or a mosquito breeding ground by Thursday.

## Inputs

| Input | ID | Type | Unit | Range or choices | Required |
|---|---|---|---|---|---|
| Catchment Area * | `rg_catchment` | number | square feet | 1 to | Yes |
| Storm Rainfall Depth * | `rg_raindepth` | number | e.g. 1" for a 1-inch, 24-hour storm | 0.1 to 20 | Yes |
| Soil Percolation Rate * | `rg_percrate` | number | inches/hour | 0.01 to | Yes |
| Maximum Ponding Depth * | `rg_pondingdepth` | number | inches, typically 6 | 1 to 24 | Yes |

## Outputs

| Output ID | Default state |
|---|---|
| `rg_catchment_err` |  |
| `rg_raindepth_err` |  |
| `rg_percrate_err` |  |
| `rg_pondingdepth_err` |  |
| `rg_results_panel` | — sq ft Recommended Rain Garden Area — Runoff Volume (cu ft) — Drain Time (hrs) — Garden : Catchment Ratio Drain Time vs. 48-Hour Mosquito Threshold Safe 48 hr limit Garden Area as % of Catchment Reference: Common Scenarios (1-inch Storm, 6-inch Ponding) Catchment Perc Rate Garden Area Drain Time Status Recommended Supplies for Success Bulk Organic Compost (soil amendment) Native Seed Mixes: Milkweed & Coneflower Heavy-Duty Bypass Loppers (root clearing) Laser Level (find lowest yard grade) How |
| `rg_results_inner` | — sq ft Recommended Rain Garden Area — Runoff Volume (cu ft) — Drain Time (hrs) — Garden : Catchment Ratio Drain Time vs. 48-Hour Mosquito Threshold Safe 48 hr limit Garden Area as % of Catchment Reference: Common Scenarios (1-inch Storm, 6-inch Ponding) Catchment Perc Rate Garden Area Drain Time Status Recommended Supplies for Success Bulk Organic Compost (soil amendment) Native Seed Mixes: Milkweed & Coneflower Heavy-Duty Bypass Loppers (root clearing) Laser Level (find lowest yard grade) How |
| `rg_out_primary` | — sq ft |
| `rg_out_interp` |  |
| `rg_out_runoff` | — |
| `rg_out_drain` | — |
| `rg_out_ratio` | — |

## Formula and method

This 3D cross-section visualizes how the calculator determines the safety window between storm capture and total drainage. Show the calculation steps Step 1: Runoff Volume The volume of stormwater generated by a single storm event is calculated by converting rainfall depth from inches to feet, then multiplying by catchment area: RunoffVolume (cu ft) = CatchmentArea (sq ft) x (RainfallDepth (in) / 12) A 1-inch rain over 1,000 sq ft produces 83.3 cubic feet of runoff. This step uses a runoff coefficient of 1.0, meaning all rainfall becomes runoff. This is appropriate for fully impervious surfaces like roofs and sealed concrete. For compacted gravel, values of 0.7 to 0.9 are more accurate but must be applied manually before entering the catchment area. Step 2: Required Garden Area The garden must hold the full runoff volume at the stated ponding depth. Area is found by dividing runoff volume by ponding depth (converted to feet): GardenArea (sq ft) = RunoffVolume (cu ft) / (PondingDepth (in) / 12) A 6-inch bowl depth is 0.5 feet. Divide your runoff volume by 0.5 to get the required surface area. This calculation assumes the entire garden bowl is available at full ponding depth instantaneously, which is a conservative assumption; in practice, some infiltration occurs during the storm event itself. Step 3: Drain Time Drain time is the expected duration for all standing water to infiltrate through the soil: DrainTime (hrs) = PondingDepth (in) / PercRate (in/hr) This is the most important output from a biological safety standpoint. The formula assumes uniform, constant percolation throughout the entire depth of the bowl. In reality, percolation rates decrease slightly as soil approaches saturation, so actual drain times in field conditions may be 10 to 20% longer than this formula predicts. Step 4: Mosquito Threshold Check If DrainTime is greater than 48 hours, a biological risk exists. Standing water warmer than approximately 50 degrees Fahrenheit can support mosquito egg-laying and initial larval development within 48 to 72 hours. The calculator flags this threshold and computes a minimum expanded garden area that would, at the same soil percolation rate, achieve a shallower effective ponding depth with a shorter drain time. Rounding Rules: Garden area is rounded to the nearest whole square foot. Drain time and runoff volume are displayed to one decimal place. All internal calculations retain full floating-point precision. Assumptions and Limits Runoff coefficient is fixed at 1.0. Partially pervious surfaces (gravel, pavers, dense turf) will produce less actual runoff and this value should be adjusted before entering catchment area. Percolation rate is assumed constant with depth and constant over time. Post-storm soil saturation typically reduces effective percolation by 10 to 25% compared to dry-soil measurements. No credit is given for evapotranspiration, precipitation occurring during the drain period, or pre-wetting of the soil from antecedent moisture conditions. The formula assumes the garden bowl is a flat-bottomed rectangular depression. Contoured, sloped, or irregularly shaped gardens will have different effective storage volumes. No subsurface drainage structures (underdrain pipes, gravel infiltration trenches) are factored in. Adding an underdrain effectively increases the functional percolation rate and can resolve slow-drain scenarios without changing the garden surface area. Results are planning-level estimates suitable for pre-design decision-making. Permitted rain garden installations in regulated stormwater management zones require licensed engineering and site-specific hydrologic analysis. The tool does not account for off-season frozen ground conditions, which can render an infiltration system temporarily non-functional during winter storm events in northern climates.

## Verified worked examples

### Example 1: Sandy Loam Soil, Standard Suburban Roof

Catchment Area: 1,200 sq ft (40 ft x 30 ft roof section) Storm Depth: 1 inch Percolation Rate: 0.5 in/hr (silt loam, moderately well-drained) Maximum Ponding Depth: 6 inches Result: Runoff Volume = 1,200 x (1/12) = 100 cu ft. Garden Area = 100 / (6/12) = 200 sq ft. Drain Time = 6 / 0.5 = 12 hours. A 200 sq ft garden drains in 12 hours, well within the safe window. This scenario represents a well-matched system. A roughly 14 ft x 14 ft garden footprint handles the full roof section with capacity to spare.

### Example 2: Heavy Clay Soil, Driveway Catchment (Mosquito Risk)

Catchment Area: 800 sq ft (driveway apron) Storm Depth: 1 inch Percolation Rate: 0.1 in/hr (heavy clay) Maximum Ponding Depth: 6 inches Result: Runoff Volume = 800 x (1/12) = 66.7 cu ft. Garden Area = 66.7 / 0.5 = 133 sq ft. Drain Time = 6 / 0.1 = 60 hours. Drain time exceeds the 48-hour threshold by 12 hours. Standing water at 60 hours presents active mosquito breeding risk. To bring drain time to 48 hours or below, the garden area must be expanded to approximately 167 sq ft or the ponding depth reduced to 4.8 inches to achieve a shallower effective bowl. Soil amendment with coarse compost is strongly recommended to raise the percolation rate before finalizing dimensions.

### Example 3: Well-Drained Soil, Larger Storm Event

Catchment Area: 2,000 sq ft (combined roof and patio) Storm Depth: 2 inches (larger design event) Percolation Rate: 1.5 in/hr (sandy loam, well-drained) Maximum Ponding Depth: 8 inches Result: Runoff Volume = 2,000 x (2/12) = 333.3 cu ft. Garden Area = 333.3 / (8/12) = 500 sq ft. Drain Time = 8 / 1.5 = 5.3 hours. Even at a 2-inch storm event and an 8-inch bowl depth, the 5.3-hour drain time is excellent. This soil type allows for deep ponding without biological risk. A 500 sq ft garden is substantial but entirely appropriate for a 2,000 sq ft impervious catchment in a well-drained location.

## Assumptions

Catchment Area: 1,200 sq ft (40 ft x 30 ft roof section) Storm Depth: 1 inch Percolation Rate: 0.5 in/hr (silt loam, moderately well-drained) Maximum Ponding Depth: 6 inches Result: Runoff Volume = 1,200 x (1/12) = 100 cu ft. Garden Area = 100 / (6/12) = 200 sq ft. Drain Time = 6 / 0.5 = 12 hours. A 200 sq ft garden drains in 12 hours, well within the safe window. This scenario represents a well-matched system. A roughly 14 ft x 14 ft garden footprint handles the full roof section with capacity to spare. This 3D cross-section visualizes how the calculator determines the safety window between storm capture and total drainage. Show the calculation steps Step 1: Runoff Volume The volume of stormwater generated by a single storm event is calculated by converting rainfall depth from inches to feet, then multiplying by catchment area: RunoffVolume (cu ft) = CatchmentArea (sq ft) x (RainfallDepth (in) / 12) A 1-inch rain over 1,000 sq ft produces 83.3 cubic feet of runoff. This step uses a runoff coefficient of 1.0, meaning all rainfall becomes runoff. This is appropriate for fully impervious surfaces like roofs and sealed concrete. For compacted gravel, values of 0.7 to 0.9 are more accurate but must be applied manually before entering the catchment area. Step 2: Required Garden Area The garden must hold the full runoff volume at the stated ponding depth. Area is found by dividing runoff volume by ponding depth (converted to feet): GardenArea (sq ft) = RunoffVolume (cu ft) / (PondingDepth (in) / 12) A 6-inch bowl depth is 0.5 feet. Divide your runoff volume by 0.5 to get the required surface area. This calculation assumes the entire garden bowl is available at full ponding depth instantaneously, which is a conservative assumption; in practice, some infiltration occurs during the storm event itself. Step 3: Drain Time Drain time is the expected duration for all standing water to infiltrate through the soil: DrainTime (hrs) = PondingDepth (in) / PercRate (in/hr) This is the most important output from a biological safety standpoint. The formula assumes uniform, constant percolation throughout the entire depth of the bowl. In reality, percolation rates decrease slightly as soil approaches saturation, so actual drain times in field conditions may be 10 to 20% longer than this formula predicts. Step 4: Mosquito Threshold Check If DrainTime is greater than 48 hours, a biological risk exists. Standing water warmer than approximately 50 degrees Fahrenheit can support mosquito egg-laying and initial larval development within 48 to 72 hours. The calculator flags this threshold and computes a minimum expanded garden area that would, at the same soil percolation rate, achieve a shallower effective ponding depth with a shorter drain time. Rounding Rules: Garden area is rounded to the nearest whole square foot. Drain time and runoff volume are displayed to one decimal place. All internal calculations retain full floating-point precision. Assumptions and Limits Runoff coefficient is fixed at 1.0. Partially pervious surfaces (gravel, pavers, dense turf) will produce less actual runoff and this value should be adjusted before entering catchment area. Percolation rate is assumed constant with depth and constant over time. Post-storm soil saturation typically reduces effective percolation by 10 to 25% compared to dry-soil measurements. No credit is given for evapotranspiration, precipitation occurring during the drain period, or pre-wetting of the soil from antecedent moisture conditions. The formula assumes the garden bowl is a flat-bottomed rectangular depression. Contoured, sloped, or irregularly shaped gardens will have different effective storage volumes. No subsurface drainage structures (underdrain pipes, gravel infiltration trenches) are factored in. Adding an underdrain effectively increases the functional percolation rate and can resolve slow-drain scenarios without changing the garden surface area. Results are planning-level estimates suitable for pre-design decision-making. Permitted rain garden installations in regulated stormwater management zones require licensed engineering and site-specific hydrologic analysis. The tool does not account for off-season frozen ground conditions, which can render an infiltration system temporarily non-functional during winter storm events in northern climates. Runoff coefficient is fixed at 1.0. Partially pervious surfaces (gravel, pavers, dense turf) will produce less actual runoff and this value should be adjusted before entering catchment area. Percolation rate is assumed constant with depth and constant over time. Post-storm soil saturation typically reduces effective percolation by 10 to 25% compared to dry-soil measurements. No credit is given for evapotranspiration, precipitation occurring during the drain period, or pre-wetting of the soil from antecedent moisture conditions. The formula assumes the garden bowl is a flat-bottomed rectangular depression. Contoured, sloped, or irregularly shaped gardens will have different effective storage volumes. No subsurface drainage structures (underdrain pipes, gravel infiltration trenches) are factored in. Adding an underdrain effectively increases the functional percolation rate and can resolve slow-drain scenarios without changing the garden surface area. Results are planning-level estimates suitable for pre-design decision-making. Permitted rain garden installations in regulated stormwater management zones require licensed engineering and site-specific hydrologic analysis. The tool does not account for off-season frozen ground conditions, which can render an infiltration system temporarily non-functional during winter storm events in northern climates. Critical Warnings The 48-Hour Biological Threshold: This is not a guideline with flexibility. Mosquitoes in warm climates can complete egg hatching stages in standing water that has been present for 48 to 72 hours. A garden built in clay with a 60-hour drain time is not a rain garden by function; it is a detention pond with native plants, and those plants will exhibit root rot symptoms within one to two growing seasons as anaerobic soil conditions kill off species not adapted to permanent saturation. Percolation Rates After Construction Compaction: Heavy equipment and foot traffic during installation compact native soils significantly. A pre-construction percolation test may read 0.4 in/hr; the same soil after a backhoe has shaped the basin may perform at 0.15 to 0.25 in/hr. Test after rough grading, not before. If drain time approaches the 48-hour threshold at your design percolation rate, build in a 25% safety margin on garden area. Ponding Depth as a Dial, Not a Fixed Number: Reducing ponding depth from 6 inches to 4 inches cuts drain time proportionally. A garden that fails at 6 inches of ponding may pass comfortably at 4 inches with a modest increase in surface area. These two variables interact; adjusting one allows the other to compensate. Overflow Routing: This calculator sizes for complete capture of the design storm. Storms exceeding the design event will overflow. Plan an overflow pathway before construction. An unplanned overflow that routes toward a foundation or septic system can cause damage that far exceeds the cost of the garden itself. Minimum Standards Drain time must remain below 48 hours under the worst credible soil condition at your site. Use the slowest measured percolation rate, not an average. Minimum setback from foundations: most extension guidelines recommend 10 feet minimum from any structure. Infiltration near foundations increases hydrostatic pressure and can compromise waterproofing systems. Maximum side slope on garden berms: 3:1 (horizontal to vertical) to prevent erosion during storms and maintain safe pedestrian access around the garden perimeter. Amended soil depth for the planting zone should be a minimum of 18 to 24 inches to support deep-rooted native species. Shallow amendment produces surface soil improvements that erode over time. For projects that include compost and erosion protection layers around the garden perimeter, the compost blanket erosion calculator can help determine the appropriate application depth and coverage. Where landscape fabric is used at the garden’s overflow outlet or edge transitions, accurate material estimation with a landscape fabric overlap calculator prevents installation gaps that accelerate erosion. Competitor Trap: Nearly every competing rain garden guide recommends sizing your garden at 5 to 10 square feet per 100 square feet of catchment area. That rule was derived from average soil conditions in specific mid-Atlantic research plots. Applied blindly to clay soils, it produces gardens that are far too small to drain safely, often by a factor of two to three times. The drain time check that this calculator performs is the single most important calculation that most published rain garden guides either omit entirely or mention only as a footnote. Build to drain time, not to a percentage of catchment. Drain time must remain below 48 hours under the worst credible soil condition at your site. Use the slowest measured percolation rate, not an average. Minimum setback from foundations: most extension guidelines recommend 10 feet minimum from any structure. Infiltration near foundations increases hydrostatic pressure and can compromise waterproofing systems. Maximum side slope on garden berms: 3:1 (horizontal to vertical) to prevent erosion during storms and maintain safe pedestrian access around the garden perimeter. Amended soil depth for the planting zone should be a minimum of 18 to 24 inches to support deep-rooted native species. Shallow amendment produces surface soil improvements that erode over time. For projects that include compost and erosion protection layers around the garden perimeter, the compost blanket erosion calculator can help determine the appropriate application depth and coverage. Where landscape fabric is used at the garden’s overflow outlet or edge transitions, accurate material estimation with a landscape fabric overlap calculator prevents installation gaps that accelerate erosion. Competitor Trap: Nearly every competing rain garden guide recommends sizing your garden at 5 to 10 square feet per 100 square feet of catchment area. That rule was derived from average soil conditions in specific mid-Atlantic research plots. Applied blindly to clay soils, it produces gardens that are far too small to drain safely, often by a factor of two to three times. The drain time check that this calculator performs is the single most important calculation that most published rain garden guides either omit entirely or mention only as a footnote. Build to drain time, not to a percentage of catchment.

## Limitations and safety

Runoff coefficient is fixed at 1.0. Partially pervious surfaces (gravel, pavers, dense turf) will produce less actual runoff and this value should be adjusted before entering catchment area. Percolation rate is assumed constant with depth and constant over time. Post-storm soil saturation typically reduces effective percolation by 10 to 25% compared to dry-soil measurements. No credit is given for evapotranspiration, precipitation occurring during the drain period, or pre-wetting of the soil from antecedent moisture conditions. The formula assumes the garden bowl is a flat-bottomed rectangular depression. Contoured, sloped, or irregularly shaped gardens will have different effective storage volumes. No subsurface drainage structures (underdrain pipes, gravel infiltration trenches) are factored in. Adding an underdrain effectively increases the functional percolation rate and can resolve slow-drain scenarios without changing the garden surface area. Results are planning-level estimates suitable for pre-design decision-making. Permitted rain garden installations in regulated stormwater management zones require licensed engineering and site-specific hydrologic analysis. The tool does not account for off-season frozen ground conditions, which can render an infiltration system temporarily non-functional during winter storm events in northern climates. Critical Warnings The 48-Hour Biological Threshold: This is not a guideline with flexibility. Mosquitoes in warm climates can complete egg hatching stages in standing water that has been present for 48 to 72 hours. A garden built in clay with a 60-hour drain time is not a rain garden by function; it is a detention pond with native plants, and those plants will exhibit root rot symptoms within one to two growing seasons as anaerobic soil conditions kill off species not adapted to permanent saturation. Percolation Rates After Construction Compaction: Heavy equipment and foot traffic during installation compact native soils significantly. A pre-construction percolation test may read 0.4 in/hr; the same soil after a backhoe has shaped the basin may perform at 0.15 to 0.25 in/hr. Test after rough grading, not before. If drain time approaches the 48-hour threshold at your design percolation rate, build in a 25% safety margin on garden area. Ponding Depth as a Dial, Not a Fixed Number: Reducing ponding depth from 6 inches to 4 inches cuts drain time proportionally. A garden that fails at 6 inches of ponding may pass comfortably at 4 inches with a modest increase in surface area. These two variables interact; adjusting one allows the other to compensate. Overflow Routing: This calculator sizes for complete capture of the design storm. Storms exceeding the design event will overflow. Plan an overflow pathway before construction. An unplanned overflow that routes toward a foundation or septic system can cause damage that far exceeds the cost of the garden itself. Minimum Standards Drain time must remain below 48 hours under the worst credible soil condition at your site. Use the slowest measured percolation rate, not an average. Minimum setback from foundations: most extension guidelines recommend 10 feet minimum from any structure. Infiltration near foundations increases hydrostatic pressure and can compromise waterproofing systems. Maximum side slope on garden berms: 3:1 (horizontal to vertical) to prevent erosion during storms and maintain safe pedestrian access around the garden perimeter. Amended soil depth for the planting zone should be a minimum of 18 to 24 inches to support deep-rooted native species. Shallow amendment produces surface soil improvements that erode over time. For projects that include compost and erosion protection layers around the garden perimeter, the compost blanket erosion calculator can help determine the appropriate application depth and coverage. Where landscape fabric is used at the garden’s overflow outlet or edge transitions, accurate material estimation with a landscape fabric overlap calculator prevents installation gaps that accelerate erosion. Competitor Trap: Nearly every competing rain garden guide recommends sizing your garden at 5 to 10 square feet per 100 square feet of catchment area. That rule was derived from average soil conditions in specific mid-Atlantic research plots. Applied blindly to clay soils, it produces gardens that are far too small to drain safely, often by a factor of two to three times. The drain time check that this calculator performs is the single most important calculation that most published rain garden guides either omit entirely or mention only as a footnote. Build to drain time, not to a percentage of catchment.

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## Provenance

- Model ID: `tyg-2749`
- Model version: `1.0.0`
- Reviewed by: Umer Hayiat
- Page modified: 2026-08-24T08:37:45
- Runtime SHA-256: `08f81a69919ff1744837dca99faaeec68f5ea9b0cc097644d13af3606015bcb3`

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