---
title: "Pond Evaporation Calculator: Size Your Auto-Fill System Before Your Pump Burns Out"
canonical: "https://theyieldgrid.com/pond-evaporation-calculator/"
model_id: "tyg-2739"
model_version: "1.0.0"
last_reviewed: "2026-08-20T11:04:25"
reviewed_by: "Umer Hayiat"
---

# Pond Evaporation Calculator: Size Your Auto-Fill System Before Your Pump Burns Out

> Canonical calculator: [https://theyieldgrid.com/pond-evaporation-calculator/](https://theyieldgrid.com/pond-evaporation-calculator/)

## What this calculator does

Home - Free Gardening Calculators & Tools - Pond Evaporation Calculator: Size Your Auto-Fill System Before Your Pump Burns Out Evaporation is not a background nuisance -- it is a thermodynamic process that operates at a measurable, predictable rate every hour your water feature is exposed to heat, wind, and dry air. A shallow 200-square-foot fountain in July does not just "lose some water." Under the right conditions, it can lose 40 or more gallons per day strictly to atmospheric evaporation, before accounting for splash and spray. That rate does not slow down on weekends. It does not pause while you are at work. If the pump intake drops below the water surface, the motor runs hot and fails.

## Inputs

| Input | ID | Type | Unit | Range or choices | Required |
|---|---|---|---|---|---|
| Water Surface Area | `pondevap_area` | text | feet | e.g. 200 | No |
| Avg Daily High Temperature | `pondevap_temp` | text |  | e.g. 95 | No |
| Average Wind Speed | `pondevap_wind` | text |  | e.g. 10 | No |
| Relative Humidity | `pondevap_humid` | text |  | e.g. 30 | No |

## Outputs

| Output ID | Default state |
|---|---|
| `pondevap_results` | Estimated Daily Water Loss — gallons/day Daily Loss Breakdown Metric Value Evaporation Severity Index Low Moderate Severe Quick Reference: Evaporation by Temperature Temp (°F) Base Rate (in/day) 200 sq ft Loss (gal) Auto-Fill (GPM) How This Calculator Works Step 1 — Base Evaporation Rate: We estimate a base evaporation rate in inches per day from temperature. A simplified thermodynamic model is used: baseRate = 0.00002 × (Temp°F)² − 0.0004 × Temp°F + 0.04 . This produces rates from ~0.05 in/day |
| `pondevap_out_primary` | — |

## Formula and method

Wind speeds exceeding 15 mph disrupt the saturated boundary layer, effectively doubling your evaporation rate. Show the calculation steps Step 1 -- Base Evaporation Rate: A temperature-driven base rate is calculated in inches per day using a quadratic relationship: base rate = (0.00002 × T²) minus (0.0004 × T) plus 0.04, where T is temperature in °F. This approximates the vapor pressure increase with temperature and produces rates from roughly 0.04 in/day at 40°F to 0.24 in/day at 110°F. Step 2 -- Humidity Scaling: The base rate is multiplied by (1 minus H / 100), where H is relative humidity in percent. At 100% humidity the output is zero; at 0% humidity the full base rate applies. This represents the vapor pressure deficit -- the driving force that pulls water molecules off the surface. Step 3 -- Wind Factor: For wind speeds at or below 15 mph, the rate is multiplied by (1 + wind speed × 0.04). For wind speeds above 15 mph, the entire adjusted rate is doubled, reflecting turbulent boundary-layer disruption that continuously replaces the saturated air layer above the water surface with dry ambient air. The 15 mph threshold is the critical split point derived from standard evaporation modeling for open water surfaces. Step 4 -- Gallons Lost per Day: gallons = surface area (sq ft) × (evaporation rate / 12) × 7.48. Dividing by 12 converts inches to feet; multiplying by 7.48 converts cubic feet to US gallons. Step 5 -- Auto-Fill GPM: Required GPM = gallons lost / (24 × 60). This gives the minimum continuous flow rate in gallons per minute that would keep the level perfectly stable over a full day. Rounding: Gallons are displayed to one decimal place. GPM is displayed to four decimal places due to the small magnitudes involved in residential-scale features. Assumptions and Limits Results model open, still-water surface evaporation only. Active spray fountains, waterfalls, and bubblers introduce additional loss (typically 15 to 30% more) through aerosolization and splash that is not captured here. The wind doubling factor above 15 mph is a simplified model. In practice, the effect is continuous and varies with fetch distance, surface roughness, and atmospheric stability -- the doubling approximation is conservative for most residential features. The calculator does not account for shading. A feature shaded for four or more hours per day during peak sun will evaporate meaningfully less than the tool suggests. Rainfall replenishment is not included. In humid climates with regular afternoon storms, actual net loss will be significantly lower than the gross evaporation figure. The formula is calibrated for air temperatures between 40 and 120°F. Inputs outside this range are rejected by the validator because the quadratic model becomes inaccurate at the extremes. Water temperature is assumed to approximate air temperature at the daily high. Deeper ponds or those in shaded settings may have cooler water temperatures and lower actual evaporation than calculated. Results are daily averages. Actual hour-by-hour loss during peak afternoon heat will exceed the daily average rate; overnight evaporation will be lower. The core relationship is: gallons lost = surface area (sq ft) times (evaporation depth in inches per day divided by 12) times 7.48. The evaporation depth depends on temperature, humidity, and wind speed. This tool calculates the depth automatically from those inputs using a thermodynamic model, then converts to gallons. Detailed formula steps are available in the "How the Calculation Works" section above.

## Verified worked examples

### Example 1: Small Backyard Fountain, Mild Summer Conditions

Surface Area: 50 sq ft Temperature: 80°F Wind Speed: 5 mph Relative Humidity: 50% Result: Evaporation rate 0.0816 in/day; daily water loss approximately 2.5 gallons; auto-fill rate 0.0018 GPM. At this scale and in these conditions, a 5-minute hose fill every two to three days is physically adequate. An auto-fill valve is a convenience item, not a mechanical necessity. A standard 1/4-inch trickle valve on city pressure handles this with ease.

### Example 2: Medium Koi Pond, Hot Dry Summer Day

Surface Area: 200 sq ft Temperature: 95°F Wind Speed: 10 mph Relative Humidity: 30% Result: Evaporation rate 0.1789 in/day; daily water loss approximately 22.3 gallons; auto-fill rate 0.0155 GPM (22.3 gallons per 24-hour period). This feature loses 156 gallons in a week. A single garden hose session delivering 50 to 75 gallons falls roughly two days short of full replenishment. Without a float-valve system or at minimum a twice-weekly fill routine, the pump intake will intermittently run dry during the second half of the week, shortening motor life significantly.

### Example 3: Large Commercial Reflecting Pool, High-Wind Arid Climate

Surface Area: 500 sq ft Temperature: 100°F Wind Speed: 20 mph (above 15 mph threshold -- evaporation doubles) Relative Humidity: 15% Result: Evaporation rate 0.340 in/day; daily water loss approximately 106 gallons; auto-fill rate 0.074 GPM. At 106 gallons lost per day, this feature requires a dedicated auto-fill circuit on a 3/4-inch supply line minimum. A standard 1/2-inch residential float valve at typical city pressure maxes out around 2 to 3 GPM, which is sufficient for the volume, but the supply line pressure drop over distance must also be verified. A site this size also loses over 3,100 gallons per month -- a material water-cost line item in arid municipalities with tiered pricing.

## Assumptions

Wind speeds exceeding 15 mph disrupt the saturated boundary layer, effectively doubling your evaporation rate. Show the calculation steps Step 1 -- Base Evaporation Rate: A temperature-driven base rate is calculated in inches per day using a quadratic relationship: base rate = (0.00002 × T²) minus (0.0004 × T) plus 0.04, where T is temperature in °F. This approximates the vapor pressure increase with temperature and produces rates from roughly 0.04 in/day at 40°F to 0.24 in/day at 110°F. Step 2 -- Humidity Scaling: The base rate is multiplied by (1 minus H / 100), where H is relative humidity in percent. At 100% humidity the output is zero; at 0% humidity the full base rate applies. This represents the vapor pressure deficit -- the driving force that pulls water molecules off the surface. Step 3 -- Wind Factor: For wind speeds at or below 15 mph, the rate is multiplied by (1 + wind speed × 0.04). For wind speeds above 15 mph, the entire adjusted rate is doubled, reflecting turbulent boundary-layer disruption that continuously replaces the saturated air layer above the water surface with dry ambient air. The 15 mph threshold is the critical split point derived from standard evaporation modeling for open water surfaces. Step 4 -- Gallons Lost per Day: gallons = surface area (sq ft) × (evaporation rate / 12) × 7.48. Dividing by 12 converts inches to feet; multiplying by 7.48 converts cubic feet to US gallons. Step 5 -- Auto-Fill GPM: Required GPM = gallons lost / (24 × 60). This gives the minimum continuous flow rate in gallons per minute that would keep the level perfectly stable over a full day. Rounding: Gallons are displayed to one decimal place. GPM is displayed to four decimal places due to the small magnitudes involved in residential-scale features. Assumptions and Limits Results model open, still-water surface evaporation only. Active spray fountains, waterfalls, and bubblers introduce additional loss (typically 15 to 30% more) through aerosolization and splash that is not captured here. The wind doubling factor above 15 mph is a simplified model. In practice, the effect is continuous and varies with fetch distance, surface roughness, and atmospheric stability -- the doubling approximation is conservative for most residential features. The calculator does not account for shading. A feature shaded for four or more hours per day during peak sun will evaporate meaningfully less than the tool suggests. Rainfall replenishment is not included. In humid climates with regular afternoon storms, actual net loss will be significantly lower than the gross evaporation figure. The formula is calibrated for air temperatures between 40 and 120°F. Inputs outside this range are rejected by the validator because the quadratic model becomes inaccurate at the extremes. Water temperature is assumed to approximate air temperature at the daily high. Deeper ponds or those in shaded settings may have cooler water temperatures and lower actual evaporation than calculated. Results are daily averages. Actual hour-by-hour loss during peak afternoon heat will exceed the daily average rate; overnight evaporation will be lower. Results model open, still-water surface evaporation only. Active spray fountains, waterfalls, and bubblers introduce additional loss (typically 15 to 30% more) through aerosolization and splash that is not captured here. The wind doubling factor above 15 mph is a simplified model. In practice, the effect is continuous and varies with fetch distance, surface roughness, and atmospheric stability -- the doubling approximation is conservative for most residential features. The calculator does not account for shading. A feature shaded for four or more hours per day during peak sun will evaporate meaningfully less than the tool suggests. Rainfall replenishment is not included. In humid climates with regular afternoon storms, actual net loss will be significantly lower than the gross evaporation figure. The formula is calibrated for air temperatures between 40 and 120°F. Inputs outside this range are rejected by the validator because the quadratic model becomes inaccurate at the extremes. Water temperature is assumed to approximate air temperature at the daily high. Deeper ponds or those in shaded settings may have cooler water temperatures and lower actual evaporation than calculated. Results are daily averages. Actual hour-by-hour loss during peak afternoon heat will exceed the daily average rate; overnight evaporation will be lower. Critical Warnings The Burnt Pump Threshold: When a water feature's daily evaporation loss exceeds the volume delivered by a realistic manual fill session (typically 50 to 75 gallons from a 10 to 15-minute hose fill), running the pump without an auto-fill valve is no longer a judgment call -- it is a predictable failure mode. A pump that ingests air even briefly during a dry-run event generates heat in the motor housing that degrades the seal and winding insulation. Motors that "run dry" intermittently typically fail within one to three seasons without any other cause. Wind Is the Hidden Multiplier: Many pond owners build their water management habits around still-air summer readings. A site that regularly experiences afternoon wind speeds above 15 mph is not operating under the same physics. The boundary-layer disruption effect at those speeds mathematically doubles the evaporation rate. A 200-square-foot pond that loses 22 gallons per day at 10 mph wind can lose 39 gallons per day when wind increases to 20 mph -- with no change in temperature or humidity. This single variable is the most commonly underestimated factor in auto-fill sizing decisions. Shallow Features Fail Faster: The evaporation rate is calculated per unit of surface area, not volume. A shallow fountain basin with 6 inches of water depth loses a proportionally larger fraction of its total volume per day than a deeper koi pond. If your feature has less than 12 inches of operating depth, even moderate evaporation rates can drain the effective pump-intake zone within 48 hours. Auto-Fill Valve Sizing: The calculated GPM output is the minimum spec, not the target spec. Float valves should be sized at 1.5 to 2 times the calculated minimum to account for pressure fluctuation, partial closing during the refill cycle, and any line-length friction loss between the supply tap and the valve. For sizing the pump that moves water through your feature, cross-reference the waterfall pump calculator to confirm the recirculation circuit handles your full flow requirements. Minimum Standards Any water feature with a calculated daily loss above 20 gallons should be equipped with an automatic water-level device. Manual maintenance at that volume is not sustainable across a full season without risk of pump damage. Supply lines for auto-fill valves serving features with more than 50 gallons per day of calculated loss should be 3/4-inch minimum to reduce friction loss and maintain adequate fill pressure. Running a high-demand valve on a long 1/2-inch line creates a flow restriction that prevents the valve from fully compensating. Submersible pumps used in features with auto-fill systems should include an auto-shutoff water-level sensor as a backup, independent of the float valve. Float valves can stick open or closed. A secondary water-level sensor protects both the pump and the surrounding landscape. If water management requirements extend to your broader irrigation design, the turf watering calculator can help you model the added demand on a shared supply line. Competitor Trap: Most pond evaporation guides and calculators online present a single static evaporation rate for a given temperature -- typically 0.25 inches per day or some similar flat number -- and apply it universally. That approach ignores humidity (which can cut the effective rate in half), ignores wind speed entirely, and does not flag the 15 mph threshold where the underlying physics change. A homeowner in Phoenix and a homeowner in Atlanta with identical 200-square-foot ponds at identical temperatures are not experiencing the same evaporation. Treating them as equivalent leads to undersized auto-fill systems in dry climates and oversized ones in humid ones. The formula in this tool adjusts for all three variables simultaneously. Any water feature with a calculated daily loss above 20 gallons should be equipped with an automatic water-level device. Manual maintenance at that volume is not sustainable across a full season without risk of pump damage. Supply lines for auto-fill valves serving features with more than 50 gallons per day of calculated loss should be 3/4-inch minimum to reduce friction loss and maintain adequate fill pressure. Running a high-demand valve on a long 1/2-inch line creates a flow restriction that prevents the valve from fully compensating. Submersible pumps used in features with auto-fill systems should include an auto-shutoff water-level sensor as a backup, independent of the float valve. Float valves can stick open or closed. A secondary water-level sensor protects both the pump and the surrounding landscape. If water management requirements extend to your broader irrigation design, the turf watering calculator can help you model the added demand on a shared supply line. Competitor Trap: Most pond evaporation guides and calculators online present a single static evaporation rate for a given temperature -- typically 0.25 inches per day or some similar flat number -- and apply it universally. That approach ignores humidity (which can cut the effective rate in half), ignores wind speed entirely, and does not flag the 15 mph threshold where the underlying physics change. A homeowner in Phoenix and a homeowner in Atlanta with identical 200-square-foot ponds at identical temperatures are not experiencing the same evaporation. Treating them as equivalent leads to undersized auto-fill systems in dry climates and oversized ones in humid ones. The formula in this tool adjusts for all three variables simultaneously.

## Limitations and safety

Results model open, still-water surface evaporation only. Active spray fountains, waterfalls, and bubblers introduce additional loss (typically 15 to 30% more) through aerosolization and splash that is not captured here. The wind doubling factor above 15 mph is a simplified model. In practice, the effect is continuous and varies with fetch distance, surface roughness, and atmospheric stability -- the doubling approximation is conservative for most residential features. The calculator does not account for shading. A feature shaded for four or more hours per day during peak sun will evaporate meaningfully less than the tool suggests. Rainfall replenishment is not included. In humid climates with regular afternoon storms, actual net loss will be significantly lower than the gross evaporation figure. The formula is calibrated for air temperatures between 40 and 120°F. Inputs outside this range are rejected by the validator because the quadratic model becomes inaccurate at the extremes. Water temperature is assumed to approximate air temperature at the daily high. Deeper ponds or those in shaded settings may have cooler water temperatures and lower actual evaporation than calculated. Results are daily averages. Actual hour-by-hour loss during peak afternoon heat will exceed the daily average rate; overnight evaporation will be lower. Critical Warnings The Burnt Pump Threshold: When a water feature's daily evaporation loss exceeds the volume delivered by a realistic manual fill session (typically 50 to 75 gallons from a 10 to 15-minute hose fill), running the pump without an auto-fill valve is no longer a judgment call -- it is a predictable failure mode. A pump that ingests air even briefly during a dry-run event generates heat in the motor housing that degrades the seal and winding insulation. Motors that "run dry" intermittently typically fail within one to three seasons without any other cause. Wind Is the Hidden Multiplier: Many pond owners build their water management habits around still-air summer readings. A site that regularly experiences afternoon wind speeds above 15 mph is not operating under the same physics. The boundary-layer disruption effect at those speeds mathematically doubles the evaporation rate. A 200-square-foot pond that loses 22 gallons per day at 10 mph wind can lose 39 gallons per day when wind increases to 20 mph -- with no change in temperature or humidity. This single variable is the most commonly underestimated factor in auto-fill sizing decisions. Shallow Features Fail Faster: The evaporation rate is calculated per unit of surface area, not volume. A shallow fountain basin with 6 inches of water depth loses a proportionally larger fraction of its total volume per day than a deeper koi pond. If your feature has less than 12 inches of operating depth, even moderate evaporation rates can drain the effective pump-intake zone within 48 hours. Auto-Fill Valve Sizing: The calculated GPM output is the minimum spec, not the target spec. Float valves should be sized at 1.5 to 2 times the calculated minimum to account for pressure fluctuation, partial closing during the refill cycle, and any line-length friction loss between the supply tap and the valve. For sizing the pump that moves water through your feature, cross-reference the waterfall pump calculator to confirm the recirculation circuit handles your full flow requirements. Minimum Standards Any water feature with a calculated daily loss above 20 gallons should be equipped with an automatic water-level device. Manual maintenance at that volume is not sustainable across a full season without risk of pump damage. Supply lines for auto-fill valves serving features with more than 50 gallons per day of calculated loss should be 3/4-inch minimum to reduce friction loss and maintain adequate fill pressure. Running a high-demand valve on a long 1/2-inch line creates a flow restriction that prevents the valve from fully compensating. Submersible pumps used in features with auto-fill systems should include an auto-shutoff water-level sensor as a backup, independent of the float valve. Float valves can stick open or closed. A secondary water-level sensor protects both the pump and the surrounding landscape. If water management requirements extend to your broader irrigation design, the turf watering calculator can help you model the added demand on a shared supply line. Competitor Trap: Most pond evaporation guides and calculators online present a single static evaporation rate for a given temperature -- typically 0.25 inches per day or some similar flat number -- and apply it universally. That approach ignores humidity (which can cut the effective rate in half), ignores wind speed entirely, and does not flag the 15 mph threshold where the underlying physics change. A homeowner in Phoenix and a homeowner in Atlanta with identical 200-square-foot ponds at identical temperatures are not experiencing the same evaporation. Treating them as equivalent leads to undersized auto-fill systems in dry climates and oversized ones in humid ones. The formula in this tool adjusts for all three variables simultaneously.

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## Provenance

- Model ID: `tyg-2739`
- Model version: `1.0.0`
- Reviewed by: Umer Hayiat
- Page modified: 2026-08-20T11:04:25
- Runtime SHA-256: `9c334adb5a6a0eb7f10c7bb746c133a7c9d48c76d1639e9d777f487f6a92e625`

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