Pool Evaporation Calculator
Pool evaporation loss in gallons or liters, with cover savings, from area and rate.
What it does
Evaporation is the quiet pool bill. An uncovered 500 ft2 pool can lose 12,000 to 31,000 gallons a year to the air, per EPA WaterSense, with heated and desert pools running highest. Owners see only the hose refills, not the vapor leaving daily.
The tool above turns area and daily evaporation depth into gallons or liters lost. Enter surface area, rate per day, and days. It returns period loss, daily loss, and the savings a cover brings at up to 95% prevention. Defaults of 400 ft2 at a quarter inch over 7 days open solved at 436.1 gallons.
The conversion is pure geometry. One inch of water over one square foot fills 144 cubic inches, and a gallon holds 231, so each square foot sheds 0.623 gallons per inch of evaporation. Metric runs even cleaner: one millimeter over one square meter is exactly one liter.
Climate sets the rate while the pool sets the multiplier. Heat, low humidity, wind, and water temperature push the daily depth up; shade, covers, and cooler nights pull it down. A quarter inch a day suits warm summers, while desert heat can double it and humid shade can halve it.
Use the table to compare weeks and months, then run the pool. If hose refills clearly beat the math, suspect splash out or a leak and run a bucket test before blaming the air.
How to use
- Pick imperial (ft2, inches, gallons) or metric.
- Enter pool surface area, not volume.
- Enter daily evaporation depth for your climate.
- Enter days in the period.
- Read period loss first, then daily rate.
- Compare cover savings before buying one.
Formula
- Area
- Pool surface area: ft2 imperial, m2 metric.
- Rate
- Daily evaporation depth: inches imperial, mm metric.
- Days
- Days in the period, 1 to 365.
- Factor
- 0.623 gal per ft2 per inch; 1 L per m2 per mm.
One inch of water over one square foot equals 0.623 gallons; one millimeter over one square meter equals exactly 1 liter. Multiply by days for the period total. Cover savings assume up to 95% prevention per EPA WaterSense. Enter a local pan rate or the 0.25 in/day starting point.
Worked example
Example: 400 ft2 pool, quarter inch a day, one week
A 400 ft2 pool evaporates 0.25 in per day. Find the 7 day loss and cover savings.
- Daily loss: 400 x 0.25 x 0.623 = 62.3 gal per day.
- One inch over one square foot is 0.623 gal; a quarter inch scales it down.
- Week total: 62.3 x 7 = 436.1 gal.
- Cover savings at 95%: 436.1 x 0.95 = 414.3 gal.
- Without a cover that water leaves as vapor plus heat.
- Metric mirror: 37.2 m2 at 6.4 mm/day loses 238.1 L per day.
Reference table
| Pool and rate | Daily | 7 days | Cover saves (7 days) |
|---|---|---|---|
| 400 ft2 at 0.25 in/day | 62.3 gal/day | 436.1 gal | 414.3 gal |
| 37.2 m2 at 6.4 mm/day | 238.1 L/day | 1666.6 L | 1583.2 L |
| 500 ft2 at 0.3 in/day, 30 days | 93.5 gal/day | 2803.5 gal | 2663.3 gal |
| 50 m2 at 5 mm/day, 30 days | 250 L/day | 7500 L | 7125 L |
Limits
- Evaporation only; splash, leaks, and backwash add more.
- Rate is climate input, not measured here.
- Cover savings are an up to value.
- Heated and windy pools beat starting rates.
Common mistakes
- Using pool volume instead of surface area.
- Running one summer rate all year.
- Blaming evaporation for a real leak.
- Ignoring that heating raises the rate.
- Pricing water without sewer and heat costs.
Try it yourself
Quick checks. Answers sit tucked under each question.
500 ft2 pool at 0.3 in/day for 30 days?
Daily 500 x 0.3 x 0.623 = 93.5 gal. Total 93.5 x 30 = 2803.5 gal. Saved 2663.3 gal.
50 m2 pool at 5 mm/day for 30 days?
Daily 50 x 5 = 250 L. Total 250 x 30 = 7500 L. Saved 7125 L.
Metric default in gallons?
37.2 x 6.4 = 238.1 L per day, which is 62.9 gal. The imperial default gives 62.3 gal for the same weather.
Questions
Where does 0.623 come from?
About 0.623 gal: one square foot times one inch is 144 cubic inches, and a gallon holds 231, so 144 / 231 = 0.623.
Does loss scale with the rate?
Yes, roughly linear for small changes: double the rate and the loss doubles. Wind and heating bend it in practice.
How much does a cover save?
Up to 95% of evaporation, per EPA. Covers also keep heat, which cuts heating bills on top of water savings.
Does the metric default match?
6.4 mm over 37.2 m2 is 238.1 L per day, or 62.9 gal. Same pool, same weather, both modes agree.
What else loses pool water?
Backwash, splash out, leaks, and filter cleaning. If refills beat the evaporation math by far, look for a leak with a bucket test.
Which pools evaporate fastest?
Shallow, hot, windy, heated, and busy pools lose fastest. Indoor pools lose less but still need makeup water and ventilation.
Sources
- EPA WaterSense: pool water efficiency and evaporation
- USGS SIR 2012-5202: estimation of evaporation from open water