Evaporation loss depends on your pool’s exposed surface area and the depth of water that evaporates—not simply its total gallons. Multiply surface area in square feet by evaporation in inches per day and by 0.6234 to estimate U.S. gallons lost per day. A solid pool cover can reduce evaporation by more than 90% while in use, but actual savings depend on coverage, weather, and how consistently you use it.
Pool Evaporation Inputs and Cover Savings
| Decision or input | Value or method | What it means for your pool |
|---|---|---|
| Exposed water surface area | Square feet; length × width for a rectangle | Use water-surface dimensions, not deck dimensions or pool depth. |
| Evaporation rate | Inches of water lost per day | Use a locally measured rate where possible; temperature, humidity, and wind affect evaporation. |
| Daily evaporation loss | Surface area × inches per day × 0.6234 | Result is U.S. gallons per day. The conversion follows from 231 cubic inches per U.S. gallon. |
| Solid-cover effectiveness | More than 90% evaporation reduction while in use | EPA’s September 2022 residential pool guide gives this reference; it is not a guarantee for every cover or usage schedule. |
| Estimated gallons saved | Uncovered evaporation loss × assumed reduction | Apply the reduction only to evaporation during covered periods—not to leaks, splash-out, or backwashing. |
The calculation converts an evaporation depth into a volume. It does not predict the evaporation rate itself. A reliable estimate therefore starts with the right surface area and a defensible measurement or clearly labeled assumption.
Calculate Gallons Lost to Evaporation
Measure surface area, not total pool volume
For a rectangular water surface:
[
A=L\times W
]
For a circular water surface:
[
A=\pi\times\left(\frac{D}{2}\right)^2
]
Here, (A) is surface area in square feet, (L) and (W) are water-surface length and width in feet, and (D) is diameter in feet. For a freeform pool, divide the surface into approximate geometric sections without double-counting overlapping areas.
Surface area matters because evaporation occurs at the water’s exposed surface. Two pools with the same surface area and evaporation depth lose the same number of gallons even if one is deeper. Depth changes total pool volume, but it does not enter this depth-to-gallons conversion. EPA likewise identifies surface area as a direct influence on the volume lost to evaporation.
Convert evaporation depth into gallons
Use:
[
G=A\times E\times\frac{144}{231}
]
or, rounded:
[
G=A\times E\times0.6234
]
Where:
- (G) = evaporation loss in U.S. gallons per day.
- (A) = exposed water surface area in square feet.
- (E) = evaporation rate in inches per day.
One square foot contains 144 square inches, so a 1-inch layer over 1 square foot contains 144 cubic inches. Dividing by 231 cubic inches per U.S. gallon gives approximately 0.6234 gallons. Do not substitute Imperial gallons or enter a depth in feet into this inches-based formula.
Worked example: uncovered and covered loss
This hypothetical example assumes a rectangular pool measuring 16 by 32 feet and an uncovered evaporation rate of 0.25 inch per day. The evaporation rate is an illustration, not a national average or a prediction for your location.
First, calculate surface area:
[
A=16\times32=512\text{ ft}^2
]
Then calculate uncovered loss:
[
G=512\times0.25\times\frac{144}{231}
\approx79.8\text{ gallons/day}
]
For planning, assume a cover reduces evaporation by 90% throughout that same measurement period:
[
\text{Gallons saved}=79.8\times0.90
\approx71.8\text{ gallons/day}
]
[
\text{Remaining evaporation}=79.8\times(1-0.90)
\approx8.0\text{ gallons/day}
]
| Result | Hypothetical daily amount |
|---|---|
| Uncovered evaporation | 79.8 U.S. gallons |
| Evaporation avoided at an assumed 90% reduction | 71.8 U.S. gallons |
| Remaining evaporation | 8.0 U.S. gallons |
These results assume comparable conditions and continuous coverage during the modeled period. Removing the cover for swimming changes the estimate. Rainfall, refilling, leaks, and other water losses are excluded.
Establish a Local Evaporation Rate
A falling waterline measures net water-level change, not evaporation alone. EPA identifies evaporation, leaks, cleaning, and splashing as separate sources of pool water use. An automatic fill can also conceal losses.
A bucket test helps screen for losses beyond evaporation. EPA’s September 2022 guide describes a 24-hour comparison between water in a partly submerged bucket and the surrounding pool.
1. Choose a day without rain or heavy winds, and keep the pool unused for 24 hours.
- Following equipment instructions, turn off automatic filling, filtration, cleaners, and other pool systems for the test. Do not operate a heater without its required circulation.
- Fill a stable 2.5- to 5-gallon bucket to about 1 inch below its rim. Place it on a submerged step or bench with at least two-thirds of the bucket underwater and its internal water level slightly above the pool level.
- Mark or measure both starting water levels using fixed reference points.
- After 24 hours, measure the drop inside the bucket and in the pool. Record both in inches.
The bucket’s drop provides a short-term evaporation reference under the test conditions. If the pool drops more than the bucket, EPA says the pool may have a leak; investigate rather than treating the entire pool drop as evaporation. The test does not locate a leak or rule out losses that occur only when equipment operates.
For a measurement lasting (t) days:
[
E=\frac{\text{measured evaporation depth in inches}}{t}
]
Repeat measurements under representative conditions before projecting longer-term losses. EPA identifies water temperature, humidity, wind, and surface turbulence as important influences, so one calm day should not be treated as a season-long forecast.
Common pitfall: A cover’s evaporation-reduction percentage is not a reduction in every source of water loss. Multiplying total refill demand by 90% overstates savings when leaks, backwashing, or splash-out contribute to that demand.
Compare Pool Covers by Evaporation Control
The figures below are reference findings reported in EPA’s September 2022 guide, which attributes its cover-effectiveness table to a 2016 study. They are not new 2026 measurements or product-specific performance guarantees.
| Cover option | Reported evaporation reduction while in use | Selection consideration |
|---|---|---|
| Solid physical cover | More than 90% | A strong option for evaporation control; account for handling and removal of water collected on top. |
| Solar bubble cover, thermal cover, or solar rings | 50–95%, depending on shape and coverage | Gaps and incomplete coverage matter. These products are not safety covers. |
| Mesh cover | EPA notes that mesh allows more evaporation than solid covers. Do not assign it solid-cover performance without supporting product data. | |
| Liquid evaporation suppressant | 15% | EPA reports substantially lower reduction than full physical covers; application follows the specific product label. |
Ease of use affects real savings. EPA recommends considering handling when selecting a cover because it works only when applied; an automatic mechanism or compatible reel can make regular deployment easier. A winter cover may suit extended closure but be impractical for daily use.
Evaporation control and safety are separate requirements. Check the specific product’s safety designation, installation instructions, and applicable local requirements rather than assuming that any physical cover can serve as a safety barrier.
Account for Time Uncovered
A 90% reduction while covered does not automatically mean 90% less evaporation over a full day.
For a simplified planning model:
[
\text{Overall reduction}\approx r\times f
]
Where (r) is the assumed reduction while covered and (f) is the fraction of otherwise expected evaporation occurring during covered periods.
If evaporation were uniform throughout the day, covering for 16 of 24 hours at an assumed 90% effectiveness would give:
[
0.90\times\frac{16}{24}=0.60
]
That is a modeled 60% daily reduction—not 90%. However, evaporation is not necessarily uniform by hour: water temperature, humidity, and wind change. Use the calculation to understand the effect of coverage time, not as a guaranteed savings estimate. EPA’s guidance emphasizes both these environmental variables and consistent cover use.
For a heated pool, reducing evaporation also reduces heat loss. EPA reports potential heating-cost savings of 50–70% with solid covers, but that range is separate from the evaporation-reduction percentage and should not be applied directly to your energy bill.
Check the Estimate Before Using It
- Measure the actual water surface, excluding the surrounding deck.
- Enter area in square feet and evaporation depth in inches.
- Label the evaporation rate as measured or assumed.
- Exclude refill events, rain, swimming, and maintenance losses from an evaporation-only measurement.
- Investigate a pool-level drop greater than the bucket-level drop.
- Use cover-effectiveness data appropriate to the cover type and actual coverage.
- Account for uncovered periods instead of applying the covered reduction to the entire day.
- Verify safety suitability and follow the cover manufacturer’s operating instructions.
Use the resulting gallons to plan replacement-water needs and compare cover scenarios. For ongoing decisions, update the evaporation input when conditions change; the unit conversion stays the same, but your pool’s evaporation rate does not.