Pool Evaporation Calculator
The rule of thumb says “a quarter inch a day” — but evaporation triples between a humid calm day and a dry windy one, and the same pool can lose 100 gallons on one day and 350 on another. This page computes your actual loss from the vapor-pressure physics, lets you compare it to what you're measuring, and answers the question that brought most people here: leak or evaporation?
Hook
“A pool loses about a quarter inch a day.”
You'll read that everywhere — and it's almost useless, because the real rate triples depending on your weather. The same pool that loses 100 gallons on a humid, still day loses 350 on a dry, windy one. If you're trying to figure out whether your water level is dropping because of evaporation or a leak, a flat rule of thumb can't tell you. The physics can.
Promise
This calculator computes your pool's actual evaporation from the vapor-pressure physics — your water temperature, the air, the humidity, and the wind — and gives you gallons per day and per week for your conditions. Compare it to what you're actually losing: if you're losing a lot more than the physics predicts, you may have a leak, not evaporation. And it shows you exactly how much a cover saves. Every term derived on the page.
Here's the deal: evaporation isn't really about temperature — it's about the difference in water-vapor pressure between your warm pool and the air above it. Warm water wants to evaporate; dry, moving air pulls the vapor away faster. Get those two right and you can predict your water loss closely enough to tell normal evaporation from a leak — and to see why a cover changes everything.
What you'll give us
The four physics inputs: water temperature, air temperature, humidity (the one the rule of thumb omits), and wind. Plus surface area for the loss footprint. Optionally, the water loss you're actually measuring — we'll compare it to the physics and tell you whether to suspect a leak. The volume calculator deep-links here with ?gal= prefilled.
The calculator
Fill the fields, hit Calculate. The result panel returns gallons per day, gallons per week, and inches per day (the rule-of-thumb unit), the equivalent BTU/hr of heat carried off, the cover-on alternative regardless of toggle, and — if you entered your actual loss — the leak-or-evaporation verdict.
Leak or evaporation? The question everyone's actually asking.
If you're here, there's a decent chance your water level is dropping and you're worried it's a leak. Here's how to tell. We compute how much your pool shouldbe evaporating given your actual conditions — your water temperature, the air, the humidity, the wind. If you're losing roughly that much, it's just evaporation, and it's normal.
If you're losing dramatically more — say, your conditions predict 130 gallons a day and you're down 400 — evaporation can't explain it, and you should run the bucket test. A flat “quarter inch a day” can't make this call, because evaporation triples between a humid calm day and a dry windy one. A climate-blind number can't distinguish normal-but-high evaporation from a leak. Ours can, because it knows your weather.
Humidity is the hidden driver
Most people think evaporation is about heat — hot day, more evaporation. Half right. What actually drives it is the differencein water-vapor pressure between your pool and the air. Warm water has a high vapor pressure; it wants to escape into the air. But if the air is already humid — already full of water vapor — there's less room for more, and evaporation slows. Dry air is thirsty air.
That's why the same pool on the same 85-degree day loses nearly twice as much water when the humidity is 30 % as when it's 90 % (178 vs 98 gallons per day). The clones' flat rule of thumb can't see this — it has no humidity input. This page does.
Why a cover wins
Across this site's heating calculators, one piece of advice keeps coming up: get a cover. This is why. Evaporation is the single biggest way your pool loses both water and heat — and a cover stops it cold by putting a barrier between the warm water and the thirsty air. It's not insulation in the blanket sense; it's an evaporation lid. Cut the evaporation and you cut the dominant loss, which is why a cover saves more water and more money than any other single change.
Three views, one physics — and we're candid about how the multipliers reconcile. A cover cuts EVAPORATION specifically by about 90 % (we use a 0.10 multiplier here). It cuts the COMBINED heat loss (evaporation + convection + radiation) by ~65 %, which is the figure the heater-sizing and cost-to-heat pages use. Both numbers describe the same cover; they describe different things about it. Conv/rad get hit too, but not as completely as evaporation does — and our cover-decomposition assertion proves the two multipliers reconcile exactly at standard conditions.
Reading your evaporation rate
Four things set your evaporation rate, and you control some more than others. Wind is the biggest swing you can do something about — sheltering or covering the pool cuts a windy day's loss by an order of magnitude. Humidity sets how much your air can absorb in the first place. Water temperature is a slower lever — and raising it costs both heat and water. Air temperature looks counterintuitive: at fixed humidity, cooler air actually evaporates harder, because cool air's saturation pressure is lower, so the same RH leaves a bigger vapor-pressure gap.
Where the numbers come from
Five steps from temperature + humidity + wind to gallons per day, all derivable on the page. The calibration step is candid — the “verified or omitted” philosophy applies hard here.
Step 1 · saturation vapor pressure (Magnus)
Saturation vapor pressure is the “wants to escape” pressure of water vapor at a given temperature. We compute it with the Alduchov–Eskridge refinement of the Magnus relation:
svp(T °C) = 6.1094 × exp(17.625 × T / (243.04 + T))in hPa, then converted to inHg. At 85 °F, svp = 1.2117 inHg; at 70 °F, svp = 0.7379 inHg.Step 2 · the vapor-pressure gap
Evaporation is driven by the gap between the water's svp and the air's effective vapor pressure:
gap = svp(Twater) − RH × svp(Tair)in inHg. At standard conditions, gap = 1.21 − 0.6 × 0.74 = 0.77inHg. When humidity rises the air's side moves up toward the water's, and the gap shrinks — same temperature, less evaporation.Step 3 · the calibrated mass-transfer coefficient (the candor step)
The mass-transfer coefficient turns “vapor-pressure gap” into “pounds of water per hour per square foot.” The textbook indoor / ASHRAE form is
0.089 + 0.0782 × windMph— but that's the OCCUPIED-pool form, where bathers strip the surface boundary layer. It overstates residential evaporation about six times. We scale by ~0.163 to the unoccupied-residential basis that reconciles with this site's heat-loss engine:C0 = 0.0145 , C1 = 0.0127.That calibration is an engineering reconciliation, NOT a fundamental constant.
Stating it honestly here — “we picked the multiplier that makes the evaporation page agree with the heater-sizing engine on the same standard pool” — is the page's credibility wedge. We'd rather show our work and be wrong by 10 % at the extremes than hand you a number with hidden assumptions.
Step 4 · rate → gallons + BTU
rate (lb/hr/ft²) = (C0 + C1 × windMph) × gap. Multiply by surface ft² to get lb/hr; by 24 ÷ 8.345 lb/gal to get gal/day; by 1050 BTU/lb (latent heat of vaporization) to get BTU/hr of heat carried off. Those last two — gallons and BTU — share the same constants as the cost calculator, so this page's water loss and that page's heat loss can't disagree about how much heat each pound of evaporation removes.
Step 5 · sanity check (E1)
Standard pool: 800 ft², 85 °F water, 70 °F air, RH 60 %, 5 mph. Engine reports ≈ 138 gal/day (≈ 966 gal/week), carrying off ≈ 50381 BTU/hrof heat. That heat figure is ~60 % of the engine's combined 84,000 BTU/hr — exactly the reconciliation step 3's calibration was designed to produce. The two pages describe the same pool.
On the cover side, 10 % is the EVAPORATION-specific cover multiplier (a cover nearly eliminates evaporation), while 35 % is the COMBINED cover multiplier the engine uses for total loss. The two decompose exactly: 0.10·0.60 + 0.72·0.40 = 0.35.
Eight worked examples
All numbers consume the asserted lib/thermal/evaporation.ts engine and are locked in assert-evaporation.mjs. Standard pool unless noted (800 ft²).
E1 — Standard pool, typical conditions (the baseline)
Water 85 °F, air 70 °F, RH 60 %, 5 mph average wind → ~138 gallons/day (~966gallons/week), carrying off ≈50,400 BTU/hr — ~60 % of the engine's total surface loss.
Takeaway:a typical pool loses about 130–140 gallons a day to evaporation — roughly an inch every two days. That's normal, and it's most of your heating bill.
E2 — Dry air (the humidity wedge, low side)
Same pool, RH 30 % instead of 60 % → ~178 gallons/day. Nearly 30 % more than the humid case at the same temperature.
Takeaway:dry air is thirsty air. In an arid climate your pool evaporates far more than the rule of thumb predicts — temperature didn't change, humidity did.
E3 — Humid air (the humidity wedge, high side)
Same pool, RH 90 % → ~98 gallons/day. About half the dry-air rate.
Takeaway:in a humid climate the air is already full, so your pool holds its water better. Same pool, same heat, half the loss — which a flat “¼ inch a day” rule can't tell you.
E4 — Windy (the wind wedge)
Same pool, 15 mph wind → ~363 gallons/day. More than 2.5× the calm-average rate.
Takeaway: wind is the biggest evaporation driver you can actually do something about. A windbreak or a cover pays off fastest on an exposed pool.
E5 — Dead calm (the still-air floor)
Same pool, 0 mph → ~26 gallons/day. A fraction of the breezy rate, because a saturated layer sits over the water and slows further evaporation.
Takeaway:still air is your friend. It's why sheltered pools and covers work, and why a windy site is the worst case.
E6 — Cool dry night with a breeze (the counterintuitive case)
Water 80 °F, air 55 °F, RH 40 %, 8 mph → ~229 gallons/day. High, despite the cool air.
Takeaway:a cool night isn't a low-evaporation night if the air is dry and moving. Your warm water against thirsty, breezy air still loses hard — which is why pools keep evaporating overnight.
E7 — Leak or evaporation? (the killer-app, worked)
Standard pool (computed ~138 gal/day), but the owner measures ~400 gal/day of actual loss. Actual is ~2.9× the computed evaporation — evaporation can't explain it. Suspect a leak; run the bucket test.
Takeaway: the number isn't “is 400 gallons a lot?” — it's “is 400 a lot for your conditions?” At 138 expected, 400 means call a leak detection service after the bucket test confirms.
E8 — The cover (corrected — uses the evaporation-specific multiplier)
Standard pool (138 gal/day uncovered) with a cover → evaporation drops to ~14 gal/day, saving ~124 gallons a day.
Takeaway: a cover nearly eliminates evaporation — about a 90 % cut, much stronger than the combined heat-loss cover figure (~65 %) the sizing/cost pages quote. Both numbers are right; they describe different physics about the same cover, and they reconcile exactly.
Reference tables
T1 · Evaporation by air temperature × humidity
ESTIMATE · standard 800 ft² pool, water 85 °F, 5 mph wind, uncovered. Gal/day at default residential-calibrated coefficients. Your absolute numbers shift with surface area and water temp.
| Air °F \ RH | 30 % | 50 % | 70 % | 90 % |
|---|---|---|---|---|
| 60 °F | 189 | 171 | 152 | 133 |
| 70 °F | 178 | 151 | 125 | 98 |
| 80 °F | 162 | 125 | 88 | 51 |
T2 · Evaporation by wind (the biggest controllable lever)
ESTIMATE · standard pool, 85 °F water, 70 °F air, RH 60 %. The model is residential-calibrated; at high wind (15 mph and above) the vapor-pressure model and the engine's linear-in-wind model can diverge 20–30 %.
| Wind | Description | gal/day | vs 5 mph baseline |
|---|---|---|---|
| 0 mph | Dead calm | 26 | 0.19× |
| 1.5 mph | Sheltered | 59 | 0.43× |
| 5 mph | Average | 138 | 1.00× |
| 15 mph | Windy | 363 | 2.63× |
T3 · Cover savings by pool size
ESTIMATE · 85/70 °F, RH 60 %, 5 mph wind. Uses the EVAPORATION-specific cover multiplier (~10 % of uncovered); the COMBINED heat-loss cover figure on the sizing/cost pages is the engine's 35 %. Both are right; they describe different physics.
| Pool size | Uncovered gal/day | Covered gal/day | Saving gal/day | Saving gal/week |
|---|---|---|---|---|
| Small (~500 ft²) | 86 | 9 | 78 | 543 |
| Standard (800 ft²) | 138 | 14 | 124 | 869 |
| Large (1,100 ft²) | 190 | 19 | 171 | 1,195 |
Tables released CC BY 4.0. All figures are residential-calibrated estimates at typical conditions — use them to compare scenarios, not as meter readings.
Sanity: E1 covered ≈ 14 gal/day at the standard pool.
Methodology & sources
Evaporation is a vapor-pressure-driven mass transfer. The model is the ASHRAE pool form: rate (lb/hr/ft²) = (C0 + C1 × windMph) × (svp_water − RH × svp_air) with saturation vapor pressure computed via the Alduchov–Eskridge refinement of the Magnus relation. The relation is accurate to better than 0.1 % over the temperature range any realistic pool sees.
The calibration, stated plainly. The raw textbook coefficient (0.089 + 0.0782 × mph) is the indoor / occupied-pool form, where bathers strip the surface boundary layer. It overstates residential evaporation about six times. We scale by ~0.163 to the unoccupied-residential basis (C0 = 0.0145, C1 = 0.0127) — the values that reconcile this page's evaporative heat loss to ~60 % of the heater-sizing engine's combined surface loss at standard conditions. That calibration is an engineering reconciliation, not a fundamental constant. Stating it is the credibility wedge — every figure on this page rides on it, and you deserve to see the assumption.
The shared physics constants — latent heat of vaporization (1050 BTU/lb), water density (8.345 lb/gal) — are imported from the same modules the heater-sizing and cost-to-heat pages use. That's why gallons-lost (here) and BTU-lost (the cost page) can't disagree about how much heat each pound of evaporation removes.
The cover decomposition. The engine uses a combined cover-loss multiplier of 0.35 (cover cuts TOTAL heat loss by ~65 %). For EVAPORATION specifically a cover is more effective: we use 0.1 (cover cuts evaporation by ~90 %). The two are consistent — at standard conditions, where evap is ~60 % of combined loss and conv/rad are ~40 %, the implied non-evap cover fraction is ~0.72 and 0.10·0.60 + 0.72·0.40 = 0.35 exactly. The assertion script proves the decomposition holds.
The leak-vs-evaporation inference and bucket test are practical methods, not certified diagnostics. We compute the expected loss for your conditions and compare; a ratio above 1.75× suspects a leak and routes to the bucket test, which is the standard practical confirmation. We never declare a leak with certainty — we suspect and route to the test.
State plainly. The output is a residential-calibrated estimate range. Two reasonable models can differ 20–30 % at high wind. Use it to compare conditions and compare to your actual loss — not as a meter reading. The honesty is the point.
Reference tables T1/T2/T3 are released under CC BY 4.0. All entries are model-generated at typical residential conditions.
Frequently asked questions
- How much water should my pool lose to evaporation?
For a standard 800 ft² pool at typical conditions — water 85 °F, air 70 °F, RH 60 %, light breeze — about 130–140 gallons per day, or roughly a quarter inch a day. Your number scales with surface area, swings up to 30 % with humidity, and can triple with wind. The rule of thumb is in the right ballpark, but it can't see your weather.
- Is my pool leaking or just evaporating?
Compute the expected evaporation for your conditions and compare to what you're actually measuring. If actual is roughly equal (under 1.3× expected), it's normal evaporation. 1.3–1.75× is elevated and worth a bucket test. Above 1.75× is a suspect-leak signal: run the bucket test, and if confirmed call a leak-detection service.
- Why does my pool evaporate more in dry weather?
Because evaporation is driven by the vapor-pressure difference between your warm water and the air, not by temperature alone. Dry air has a low effective vapor pressure, so the gap to the water is bigger and evaporation runs faster. Same pool at 85 °F loses ~178 gal/day at 30 % humidity vs ~98 gal/day at 90 % humidity.
- Does wind really increase evaporation that much?
Yes. Wind strips the saturated boundary layer above the water and lets the next batch of dry air contact the surface. Same pool at 5 mph average wind loses 138 gal/day; at 15 mph windy it loses 363 — about 2.6× more. A windbreak or a cover pays off fastest on an exposed pool.
- How much evaporation does a pool cover stop?
About 90 %of evaporation specifically — a cover is an evaporation lid. That's stronger than the combined heat-loss reduction (~65 %) the sizing and cost pages quote, because covers block evaporation harder than they block radiation or convection. Both numbers are right; they describe different things about the same cover. The two multipliers reconcile exactly at standard conditions.
- Does my pool evaporate at night?
Yes — sometimes hard. A cool dry night with a breeze (water 80 °F, air 55 °F, RH 40 %, 8 mph) still loses ~229 gal/dayon a standard pool, because your warm water is up against thirsty moving air. Night isn't a low-evaporation regime by default; it's just one where humidity often climbs and slows things down.
- How accurate is this evaporation estimate?
It's a residential-calibrated estimate, designed for comparison to your actual water loss — not as a meter reading. The mass-transfer coefficient is reconciled to the heat-loss engine so the two pages describe the same physics; the raw textbook indoor coefficient would overstate residential evaporation about six times. Two reasonable models can still differ 20–30 % at high wind. Use the figure to compare conditions and compare to your measured loss, not to forecast to the gallon.
- How do I do the bucket test?
- Float a bucket of pool water on the pool steps with the bucket water level matching the pool water level.
- Mark both levels on the bucket with a piece of tape.
- Wait 24 hours.
- Compare the drops: if the pool dropped more than the bucket, the difference is your leak rate (evaporation hits both equally). Repeat with the pump off if you suspect a plumbing leak.
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