Stepwise.

Pool Water Loss Calculator

Tell a leak from ordinary evaporation using the bucket test, convert the loss into gallons and money, and check it against what evaporation should be.

How to use this calculator

  1. 1Mark the water level against something fixed — tape on the skimmer throat is easiest, because the water sits still there.
  2. 2Do the bucket test rather than relying on an estimate. It measures evaporation directly under exactly your conditions, and it costs a bucket and a day.
  3. 3Stand the bucket on a step so it is partly submerged. Bucket water at a different temperature from the pool invalidates the test.
  4. 4Note any rain and add it back. Rain refills the pool while the loss continues underneath, and it is the commonest reason a leak looks intermittent.
  5. 5Count backwashing and splash-out separately. A sand filter backwash is 200 to 500 gallons and will show up as a leak on a weekly measurement.
  6. 6If it is leaking, repeat with the pump off for 24 hours. A leak that changes with the pump tells you which side of the system to look at.

How the calculation works

Gallons per inch of depth = surface area × 0.623 True loss = observed drop + rainfall Leak = pool loss per day − evaporation per day where evaporation comes from the bucket test, or failing that from E (lb/h·ft²) = Fa × (95 + 0.425 v) × (Pw − Pa) ÷ 1000 inches per day = E × 24 × 0.1922 Pw and Pa are the vapour pressures at the water surface and in the air.
0.623
Gallons per square foot per inch of depth — the same constant as rainfall catchment
Bucket test
A bucket of pool water on a step evaporates at the pool’s rate. The difference between the two is the leak
Pw − Pa
The vapour pressure difference driving evaporation. Warm water and dry air maximise it
v
Air velocity at the water surface, in feet per minute. Wind roughly doubles evaporation between still and breezy
Fa
ASHRAE activity factor — 1.0 for a public pool, 0.5 for an unoccupied residential one. The equation was derived for the former

Only the surface area matters for evaporation. Depth changes the volume but not the rate at which water leaves the top.

The bucket test is a measurement and the conditions-based estimate is a prediction. Where they disagree, the bucket is right.

Rain must be added back to the observed drop, since it refills the pool while the loss continues underneath it.

Worked example

A 32 × 16 ft pool losing 1.5 inches in a week, bucket down 0.9

  1. 1.Surface area = 32 × 16 = 512 sq ft, so one inch of depth is 512 × 0.623 = 319 gallons.
  2. 2.The pool dropped 1.5 in over 7 days, which is 0.214 in a day — about 68 gallons a day.
  3. 3.The bucket dropped 0.9 in over the same 7 days, or 0.129 in a day. Both were in the same sun, air and wind, so both evaporated at the same rate.
  4. 4.The difference is 0.214 − 0.129 = 0.086 in a day, and that is the leak.
  5. 5.In gallons: 0.086 × 319 = about 27 gallons a day.
  6. 6.At 1.2 cents a gallon that is roughly $10 a month in water — and the water is the cheap part, because the leak also carries away heat and chemicals.
  7. 7.Next step is to repeat the measurement with the pump off for 24 hours. If the leak shrinks, it is on the pressure side — returns, filter or heater. If it stays or grows, look at the suction side and the main drain.

Result: 27 gallons a day of genuine leak, on top of 41 of evaporation

The same pool with no bucket test, in autumn with the heater on

  1. 1.The pool dropped 3.75 in over 7 days — 0.536 in a day, or 171 gallons. That looks alarming next to the previous example.
  2. 2.But the conditions are completely different: 80°F heated water under 50°F air with 5 mph of wind.
  3. 3.Cold air holds very little moisture, so the vapour pressure difference driving evaporation is large — larger than on a hot summer day with humid air.
  4. 4.The estimate for these conditions is about 0.53 inches a day, essentially all of the observed loss.
  5. 5.So this pool is almost certainly not leaking. It is a heated pool in autumn, which is the single most evaporative situation a pool experiences.
  6. 6.This is exactly the case where owners call a leak detection service in October and are told there is nothing wrong.
  7. 7.A cover would cut around 90% of it — worth about $55 a month in water here, and far more in the heat that is going up with the vapour.

Result: 0.53 in a day estimated against 0.54 measured — evaporation, not a leak

The test that settles it in a day

The bucket test is the closest thing pool care has to a free diagnostic, and it works because it removes the only variable that makes water loss ambiguous.

Evaporation depends on water temperature, air temperature, humidity and wind, and all four change constantly. Predicting it is possible but imprecise. Measuring it is trivial: put a bucket of pool water where it experiences exactly the same conditions as the pool, and whatever it loses, the pool lost to evaporation too.

The mechanics matter. The bucket goes on a step, partly submerged, so the water inside stays at pool temperature — a bucket on the deck warms up in the sun and evaporates faster, which makes the pool look fine when it is not. Mark both levels on the bucket itself so the comparison is direct. Leave the pump running as normal, because a leak on the pressure side only leaks when the pump runs. And do not swim, backwash, or run the test through rain.

After 24 hours, both marks drop by the same amount if the pool is sound. Any extra drop in the pool is a leak, measured in inches, which converts straight to gallons through the surface area.

It is worth doing before spending anything. Leak detection services are not cheap, and a substantial fraction of the pools they are called to are evaporating normally.

What normal actually looks like

A quarter to half an inch a day is ordinary for an uncovered pool in summer. On a 500 square foot surface that is 160 to 320 gallons a day, and it is the gallon figure that alarms people.

The conversion is the reason. One inch of depth over 500 square feet is 311 gallons, so a loss that is barely visible against the tile is hundreds of gallons — enough to sound like a plumbing failure when it is described in the units the water company uses.

What drives it is the difference in vapour pressure between the water surface and the air above, multiplied by how fast that air is moving. Warm water raises the first term. Dry air lowers the second, which increases the difference. Wind removes the saturated layer sitting just above the surface and replaces it with fresh dry air, which is why wind matters so much — an exposed pool can lose twice what a sheltered one does in identical weather.

The counterintuitive case is autumn. A heated pool at 80°F under 50°F air evaporates faster than the same pool under 90°F air in August, because cold air holds very little moisture and the vapour pressure difference is therefore larger. Heated pools lose remarkable amounts of water in October, and their owners reliably suspect leaks.

Finding a real leak

Once the bucket test confirms a leak, a few cheap observations narrow it down a long way before anyone with equipment is needed.

  • Pump on versus pump off — repeat the 24-hour measurement with the pump off. A leak that is much worse with the pump running is on the pressure side — returns, filter, heater or the pipework between them. One that is worse with the pump off is usually the suction side or the main drain, since those are under vacuum when running and static pressure when not.
  • Where the level settles — if the water stops dropping at a particular height and stays there, the leak is at that level. Water stopping at the skimmer throat points at the skimmer; at the light, the light niche; part way down a wall, a fitting or crack at that depth.
  • The skimmer first — the joint between a plastic skimmer and its pipe, and cracks in the skimmer throat itself, are the single commonest source of pool leaks. It is also the easiest to repair.
  • Wet ground and settling — a persistently damp patch of lawn, sinking pavers, or a deck section that has dropped points at underground pipework — and that damage is often more expensive than the water.
  • Dye testing — with the pump off and the water perfectly still, release a little dye beside a suspected fitting and watch. Water moving into a crack draws the dye with it. Slow, methodical, and effective.
  • The autofill — a pool with an automatic filler will hide a leak completely and simply run the water meter up. If the water bill has climbed and the level always looks fine, turn the autofill off before testing anything.

Covers, and what evaporation really costs

A cover reduces evaporation by roughly 90%, and on a heated pool that is the single largest economy available.

The water itself is modest — a few tens of dollars a month at typical rates. The heat is not. Evaporation is a phase change, and vaporising water takes about 970 BTU per pound. A pool losing a quarter inch a day over 500 square feet is losing around 78 gallons, which is 650 lb of water, which carries away roughly 630,000 BTU. On a heated pool that is the majority of the heating bill, and it is why covers on heated pools pay for themselves within a season.

It also carries away chemicals. Every gallon that leaves takes its share of chlorine, stabiliser and everything else, and the top-up dilutes what remains — which is one of the few things that lowers cyanuric acid without a deliberate drain.

Beyond covers, the levers are limited but real. A windbreak on the prevailing side reduces the velocity term directly. Turning off water features when the pool is not in use removes a surprising amount of surface agitation and spray. And lowering the water temperature by a couple of degrees reduces the vapour pressure at the surface, which cuts evaporation and the heating bill at the same time.

What this assumes, and where it stops

Assumptions

  • Only the pool surface area matters for evaporation; depth affects volume but not the rate.
  • One inch of depth over one square foot is 0.623 gallons.
  • The bucket test measures evaporation directly and takes precedence over the conditions-based estimate.
  • The evaporation estimate uses the standard pool engineering form E = Fa (95 + 0.425 v)(Pw − Pa) ÷ 1000, with ASHRAE’s residential activity factor of 0.5.
  • A leak is reported when losses exceed evaporation by more than an eighth of an inch a day.
  • A cover is taken to reduce evaporation by about 90%.

Limitations

  • The conditions-based estimate is far less reliable than the bucket test. The Carrier equation it uses was derived for indoor commercial pools, and even with the residential activity factor applied it is sensitive to wind speed in a way that outdoor pools are not — a weather-station wind figure will overstate what the water surface actually experiences.
  • Splash-out from swimming and backwash volumes are not distinguished from leaks and must be counted separately.
  • Automatic fillers hide level changes entirely; they must be switched off before any measurement means anything.
  • A leak rate can vary with water level, temperature and pump operation, so a single measurement may not represent the whole picture.
  • The cost figures cover water only. Heat, chemicals and any structural damage from a leak are all excluded and are usually larger.

Common questions

How do I know if my pool is leaking or just evaporating?

The bucket test. Fill a bucket with pool water, stand it on a step so it stays at pool temperature, mark the level inside and the pool level outside, and wait 24 hours with the pump running as normal. Both drop equally to evaporation; any extra drop in the pool is a leak, measured directly.

How much water should a pool lose per day?

A quarter to half an inch a day for an uncovered pool in summer — on a 500 sq ft surface that is 160 to 320 gallons. It sounds alarming in gallons and is entirely normal. Above three quarters of an inch a day, do the bucket test.

Why does my heated pool lose so much water in autumn?

Because cold air holds very little moisture. Evaporation is driven by the vapour pressure difference between the water surface and the air, so 80°F water under 50°F air evaporates faster than the same pool under 90°F humid air in August. Heated pools lose the most water in autumn, and their owners reliably suspect leaks.

How many gallons is an inch of water in my pool?

Surface area multiplied by 0.623. A 32 by 16 ft pool has 512 sq ft of surface, so one inch is 319 gallons. Only the surface area matters — depth changes the total volume but not how fast water leaves the top.

Where do pool leaks usually happen?

The skimmer first — cracks in the throat and the joint between skimmer and pipe are the commonest source by a wide margin. Then light niches, return fittings, and underground pipework. If the level stops falling at a particular height, the leak is at that height, which narrows it down immediately.

Does a pool cover really reduce water loss?

By around 90%, and the water is the smaller saving. Evaporation takes about 970 BTU per pound of water vaporised, so on a heated pool it accounts for most of the heating bill — which is why covers on heated pools typically pay for themselves within a season.

Sources

Formula and content last reviewed on .

Results are estimates for information only, not professional advice.

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