Pools
Pool volume calculator
Every pool calculator asks for an average depth. That formula is exactly right for a floor that ramps evenly from end to end — and almost nobody has that pool. A 16 × 32 ft pool with the hopper bottom of a vinyl liner holds 17,922 gallons where the usual arithmetic says 21,065. That is 3,143 gallons of water that are not there, two extra bags of salt, and salt is the one thing you cannot take back out.
Written and edited by Pablo Ruiz Quintero
Last reviewed 2026-09-22 · Nothing you type leaves your browser
The shape of the bottom, not the average of two numbers
Every pool calculator asks for an average depth. That formula is exactly right for a floor that ramps evenly from one end to the other, and wrong for almost every other pool ever built.
Length times width, and nothing to argue about. Most inground pools and every lap pool.
Inside the walls, at the water line.
Inside the walls, at the widest point.
The bottom
This is the decision the other calculators do not let you make, and it is worth more than every other input on this page put together.
Vinyl-liner pools, which is most inground pools in America. The deep end is not a floor. It is four sloping walls converging on a small flat panel — a funnel. It holds far less water than a deep end that looks the same from the surface, and it is why the average-depth formula is most wrong on the commonest kind of inground pool.
Water depth, not the wall height. The two differ by four to six inches on most pools.
At the deepest point — the bottom of the hopper, if there is one.
How much of the pool is flat at the shallow depth before the floor starts to fall away. Pace it out or look at where the slope begins — around 40 to 50 per cent is usual.
Gallons is how salt, shock and every chemical label is written in the US. Litres is here because the question gets asked in litres too.
Rectangular, hopper bottom
17,922 gal
512 ft² of surface · 4.68 ft average depth · 2,396 ft³
The average-depth formula says 21,065
It is 3,143 gallons out — 17.5 per cent of the pool. Every chemical you ever add gets dosed against this number, so the error does not happen once. It happens every time.
What the error actually costs
3,761 ppm instead of 3,200
Dose the salt against the average-depth figure and you put in 14 bags where 12 were needed. That is above the 3,400 ppm top of the band a salt cell is built to run in.
Salt does not evaporate and nothing consumes it. The only way down is to take water out and put fresh water in — about 2,674 gallons of it.
The pool in section, and the water that is not there
Cut along the length. The pale band is what the formula assumes.
| If the bottom were… | Gallons | Average depth says | Usually found on |
|---|---|---|---|
| The same depth all over | 11,490 | exactly right | Above-ground pools, and small plunge pools |
| Sloping evenly from end to end | 21,065 | exactly right | Some small gunite pools and lap pools |
| Shallow shelf, slope, deep floor | 19,246 | +9.5% | Gunite and concrete pools |
| Hopper bottom · yours | 17,922 | +17.5% | Vinyl-liner pools, which is most inground pools in America |
Same pool, same two depths, four different floors. If you do not know which one you have, the construction tells you: vinyl liner means a hopper, gunite usually means a shelf and a slope, above-ground means one depth all over.
What the number is for
- Salt to 3,200 ppm
From fresh water, in 40 lb bags. This is the dose that a wrong volume makes expensive, because salt does not evaporate and does not get used up.12 bags - Shock to 10 ppm free chlorine
Calcium hypochlorite at 73 per cent available chlorine. Roughly what it takes to bring a green pool back.2 lb - Pump flow for one turnover
37 gpm turns the pool over in 8 hours and 25 gpm in 12. That is the figure a pump is chosen against — not the horsepower on the box.25–37 gpm - Interior surface
Floor plus walls, which is the area that gets plastered, painted or lined. The plan area alone is about half of it.961 ft²
The pool, measured
Everything that follows from the shape, before any chemical is opened.
| Water volume | 17,922 gal |
| In cubic feet | 2,396 ft³ |
| Surface area | 512 ft² |
| Perimeter | 96 ft |
| Interior surface | 961 ft² |
| True average depth | 4.68 ft |
The perimeter is what coping and tile are bought by, and the interior surface is what gets plastered or painted — so if the pool is due resurfacing, that is the figure the quote should be built on rather than the surface area of the water.
Worth knowing
- Averaging the two depths gives 21,065 gallons. The floor of this pool actually holds 17,922 — a difference of 3,143 gallons, or 17.5 per cent. A hopper is a funnel, not a floor: four walls converging on a small panel. It looks identical from the surface to a deep end that holds far more.
- 17,922 gallons is 478 lb of salt to reach 3,200 ppm from fresh water — 12 bags of 40 lb. Dose against the average-depth figure instead and you put in 14 bags and land at 3,761 ppm.
- 3,761 ppm is above the 3,400 ppm top of the band a salt cell is built to run in. Salt does not evaporate and it does not get used up — the only way down is to take water out and put fresh water in, about 2,674 gallons of it. That is the whole cost of an arithmetic mistake nobody checks.
- Turning the whole pool over through the filter takes 37 gallons a minute at 8 hours, or 25 at 12. That figure, not the horsepower on the box, is what a pump has to be chosen against.
On this page — 4 sections
Every pool calculator asks the wrong question
The short version
- Average depth is right for one shape of floor — an even ramp, end to end — and high on almost every other pool.
- A hopper bottom is a funnel, not a floor. On this pool the formula is 17.5 per cent out.
- 3,143 gallons that are not there is two extra bags of salt, every time you dose.
- Salt does not evaporate. The only way back from 3,761 ppm is 2,674 gallons of fresh water.
Ask the internet how many gallons your pool holds and every answer runs the same way: length times width times average depth, where average depth is the shallow end and the deep end added together and halved.
That formula is not a rule of thumb. It is exactly right — for a floor that ramps evenly from one end to the other, so that every foot of extra depth at the deep end is paid for by a foot of shallow at the other. Pour that pool and average depth gives you the volume to the gallon.
Almost nobody has that pool.
What most people have is a hopper bottom: the standard deep end of a vinyl-liner pool, which is most inground pools in America. The floor does not ramp. It drops away on all four sides into a small flat panel — a funnel. From the surface it looks identical to a deep end that runs wall to wall. It holds nothing like the same water.
| The floor | Gallons | Average depth says | Out by | Found on |
|---|---|---|---|---|
| The same depth all over | 15,320 | 15,320 | nothing | Above-ground pools, and small plunge pools |
| Sloping evenly from end to end | 21,065 | 21,065 | nothing | Some small gunite pools and lap pools |
| Shallow shelf, slope, deep floor | 19,246 | 21,065 | 9.5% | Gunite and concrete pools |
| Hopper bottom | 17,922 | 21,065 | 17.5% | Vinyl-liner pools, which is most inground pools in America |
A 16 × 32 ft pool, 3 to 8 ft deep — except the first row, which is a flat pool at 4 ft. The measurements are identical down every row. The pools are not.
MaterialsEstimator, Pool bottom profiles by construction type, and the volume error in the average-depth method · checked 2026-09-22
Why being 17 per cent out is expensive
A wrong volume would be trivia if the number were only ever used to answer a question at a barbecue. It is not. Every chemical that goes into a pool is dosed against it, which means the error does not happen once — it happens every time, in the same direction, for as long as you own the pool.
The arithmetic behind a dose is simple enough to do on paper. Parts per million is parts by weight, a US gallon of water weighs 8.34 lb, so the pounds of anything needed to move a pool by a given ppm depend on the volume and on nothing else at all. Get the volume wrong and every dose is wrong by the same proportion, for ever.
Most of the time that is survivable. Chlorine gets consumed. Acid gets consumed. Over-dose and you wait, and the pool comes back. Salt is the exception, and it is the exception that makes this page worth writing.
| Dosed on the real volume | Dosed on average depth | |
|---|---|---|
| Volume believed | 17,922 gal | 21,065 gal |
| Salt added | 12 bags | 14 bags |
| Where the pool lands | 3,200 ppm | 3,761 ppm |
| Inside the cell's band | Yes | No — above 3,400 ppm |
| How you fix it | — | Drain and refill 2,674 gal |
| What that costs | — | Water, and a day |
17,922
What the pool holds
16 × 32 ft, hopper bottom
17.5%
How wrong the formula is
3,143 gallons of nothing
4.68 ft
The true average depth
Not 5.5, which is what gets used
2,674 gal
To undo the salt
The only remedy there is
Hayward, Pentair and Jandy, Salt chlorine generator operating specifications · checked 2026-09-22
What the number is actually for
A volume on its own is a fact. It becomes useful in about five places, and it is worth knowing which, because they are the reasons to write it on the pump housing in marker rather than work it out again every spring.
Everything downstream of the gallons
- Salt, to about 3,200 ppm — and the one dose you cannot walk back.
- Shock, to around 10 ppm of free chlorine when the pool has gone green.
- Acid and alkalinity adjustments, which are dosed per 10,000 gallons on every label ever printed.
- Stabiliser, which is the other chemical that accumulates and does not leave on its own.
- The pump, chosen against flow: one turnover in 8 to 12 hours, not against the horsepower on the box.
- The heater, sized on volume and surface area together — the volume is what has to be raised, the surface is where the heat leaves.
- A winter cover and a solar cover, which are bought on the plan area rather than the volume.
- Resurfacing and painting, which are bought on the interior surface — floor plus walls — and that is roughly double the plan area.
That last pair is worth separating out, because it is the one place where people reach for the wrong one of these numbers. The water surface is not the surface that gets plastered. A 16 × 32 ft pool has 512 square feet of water and about 961 square feet of interior once the walls are counted — near enough double. A resurfacing quote built on the first number is missing half the job.
Pool & Hot Tub Alliance, Residential pool water circulation and chemistry guidance · checked 2026-09-22
What goes wrong
01Averaging the two depths
It assumes the floor is one even ramp. On a hopper bottom it is 17.5 per cent high — 3,143 gallons — and every dose after that inherits the error.
Instead — Work out which floor you have first. Vinyl liner means a hopper, gunite means a shelf and a slope, above-ground means one depth.
02Measuring the wall instead of the water
There is four to six inches of dry wall above the water line on most pools. On a 500 ft² pool five inches of phantom water is over 1,500 gallons, and it is added to every calculation for ever.
Instead — Measure the water depth. Mark a pool pole and stand it on the floor if there is no easier way.
03Trusting a single published figure for your pool size
"A 16 by 32 pool holds X gallons" cannot be true, because the floor is not part of the description. The honest range on this one is 17,922 to 21,065.
Instead — Treat the size as half the question. The floor is the other half, and it moves the answer by more than any measurement error you are likely to make.
04Over-salting because the volume was over-estimated
Salt does not evaporate and nothing consumes it. 3,761 ppm is outside what a cell is built for, and the only remedy is draining 2,674 gallons and re-balancing the fresh water that replaces it.
Instead — Add salt to about three quarters of the calculated dose, run the pump for a day, then test and top up. It is the one chemical where the cautious approach is genuinely worth the extra day.
05Quoting resurfacing off the water surface area
The interior is the floor plus the walls — about 961 ft² against 512 ft² of water on this pool. A quote built on the smaller number is about half a job.
Instead — Use the interior surface for anything applied to the shell, and the plan area only for covers and for heat loss.
06Sizing a pump by horsepower
Horsepower is not a flow rate. This pool needs 25 to 37 gpm for a turnover, and a pump that delivers much more is pushing water through the filter faster than the filter can use.
Instead — Work out the gallons a minute for a turnover in 8 to 12 hours and hold a pump curve against it.
07Approximating a freeform pool and then trusting it
There is no formula for a shape somebody drew freehand. The trade allowances get within a few per cent, and a few per cent is a bag of salt.
Instead — Approximate it to get started, then read the water meter on the next refill and replace the estimate with a measurement.
08Working it out again every spring
The volume does not change and the re-derivation does. Two people in the same household will produce two different numbers, and both will dose against them.
Instead — Write it on the pump housing in permanent marker. It is the cheapest piece of pool equipment there is.
- 01Measure length and width inside the wallsAt the water line, not the outside of the coping. For a round pool, straight across the widest point.
- 02Measure the water depth, not the wallThere is four to six inches of dry wall above the water on most pools, and counting it adds water that is not there.
- 03Find out what the floor is doingWalk the shallow end until it starts to fall away, and note roughly where. Look at where the deep end bottoms out — if it is a small panel rather than a floor across the whole width, it is a hopper.
- 04Let the construction tell you if you cannot seeA vinyl liner almost always means a hopper. Gunite usually means a shelf and a slope. An above-ground pool is one depth all over.
- 05Take the gallons from the shape, not the averageAveraging the two depths is right on a floor that ramps evenly end to end and high on everything else.
- 06Check it against the water meter if it mattersRead the meter before and after a refill. It is the only exact measurement of a curved pool that exists, and it is free.
- 07Write the number on the pump housingIn marker. Every chemical you ever add is dosed against it, and it is the number that will otherwise be re-guessed every spring.
The perimeter, the plan area and the interior surface all come out of this page, but if you are measuring a deck, a patio or anything else around the pool, that is a different calculation with its own traps — chiefly that nobody counts the bits they walk on without thinking.
If it is areas you are after- Hopper bottom
- A deep end made of four sloping walls converging on a small flat panel, like a funnel. It is how nearly every vinyl-liner pool is built, and it holds far less water than a deep end that looks the same from the surface.
- Average depth
- The shallow and deep depths added and halved. It is exactly right for a floor that ramps evenly from end to end, and it is high on every floor with a shelf or a hopper in it.
- True average depth
- The volume divided by the surface area — what the average depth actually is once the floor is integrated as the solid it is. On a hopper pool it sits well below the halfway point between the two depths.
- Turnover
- Passing the whole volume of the pool through the filter once. Residential pools are circulated at one turnover in eight to twelve hours, and it is what converts gallons into the flow rate a pump has to deliver.
- ppm
- Parts per million, which is milligrams per litre. Because a gallon of water weighs 8.34 lb, the pounds of anything needed to move a pool by a given ppm follow from the volume and nothing else — which is why the volume has to be right.
- Salt cell
- The chlorine generator on a saltwater pool. It is built to run in a band around 3,200 ppm, and above roughly 3,800 it stops generating to protect itself.
- Shock
- A deliberately large dose of chlorine, usually to about 10 ppm free chlorine, to clear a pool that has gone green. Dosed off the volume like everything else.
- Coping
- The capping that finishes the top edge of the pool wall. Bought by the perimeter, which is why the perimeter is worth knowing alongside the volume.
- Interior surface
- The floor plus the walls — what gets plastered, painted or lined. It is roughly double the surface area of the water on an average inground pool, and it is what a resurfacing quote should be built on.
- Plan area
- The area of the water seen from above. It is what the sun heats and what a cover has to span, and it is not what the pool holds.
- Gunite
- Sprayed concrete, the usual construction for a custom inground pool. Gunite pools normally have a shallow shelf, a transition slope and a deep floor that runs the full width.
- Freeboard
- The gap between the water line and the top of the wall. It is why water depth and wall height are not the same number, and measuring the wall instead of the water adds four to six inches of pool that is not wet.
Common questions
- How do I calculate the volume of my pool?
- Surface area times the true average depth, times 7.480519 to get gallons. The catch is the true average depth: it is not the shallow and deep depths halved unless the floor ramps evenly from one end to the other. A 16 × 32 ft pool from 3 to 8 ft holds 21,065 gallons if it is an even ramp, 19,246 with a shallow shelf and a deep floor, and 17,922 with a hopper bottom. Same measurements, three different pools.
- Why is the average depth formula wrong?
- It is not wrong — it is right for one shape and gets applied to all of them. Averaging two depths assumes the floor is a single even ramp, so that every foot of extra depth at one end is matched by a foot of shallow at the other. A hopper bottom is not a ramp at all: it is a funnel, four walls converging on a small panel, and most of the deep end is air. On this pool the formula reads 21,065 gallons where there are 17,922 — 17.5 per cent high.
- What is a hopper bottom and do I have one?
- It is the standard deep end of a vinyl-liner pool: the floor drops away on four sides into a flat panel at the bottom, typically about half the width of the pool and a couple of yards long. If you have a vinyl liner, you almost certainly have one. Stand at the deep end and look down — if the bottom is a small rectangle with the walls sloping in to meet it rather than a floor running wall to wall, that is a hopper.
- How many gallons is a 16x32 pool?
- Between 17,922 and 21,065 depending entirely on the floor, at 3 to 8 ft. That range is 18 per cent wide, which is why the single number published everywhere for "a 16 by 32 pool" is not much use. A 24 ft round above-ground pool at 4 ft is a much easier question: 13,536 gallons, exactly, because it is one depth all over.
- How much salt does my pool need?
- To reach 3,200 ppm from fresh water: about 478 lb, or 12 bags of 40 lb, for this pool. The arithmetic is simple — a gallon of water weighs 8.34 lb, and ppm is parts per million by weight — so the whole answer depends on the volume being right. Dose against the average-depth figure here and you would put in 14 bags and land at 3,761 ppm.
- What happens if I add too much salt to my pool?
- Above about 3,400 ppm you are outside the band a salt cell is built for, and in the high 3,800 range cells stop generating to protect themselves. The problem is that there is no undoing it: salt does not evaporate with the water and nothing in the pool consumes it. The only remedy is to drain part of the pool and refill with fresh water — about 2,674 gallons in the example above. An over-estimated volume is the commonest way people get there.
- Should I measure the water depth or the wall height?
- The water. There is normally four to six inches of dry wall above the water line, and using the wall height adds that to every square foot of the pool. On a 500 ft² pool, five inches of phantom water is over 1,500 gallons.
- How do I calculate the volume of a kidney or freeform pool?
- There is no exact formula, and anyone giving you one is giving you an approximation with a confident face. The trade allowance is about 75 per cent of the bounding rectangle for a kidney and about 85 per cent for a freeform shape, which gets within a few per cent. If the number has to be right — and for salt it does — read your water meter before and after a refill. That is exact, it is free, and it is the only measurement of a curved pool that is either.
- What size pump do I need for my pool?
- Pumps are chosen against flow, not horsepower. One turnover of this pool in 8 hours is 37 gallons a minute, and in 12 hours it is 25. That is the figure to hold a pump curve against — and it is why an oversized pump on a small pool is not "extra margin", it is money spent pushing water through a filter faster than the filter wants it.
- How many gallons is an above ground pool?
- Easier than any inground pool, because it is one depth everywhere: area times depth. A 24 ft round at 4 ft of water is 13,536 gallons and there is no approximation in that at all. The commonest mistake here is not the shape — it is measuring the 52 in wall instead of the 48 in of water in it.
- How much shock does my pool need?
- To reach 10 ppm of free chlorine — roughly what it takes to bring a green pool back — about 2 lb of calcium hypochlorite at 73 per cent available chlorine for this pool. Same arithmetic as the salt, same dependence on the volume, and the same reason to get the volume right once rather than guess it every time.
- Is it in gallons or litres?
- Gallons, in the US, because that is the unit every chemical label and every dosing chart is written in. The calculator gives litres too, since the question gets asked both ways and converting afterwards is how people drop a factor of four.
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About the author
Pablo Ruiz Quintero — Editor
Editor of MaterialsEstimator. Decides which documents count as a source, what every default value is, and where the line sits between what the reader parses and what the engines compute.
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