Flat part before the slope.
Where the floor drops.
0 for a circle, or the long axis of an oval.
From the builder or the plaque.
Salt and chlorination
From a test strip or meter. 0 for a new fill.
Check YOUR chlorinator manual.
0 to skip the turnover check.
This pool volume calculator works from the actual floor profile — shallow shelf, slope, deep end — rather than a single average depth, then works out the salt your chlorinator needs. Both matter, because every chemical dose you ever make depends on getting the volume right.
The short answer
Measure the shallow shelf, the slope and the deep end separately. A 10 × 5 m pool running 1.1 to 2.2 m deep holds about 74,250 litres — 19,615 US gallons.
The average-depth shortcut gives 82,500 L for that pool. It’s 11% high, and every chemical dose inherits the error.
On this page
How to use this pool volume calculator
Four ways in, depending on your pool:
- Shallow & deep end — the accurate method. Shelf, slope and deep section measured separately
- Constant depth — lap pools, plunge pools, above-ground
- Round or oval — enter one diameter for a circle, two for an oval
- I know the volume — skip straight to salt dosing
The header toggle switches between metres and feet, and results always show litres, cubic metres, US gallons, imperial gallons and cubic feet — because pool chemicals are labelled in all of them depending where you are.
The salt panel is open by default, since most people arriving at pool volume are on their way to a dosing question.
When the average-depth shortcut is wrong
Worth being precise here, because a lot of pool advice overstates this.
For a pool whose floor drops in one straight line from the shallow wall to the deep wall, length × width × (shallow + deep) ÷ 2 is mathematically exact. Not an approximation — exact. Anyone telling you that formula is inherently wrong has it backwards.
The problem is that almost no pool is built that way. A real pool has a flat shallow shelf where people stand, a transition slope, then a flat deep end. And the flat shelf sits below the average depth for its entire length, so the shortcut overstates.
| 10 × 5 m pool, 1.1–2.2 m | True volume | Average depth says | Over by |
|---|---|---|---|
| Single even slope | 82,500 L | 82,500 L | 0% |
| 4 m shelf + 3 m slope | 79,750 L | 82,500 L | 3.4% |
| 5 m shelf + 3 m slope | 74,250 L | 82,500 L | 11.1% |
| 6 m shelf + 2 m slope | 71,500 L | 82,500 L | 15.4% |
| 6 m shelf + 2.5 m slope | 70,125 L | 82,500 L | 17.6% |
The longer the shallow shelf, the bigger the error — and family pools are built with generous shelves precisely because that’s where people stand. A pool with a short shelf and a large deep end swings the other way and the shortcut understates, by around 14%.
An 11% error on 74,000 litres is 8,000 litres of water you’re dosing for and don’t have. Every chemical addition for the life of the pool inherits it.
The formulas
Volume = Width × [ (Shelf × Shallow) + (Slope × (Shallow + Deep) ÷ 2) + (Deep length × Deep) ]
Three sections, measured separately. The middle term is the sloped wedge
Constant depth = L × W × D
Circular = π × r² × D
Oval = π × (a ÷ 2) × (b ÷ 2) × D
a and b are the two diameters of the oval
Worked example — 10 × 5 m, 1.1 m shallow to 2.2 m deep
Litres, gallons and cubic metres
| Unit | Equals |
|---|---|
| 1 cubic metre | 1,000 L · 264.2 US gal · 220.0 imp gal · 35.31 ft³ |
| 1 cubic foot | 28.32 L · 7.481 US gal |
| 1 US gallon | 3.785 L · 8.34 lb of water |
| 1 imperial gallon | 4.546 L · 1.201 US gal |
An imperial gallon is 4.546 L against the US gallon’s 3.785 — a 20% difference. Pool chemical dosing tables printed for one market and used in the other are wrong by that much.
Australian and UK products are labelled in litres, which avoids the problem entirely. If a chart says “gallons” without specifying, find out which before dosing anything.
Salt dosing
The arithmetic here is exact, because 1 ppm is 1 milligram per litre by definition. No estimation involved.
kg = Litres × (Target − Current) ÷ 1,000,000
lb = US gallons × 8.34 × (Target − Current) ÷ 1,000,000
Always subtract what’s already in the pool
| Pool | Current → target | Salt | 20 kg bags |
|---|---|---|---|
| 30,000 L | 0 → 3,000 | 90.0 kg | 5 |
| 50,000 L | 0 → 3,200 | 160.0 kg | 8 |
| 74,250 L | 0 → 3,200 | 237.6 kg | 12 |
| 50,000 L | 2,000 → 3,500 | 75.0 kg | 4 |
| 75,000 L | 2,800 → 3,500 | 52.5 kg | 3 |
Notice the last two rows. A pool nearly at target needs remarkably little salt — 52.5 kg for a 75,000 L pool that’s only 700 ppm short. Test first. Dosing a topped-up pool as though it were a fresh fill is how people end up over target.
Target salinity differs by chlorinator
| Unit | Target range |
|---|---|
| Most US units, Hayward | 2,700–3,400 ppm |
| Pentair IntelliChlor | 3,000–3,500 ppm |
| Zodiac and many AU units | 3,000–6,000 ppm |
| Some high-salt units | 4,000–6,000 ppm |
These ranges genuinely differ between manufacturers, and a unit run below its range simply stops producing chlorine while one run above it can suffer corrosion and cell damage.
The general “3,200 ppm” figure quoted everywhere suits most US units and is well below what some Australian chlorinators want. Find the plate on your cell or the manual before you order salt.
Salt does not evaporate
This is the single most useful thing to understand about a saltwater pool, and it’s counterintuitive.
Water leaves. Salt stays. So evaporation doesn’t lower your salinity — it raises it, by concentrating the same salt into less water. A pool that’s been evaporating all summer will test high, not low.
| What happens | Effect on salinity |
|---|---|
| Evaporation | Raises it — salt concentrates |
| Topping up with fresh water | Lowers it — dilution |
| Splash-out and people leaving wet | Lowers it — salt leaves with the water |
| Backwashing the filter | Lowers it — a real volume of salted water gone |
| Heavy rain overflowing | Lowers it — dilution plus overflow |
| Running the chlorinator | No effect — salt is recycled, not consumed |
That last row surprises people. The chlorinator splits salt into chlorine, which sanitises and then recombines back into salt. You’re not consuming it. Salt only leaves the pool when water physically leaves the pool.
The practical rule: topping up 10% of your volume with fresh water drops salinity by about 10%. On a 74,000 L pool at 3,200 ppm, replacing 7,400 L takes you to roughly 2,880 — enough to need topping up.
If you’ve added too much
There is no additive that takes salt out. The only route down is dilution: drain part of the pool and refill with fresh water.
To go from 4,000 ppm to 3,200 you need to replace 20% of the volume — on a 74,000 L pool that’s nearly 15,000 litres drained and refilled, plus the cost of the water and rebalancing everything else afterwards.
Which is why the standard advice is to add in stages. Put in about three quarters of the calculated amount, run the pump for 24 hours, then test and add the rest. It costs you a day and removes the possibility of an expensive mistake.
How to add salt
- Test first, even on a pool you think is at zero. Fill water often carries some salt.
- Balance pH and alkalinity first — salt affects readings, and it’s easier to correct beforehand.
- Run the pump throughout, and for 24 hours afterwards.
- Broadcast across the shallow end, spread out rather than in one heap.
- Brush it along the floor to dissolve. Undissolved salt sitting on plaster can etch it.
- Never pour salt into the skimmer. A concentrated slug through the pump and cell damages both.
- Wait 24 hours before retesting. Reading early gives a false low and tempts you into over-dosing.
Use pool-grade salt — sodium chloride at 99%+ purity, no anti-caking agents or iodine. Water softener salt is usually acceptable; table salt is not.
Pool volume mistakes to avoid
- Averaging shallow and deep across the whole length. Overstates by 11–18% on a typical shelf.
- Measuring wall height instead of water depth. The waterline sits below the tile.
- Confusing US and imperial gallons. A 20% difference in every dose.
- Dosing salt without testing first. Existing salinity is rarely zero.
- Using a generic 3,200 ppm target. Some units want 5,000+.
- Adding all the salt at once. Over-dosing can only be fixed by draining.
- Assuming evaporation lowers salinity. It raises it.
- Pouring salt into the skimmer. Damages the pump and the cell.
- Retesting within a few hours. Salt takes 24 hours to disperse.
Frequently asked questions
How do I calculate my pool volume?
Measure the shallow shelf, the slope and the deep end separately, then multiply each by the width and add them. A 10 × 5 m pool with a 5 m shelf at 1.1 m, a 3 m slope and a 2 m deep end at 2.2 m holds 74,250 litres, or 19,615 US gallons.
Is the average depth formula wrong?
Not inherently. If the floor slopes evenly from one end to the other, length × width × average depth is mathematically exact. The problem is that real pools have a flat shallow shelf, then a slope, then a flat deep end — and the shortcut then overstates by 11 to 18% depending how long the shelf is.
How much salt does my pool need?
Litres × (target ppm − current ppm) ÷ 1,000,000 gives kilograms. A 50,000 L pool going from zero to 3,200 ppm needs 160 kg, which is 8 bags of 20 kg. Always subtract the salt already present — a pool 700 ppm short needs far less than a fresh fill.
What salinity should my pool be?
Whatever your chlorinator’s manual says, because the ranges genuinely differ. Most US units want 2,700–3,400 ppm, Pentair IntelliChlor 3,000–3,500, and many Australian units 3,000–6,000. Running below range stops chlorine production; running above risks corrosion.
Does salt evaporate from a pool?
No. Water evaporates and salt stays behind, so evaporation actually raises salinity by concentrating the same salt into less water. Salt only leaves through splash-out, backwashing, rain overflow or draining. Running the chlorinator doesn’t consume it either — salt is recycled, not used up.
How do I lower salt levels in my pool?
Only by dilution — there’s no chemical that removes salt. Drain part of the pool and refill with fresh water. Going from 4,000 ppm to 3,200 means replacing 20% of the volume. This is why you should add salt in stages: three quarters first, then test after 24 hours.
How many litres in a cubic metre of pool water?
Exactly 1,000, which also equals 264.2 US gallons or 220.0 imperial gallons. A cubic foot is 28.32 litres or 7.481 US gallons.
What is the difference between US and imperial gallons?
An imperial gallon is 4.546 litres and a US gallon is 3.785 — about 20% apart. Chemical dosing tables written for one market and used in the other are wrong by that margin, which is a real risk with pool chemicals. Australian and UK products use litres, avoiding it.
Where should I add pool salt?
Broadcast it across the shallow end with the pump running, spread out rather than in a heap, then brush it along the floor to dissolve. Never pour salt into the skimmer — a concentrated slug through the pump and cell damages both. Wait 24 hours before retesting.
What kind of salt do I use?
Pool-grade sodium chloride at 99% purity or better, with no anti-caking agents, iodine or additives. Water softener salt is usually acceptable and often cheaper. Table salt is not — the additives cause staining and cloudiness.
How do I measure pool depth correctly?
Measure the water depth, not the wall height. Most pools run 100–150 mm below the top of the tile line, so using wall height overstates volume by roughly that fraction. Measure at the shallow shelf and at the deepest point, and note where the slope starts and ends.
How long does it take to turn over my pool water?
Volume divided by flow rate. A 50,000 L pool with a 300 L/min pump takes about 2.8 hours for one full turnover. Most guidance suggests one to two turnovers a day, though this varies with bather load and climate. Enter your flow rate to see the figure for your pool.
Sources and method
- Volume computed by section — flat shelf, sloped wedge, flat deep end — rather than a single average depth
- Salt dosing from the definition of ppm: 1 ppm = 1 mg/L, so kg = litres × ppm ÷ 1,000,000. Metric and imperial paths cross-checked to within 0.1%
- Conversions: 1 m³ = 1,000 L = 264.172 US gal = 219.969 imp gal; 1 ft³ = 28.3168 L; water at 8.34 lb per US gallon
- Salinity target ranges vary by manufacturer — the unit’s own manual takes precedence over any general figure
- This calculator covers volume and salt only. Chlorine, pH, alkalinity, stabiliser and calcium hardness all need separate testing and dosing
Related calculators
How this pool volume calculator works out its numbers
The profile mode splits the pool into three sections and computes each separately: the shelf as a flat prism, the transition as a wedge using the average of its two end depths, and the deep end as a flat prism. Adding them gives the true volume. The average-depth figure is calculated alongside so you can see the difference on your own pool.
If your shelf and slope lengths add up to more than the pool length, both are scaled down proportionally rather than returning a nonsense figure — but it’s worth re-measuring, since one of the three numbers is wrong.
Salt uses the exact definition of parts per million, so there’s no rounding or fudge factor. The metric and imperial paths were cross-checked and agree to within 0.1%. Current salinity is subtracted before dosing, which is the step most likely to be skipped and the one most likely to cause over-dosing.
This handles volume and salt only. Chlorine, pH, total alkalinity, stabiliser and calcium hardness each need their own testing and dosing, and they interact — salt affects pH readings, and pH affects how well chlorine works. Balance the water before adding salt, not after. And confirm your chlorinator’s target range from its own manual, because the spread between manufacturers is wide enough to matter.