How Many Gallons Is My Pool?

Almost every pool chemical is dosed per 10,000 gallons. Guess the volume and you are guessing the dose, which is how people end up with cloudy water and a wasted afternoon.

ConstructionBy Aug 15, 20266 min read
How Many Gallons Is My Pool? — ListCalc

The number everything else depends on

Pool volume is not a fact you look up once and forget. It is the input for every dose of chlorine, every shock treatment, every algaecide, and the run time on your pump. Get it wrong by a quarter and every chemical decision you make for the rest of the season is wrong by a quarter too.

The formula

For a rectangular pool it is three numbers multiplied together:

Length × width × average depth × 7.48 = US gallons

The 7.48 is the number of US gallons in a cubic foot. Everything before it gives you the volume in cubic feet.

Take a common backyard size — 32 ft by 16 ft, sloping from 3 ft at the steps to 8 ft at the far end. The surface is 512 ft². The average depth is (3 + 8) ÷ 2 = 5.5 ft. So the volume is 512 × 5.5 × 7.48, which comes to roughly 21,065 gallons, or about 79,740 litres.

Average depth is where people go wrong

The instinct is to use the deep end, because that is the number people quote when they talk about their pool. Using 8 ft instead of 5.5 ft on the pool above gives 30,640 gallons — a 45% overestimate, and enough to make every chemical dose meaningfully wrong.

The shallow-plus-deep-halved shortcut works because most pools slope fairly evenly. If yours has a long flat shallow area and then drops off sharply, the true average sits below the halfway point and you will be overestimating slightly.

Round and oval pools

Only the surface area calculation changes. For a circle it is π × radius², so a 24 ft round pool is π × 12² = about 452 ft². For an oval, multiply the long axis by the short axis by 0.785. Then multiply by average depth and 7.48 exactly as before.

What the number is actually for

Once you have it, the dosing arithmetic gets easy. Divide by 10,000 and you have the multiplier for almost every product on the shelf. At 21,065 gallons that is 2.11 — so a dose written as "one pound per 10,000 gallons" means 2.11 pounds for your pool.

It also tells you how long filling will take. A garden hose moving 8 gallons a minute needs about 44 hours to fill that pool from empty, which is most of two days and worth knowing before you start.

Run your own numbers

Pool volume Pool chlorine Volume

FAQ

What is the formula for pool volume in gallons?
Surface area in square feet, times the average depth in feet, times 7.48. The 7.48 converts cubic feet to US gallons. A 32 by 16 ft pool averaging 5.5 ft deep is 512 × 5.5 × 7.48 = about 21,065 gallons.
How do I find average depth if the floor slopes?
Add the shallow-end depth to the deep-end depth and halve it. A pool that runs 3 ft to 8 ft averages 5.5 ft. This is close enough for chemical dosing on most residential pools, though a long flat shallow section followed by a sudden drop will read slightly deep.
Does a round pool use a different formula?
Yes. Use π × radius² for the surface area instead of length × width, then multiply by average depth and 7.48 as before. A 24 ft round pool has a surface area of about 452 ft².
Why does my chemical label say per 10,000 gallons?
It is the industry's round number for a dose unit. Divide your volume by 10,000 to get the multiplier. At 21,065 gallons you need 2.11 doses of anything labelled that way.
Is a US gallon the same as an imperial gallon?
No, and the gap is large. One US gallon is about 0.833 imperial gallons, so a 21,065 US gallon pool is about 17,540 imperial gallons. Chemical labels sold in the US use US gallons.

Sources

Primary references used for the figures and rules on this page.

  1. —
  2. —