Pool Volume Calculator (Gallons) for Rectangle, Round and Oval Pools

Why the volume matters so much

Every dosing chart, label and calculator works in "so much product per so many gallons". If you think your pool holds 20,000 gallons but it really holds 15,000, every dose you add is a third stronger than you meant. Chlorine overshoots, acid drops the pH too far, and stabilizer goes up by more than you wanted, which can only be fixed by draining water. Spending ten minutes with a tape measure is the cheapest improvement most pool owners can make.

How the calculator works

The tool treats your pool as a simple solid, finds its volume in cubic feet, and converts that to US gallons.

• Rectangle: length × width × average depth.
• Round: π × radius × radius × average depth, where the radius is half the diameter.
• Oval: π × half the length × half the width × average depth. This is the area of an ellipse, which is what most "oval" above-ground pools are close to.

It then multiplies cubic feet by 7.48052. That number is not a rule of thumb: a US gallon is defined as exactly 231 cubic inches, and a cubic foot is 12 × 12 × 12 = 1,728 cubic inches, so one cubic foot holds 1,728 ÷ 231 = 7.4805 gallons. For litres, the tool multiplies gallons by 3.78541.

Average depth

For a pool with a shallow end and a deep end, the calculator uses the simple average of the two depths. That is exact for a pool whose floor slopes evenly from one end to the other. Many in-ground pools have a flat shallow section, a steep "break" and a hopper at the deep end. For those, a better estimate is to split the pool into sections, work out each one separately and add them up. If the deep area is short, the simple average slightly overstates the volume.

Worked example

A round above-ground pool 24 ft across with 4 ft of water: radius 12 ft, so 3.1416 × 12 × 12 × 4 = 1,810 ft³, or about 13,540 gallons.

How to measure accurately

1. Measure the water, not the frame. For length and width, measure the inside of the pool at the waterline. For above-ground pools, the size on the box is usually the outside diameter of the frame, which can be several inches more than the water.
2. Use water depth. Drop a tape or a marked pole to the floor at the shallow and deep ends and read the depth at the surface. Most pools are filled to about the middle of the skimmer opening.
3. Note odd shapes. For L-shapes, split the pool into two rectangles. For kidney or freeform pools, break the outline into a rectangle plus circles or ovals, or use an average length and width as a rough first estimate, then refine it with the check below.
4. Write it down. Put the number inside your test kit lid or in our water log.

Check your number with a known dose

Measurements are only an estimate, especially for freeform pools. You can check your figure with chemistry. Test free chlorine, add a measured amount of liquid chlorine, run the pump for an hour, and test again. Our chlorine calculator tells you how much the dose should have raised chlorine in your estimated volume. If the real rise is noticeably bigger, your pool is smaller than you thought, and vice versa. Repeat once or twice before trusting the correction, because test kits have their own margin of error and sunlight uses up chlorine while you wait.

Hot tubs and spas

Use the manufacturer's stated water capacity for a hot tub; seats, footwells and jets make shapes hard to measure. If the capacity is unknown, measure the water on a filled tub using the rectangle or round option with the average water depth, then subtract roughly the volume of the seating. Small volumes make dosing errors larger, so add hot tub chemicals in small steps and retest.

Limits

This is a geometry estimate. It does not account for steps, benches, sun shelves, rounded corners or sloped walls, all of which make the real volume a little smaller. It also assumes US gallons; Imperial (UK) gallons are about 20% larger.

Common questions

How many gallons are in a cubic foot?

One cubic foot holds about 7.48 US gallons, because a US gallon is defined as 231 cubic inches and a cubic foot is 1,728 cubic inches.

Should I measure the wall height or the water depth?

Measure the water depth. Most pools are filled to the middle of the skimmer opening, several inches below the top of the wall.