Pool Volume in Gallons: How to Calculate It for Any Pool Shape

The single number that runs your whole pool season is the one almost nobody knows off the top of their head: how much water it holds. Get that figure wrong and every chlorine dose, every heater estimate, and every fill-cost guess inherits the error. Get it right once and you can stop second-guessing the cap on the chlorine jug.

This guide walks through the actual math for the four shapes that cover the vast majority of backyard pools, shows a fully worked example, and explains why the gallon count matters more than the brochure spec. If you just want the answer fast, the Pool Volume Calculator does all of this in your browser and reports US gallons, liters, and cubic meters at once. But it helps to know what it is doing under the hood.

Why volume is the number everything depends on

Pool chemistry is dosed per unit of water. The label on a bottle of liquid chlorine talks in ounces per 10,000 gallons. Algaecide, clarifier, stabilizer, pH-up — all of it scales with volume. If your real pool is 19,000 gallons but you have been treating it like a 25,000-gallon pool because that is what the previous owner claimed, you have been overdosing for years.

Heating is the same story. A heater warms a body of water at a rate set by its BTU output, and the time to raise the temperature is proportional to the gallons. Double the water and you double the warm-up time. Sizing a heater or a heat pump without the volume is guessing.

And then there is the water bill. Filling a fresh pool from a garden hose is a real expense, and water utilities bill by the hundred cubic feet (CCF), where 1 CCF is 748 gallons. Knowing the gallons up front turns a vague worry into a line-item estimate.

The base formula: convert cubic feet to gallons

Every shape boils down to computing a volume in cubic feet, then converting to gallons. The conversion constant is the one to memorize:

1 cubic foot = 7.48 US gallons.

So the master formula for any flat-bottomed pool is:

gallons = (surface area in ft²) × (depth in ft) × 7.48

For a rectangle, surface area is simply length × width, which gives the clean working formula:

rectangular gallons = length × width × average depth × 7.48

Everything else is just swapping in the right area formula for the shape.

Rectangular pools

This is the easy case and the most common. Multiply length, width, and depth, then by 7.48. If your floor is flat, depth is a single number. If it slopes, use the average depth (more on that below).

A 20 × 40 ft rectangle at a flat 4 ft deep is 20 × 40 × 4 × 7.48 = 23,936 gallons. The whole calculation is one line of arithmetic, which is exactly why rectangular pools are the friendliest to maintain — you can re-derive the volume in your head whenever you forget it.

Round pools

Round pools trip people up because of one mistake: squaring the diameter instead of the radius. The area of a circle is π × radius², and the radius is half the diameter. So:

round gallons = π × radius² × depth × 7.48

A round pool 18 ft across (radius 9 ft) and 5 ft deep works out to π × 9² × 5 = π × 81 × 5 ≈ 1,272 cubic feet, or about 9,518 gallons. If you had squared the 18 ft diameter instead of the 9 ft radius, you would have overstated the water by a factor of four — a 38,000-gallon answer for a pool that holds under 10,000. Measure the full width across the center, then halve it.

Oval pools

Ovals (technically ellipses) have their own area formula: π × half-length × half-width. In practice:

oval gallons = (length / 2) × (width / 2) × π × depth × 7.48

A 30 × 15 ft oval at 4.5 ft deep is 15 × 7.5 × π × 4.5 ≈ 1,590 cubic feet, or roughly 11,895 gallons. Note that an oval of the same bounding box as a rectangle holds about 79% of the water, because the rounded ends cut the corners off — a detail worth catching before you buy chemicals.

Average depth for sloped floors

Most in-ground pools are not a single depth. They start shallow and slope down to a deep end, and treating the whole pool as if it were the deep-end depth is the most expensive volume mistake you can make. The fix is the average depth:

average depth = (shallow depth + deep depth) / 2

For a steady, even slope this is exact. A 3 ft shallow end and an 8 ft deep end average to 5.5 ft, and you feed that single number into whichever shape formula applies. If your pool has a flat shallow shelf followed by a sharp drop into a diving well, the simple average slightly overstates the volume — for those, split the pool into two boxes, compute each, and add them. The Pool Volume Calculator has a variable-depth mode that does the averaging and shows the exact average it used, so you can sanity-check it against your tape measure.

A worked example, start to finish

Say you have a 32 × 16 ft rectangular pool that slopes from a 3 ft shallow end to a 7 ft deep end. Here is the full chain:

Average depth: (3 + 7) / 2 = 5 ft.
Surface area: 32 × 16 = 512 ft².
Cubic feet: 512 × 5 = 2,560 ft³.
Gallons: 2,560 × 7.48 ≈ 19,148 gallons.

That is roughly 72,480 liters, or about 72.5 cubic meters. Now the season runs on real numbers: a shock dose, a heater estimate, and a fill cost all flow from that one figure of 19,148.

A note from my own pool

I learned this the slow way. The pool at the house I rented had a faded sticker on the pump that read "30,000 gal," and for a full summer I dosed chlorine against that number. The water stayed cloudy and I kept adding more, blaming the heat. When I finally measured the thing — a plain 30 × 15 ft rectangle averaging about 4.5 ft — it came out to roughly 15,150 gallons, half of what the sticker claimed. I had been pouring in twice the chlorine the pool needed and wondering why the chemistry never settled. Five minutes with a tape measure would have saved me a season of frustration and a stack of chlorine jugs.

Turning gallons into chemicals and heat

Once you have the volume, the practical references fall into place. A common starting point: about 10.7 fluid ounces of 12.5% liquid chlorine per 10,000 gallons raises free chlorine by roughly 1 ppm. For the 19,148-gallon example, a 1 ppm bump is around 20.5 ounces — a ballpark for a purchase, not a substitute for testing the water first.

Heating scales the same way. The energy to warm water is proportional to gallons times the temperature rise, so a bigger pool simply takes longer with the same BTU heater. If your spec sheets are quoted in metric and you need to reconcile BTU, liters, and cubic meters, the Unit Converter handles the cross-unit arithmetic so you are comparing like with like.

Measure once, write the number on the inside of the pump-room door, and let every dosing and heating decision for the rest of the season inherit a figure you can actually trust.

Made by Toolora · Updated 2026-06-13