How to calculate your pool volume (gallons) by shape
Every salt dose, chlorine dose and heater size starts from one number: how many gallons your pool holds. Here is how to get it right for any shape.
Pool volume is just surface area × average depth, converted from cubic feet to gallons. One US cubic foot holds 7.48 gallons, so once you know the water’s area and how deep it is on average, the gallons follow directly. The only thing that changes between a rectangle, a circle and an oval is how you work out the area.
Step 1 — find the average depth
Most pools slope from a shallow end to a deep end, so you do not use a single depth. Take the average of the two:
average depth = (shallow depth + deep depth) ÷ 2
A pool that is 3 ft in the shallow end and 7 ft in the deep end has an average depth of (3 + 7) ÷ 2 = 5 ft. If your pool has a constant depth (many above-ground pools do), the “average” is simply the water depth — measure to the actual water line, not the wall height.
Step 2 — pick the formula for your shape
Rectangular / square: area is length × width, so
gallons = length × width × average depth × 7.48
Round / above-ground: a circle’s area is π × radius², and the radius is half the diameter, so
gallons = π × (diameter ÷ 2)² × average depth × 7.48
Oval: an ellipse uses both half-widths:
gallons = π × (length ÷ 2) × (width ÷ 2) × average depth × 7.48
Kidney / freeform (estimate): there is no single exact formula, so use the two widest widths (A and B) and the overall length:
gallons ≈ 0.45 × (A + B) × length × average depth × 7.48
The quick multipliers (7.5 and 5.9)
Pool pros rarely reach for π. Instead they fold the geometry and the 7.48 conversion into a single number and multiply length × width × average depth by it:
- Rectangular ≈ 7.5 — this is just 7.48 rounded, because a box’s area is exactly length × width.
- Round and oval ≈ 5.9 — a circle or ellipse fills only π ÷ 4 ≈ 0.785 of its bounding box, and 0.785 × 7.48 ≈ 5.87, rounded to 5.9.
So gallons ≈ 5.9 × length × width × average depth for round and oval pools, and gallons ≈ 7.5 × … for rectangles. The multiplier method and the π method agree to within about a percent — well inside the accuracy of a tape-measured pool.
Worked examples
Rectangle, 32 × 16 ft, average depth 5 ft: 32 × 16 × 5 × 7.48 = 19,149 gallons. For chemistry you would plan around ~19,000 gallons.
Round, 24 ft diameter, average depth 4 ft: π × 12² × 4 × 7.48 = 13,536 gallons. The radius is 12 ft, and 12² = 144.
Oval, 32 × 16 ft, average depth 4 ft: π × 16 × 8 × 4 × 7.48 = 12,032 gallons — smaller than the same-footprint rectangle because the corners are cut off.
Kidney, widths 15 and 18 ft, length 30 ft, average depth 4 ft: 0.45 × 33 × 30 × 4 × 7.48 ≈ 13,329 gallons (estimate).
Why the number matters
Volume is the multiplier under nearly every other pool calculation. Salt, chlorine, shock, alkalinity and calcium doses are all “pounds (or ounces) per so-many gallons,” and heater sizing and heat-up time scale directly with gallons. A 10% error in volume becomes a 10% error in every dose and every runtime. It pays to measure carefully once and reuse the figure.
Two habits keep you accurate: measure to the water line rather than the top of the wall, and treat kidney and freeform results as estimates — real curves rarely match the simple approximation. When in doubt, round your working volume to a clean number and dose to about three-quarters of the calculated amount, then re-test.
Common measuring mistakes
Most volume errors come from the tape measure, not the formula. A few habits keep you honest:
- Measure to the water line. Above-ground pools are filled a few inches below the rim, so using the wall height overstates depth — and therefore gallons — by a noticeable margin.
- Average depth, not deepest depth. A pool with a deep-end hopper is not that deep across its whole floor. Average the shallow and deep readings; for a stepped bottom, measure a few points and average them.
- Convert inches properly. Depths are rarely whole feet. Divide inches by 12 to get decimal feet — 4 ft 6 in is 4.5 ft, not 4.6 ft.
- Watch tape sag. On a long pool a loose tape reads short. Pull it taut across the true length and width at the water surface.
- Count attached features separately. A spillover spa, tanning ledge or swim-out adds water the main-shape formula misses; estimate each piece and add it in.
Finally, if you ever need liters instead of gallons, multiply gallons by 3.785. The chemistry is identical either way — ppm is ppm — so most US owners simply keep everything in gallons and pounds.
Run any of these shapes through the pool volume calculator, or use the dedicated round, oval and rectangular tools for a server-rendered answer with the working shown.