How to Calculate Pool Volume: Every Shape, Easy Formulas

Knowing your pool's volume in gallons is the single most useful number in pool care. Every chemical dose — chlorine, acid, stabilizer, salt — is figured per gallon of water, so if your volume is wrong, every dose is wrong. The good news: you do not need calculus. You need a tape measure, one multiplication, and about two minutes.

The one formula that covers most pools

For almost any pool, volume in gallons is:

Length × Width × Average Depth × Multiplier = Gallons

Measure everything in feet. The multiplier turns cubic feet of water into gallons and adjusts for your pool's shape:

Why the different numbers? 7.5 is the exact number of gallons in one cubic foot of water. The smaller multipliers for round, oval, and kidney pools simply account for the curved edges — a circle holds less water than the square it sits inside.

Step 1: Find your average depth

If your pool has a shallow end and a deep end, you can't just use one depth. Take the simple average:

Average depth = (Shallow depth + Deep depth) ÷ 2

A pool that's 3 ft in the shallow end and 8 ft in the deep end has an average depth of 5.5 ft. If your pool is one constant depth (common for above-ground pools), that single depth is your average — just measure the water depth, not the wall height.

Step 2: Measure length and width (in feet)

Measure the water surface, edge to edge, in feet. For a rectangle, that's straightforward. For round pools, measure the diameter (straight across the center). For odd shapes, see the freeform section below.

Step 3: Multiply — worked examples

• Rectangle — 32 ft × 16 ft, shallow 3 ft / deep 8 ft (avg 5.5): 32 × 16 × 5.5 × 7.5 = 21,120 gallons.
• Round above-ground — 24 ft across, 4 ft deep: 24 × 24 × 4 × 5.9 = 13,594 gallons.
• Oval — 30 ft × 15 ft, 4 ft deep: 30 × 15 × 4 × 5.9 = 10,620 gallons.
• Kidney — widest point (A) 18 ft, narrowest (B) 12 ft, length 30 ft, avg depth 5 ft: (18 + 12) × 30 × 5 × 3.38 = 15,210 gallons.

Odd, freeform, or "no idea where to even measure" shapes

If your pool curves and bends, you have two easy options:

• Break it into pieces. Mentally split the pool into a rectangle plus a circle (or two), calculate each, and add them up.
• Measure the real distances. For long or curvy pools, a walk-along measuring wheel makes measuring length and perimeter painless — you just roll it along the edge and read the distance, no fighting a floppy tape measure across the water.

Converting units

• Meters → feet: multiply meters by 3.28 before using the formulas.
• Gallons → liters: multiply gallons by 3.785.
• Cubic feet → gallons: multiply by 7.48 (we round to 7.5).

Why getting it close matters

Pool dosing isn't surgery — being within about 10% is plenty accurate for chlorine, pH, and the rest. But "it's big" isn't close enough: guess 30,000 gallons when you actually have 18,000 and you'll badly over-dose every chemical, waste money, and chase your water chemistry in circles. Two minutes with a tape measure pays for itself all season.

Once you have your number, plug it into the all-in-one pool calculator and every chemical dose is sized to your pool.

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