Calculate your pool's water volume in gallons. Rectangular, round, oval & kidney shapes — with shallow-to-deep averaging.
How to Use This Pool Volume Calculator
Select your pool shape from the dropdown — choose the one that best matches. Rectangular pools are the most common in-ground shape, and round/circular pools are typical for above-ground models. If you have an oval pool, use the Oval setting; if your pool has a kidney or freeform shape, the Kidney option provides a standard industry approximation. Enter your pool's length and width in feet. For depth, pick either Constant Depth (flat-bottom pools, typical for above-ground) or Shallow End + Deep End (hopper-bottom in-ground pools). The calculator automatically averages the two depths and shows gallons, cubic feet, and total water weight.
Pool Volume Guide: Gallons, Formulas & What to Do With the Number
Knowing exactly how many gallons your pool holds isn't trivia — it determines everything: how much chlorine to add, whether your filter pump is big enough, what size heater to buy, and whether your deck can support an above-ground pool. Guess wrong by even 3,000 gallons and you're over-or under-dosing chemicals all summer. Here's how to get the number right once and then use it for everything else.
Pool Volume Formulas by Shape
All pool volume calculations follow the same pattern: find the surface area in square feet, multiply by the average depth in feet to get cubic feet, then multiply by 7.48 to convert to gallons. One cubic foot = 7.48 US gallons = 62.4 lbs of water.
| Shape | Surface Area Formula | Example (16×32 ft, 5 ft avg) |
|---|
| Rectangular | L × W | 16 × 32 = 512 sq ft → 19,149 gal |
| Circular | π × r² | π × 12² = 452 sq ft → 16,919 gal (24 ft diam) |
| Oval | π × (L/2) × (W/2) | π × 16 × 8 = 402 sq ft → 15,038 gal |
| Kidney | 0.45 × (W + L)² | 0.45 × 48² = 1,037 sq ft → 38,771 gal |
Common Pool Sizes — Quick Reference Gallon Chart
| Pool Type | Dimensions | Avg Depth | Gallons (approx.) |
|---|
| Above-ground round | 18 ft diam | 4 ft | 7,600 |
| Above-ground round | 24 ft diam | 4 ft | 13,500 |
| Above-ground round | 27 ft diam | 4 ft | 17,100 |
| In-ground rectangular | 16×32 ft | 5.5 ft (3→8) | 21,100 |
| In-ground rectangular | 18×36 ft | 5.5 ft (3→8) | 26,600 |
| In-ground rectangular | 20×40 ft | 5.5 ft (3→8) | 32,900 |
How to Handle Deep-End / Shallow-End Pools
Most in-ground pools have a hopper bottom — a shallow end (typically 3-4 ft) that slopes down to a deep end (6-9 ft) with a diving well or hopper. The simplest approach is to average the two depths: (shallow + deep) ÷ 2. This works well when the slope is uniform and the two depth zones occupy roughly equal area. However, if your deep end is a small diving well occupying only the last third of the pool, the simple average overestimates volume. In that case, split the pool into two sections at the break line (where the floor starts to slope), calculate each section separately with its own depth, and add the volumes. Our calculator uses the average-depth method by default — for two-section calculations, use the Rectangular mode twice and add the results manually.
Pool Heater Sizing: How Many BTUs Do You Need?
Pool heaters are rated in BTUs (British Thermal Units) per hour, and sizing one correctly starts with your pool volume. One BTU is the energy required to raise one pound of water by 1°F. Since a gallon weighs 8.34 lbs, a 20,000-gallon pool contains 166,800 pounds of water. To heat it 25°F (e.g., from 55°F to 80°F) requires 166,800 × 25 = 4.17 million BTUs. A 333,000 BTU heater, running at 80% efficiency, delivers about 266,000 BTU/hr and would take about 15-16 hours for the initial heat-up. After that, maintaining temperature takes a fraction of the energy. Residential pool heater sizing guidelines: 200K-250K BTU for pools up to 15,000 gallons; 300K-400K BTU for pools 15,000-25,000 gallons. See our HVAC BTU Calculator to cross-check heater sizing.
Chemical Dosing: Why Volume Accuracy Matters
Every pool chemical — chlorine shock, algaecide, pH increaser/decreaser, stabilizer, calcium hardness increaser — has a dosage rate labeled in ounces per 10,000 gallons. If your pool actually holds 18,000 gallons but you think it's 15,000, you'll under-dose by 17% all summer long and wonder why algae keeps coming back. Worse, if you overestimate and use 25,000 gallons when it's really 18,000, you'll over-dose by 39% — wasting chemicals and potentially creating unsafe swimming conditions. Take ten minutes with a tape measure and get the real number. It pays off every time you open a bottle of shock.
Water Weight: Can Your Deck Support an Above-Ground Pool?
A 15-foot above-ground pool with 4 feet of water holds about 5,300 gallons and weighs over 44,000 pounds (22 tons) — roughly the weight of a loaded dump truck. That weight is distributed across the pool's footprint (about 177 sq ft), yielding approximately 250 lbs per square foot. A standard wood deck designed for 40-50 lbs/sq ft live load is nowhere near sufficient. Above-ground pools must sit on level, compacted ground or a reinforced pad — never on a standard deck, balcony, or rooftop without a structural engineer's assessment.