Article — Swimming Pool Volume Calculator
A standard 20 ft by 40 ft rectangular pool with an average depth of 5.5 ft holds about 32,900 US gallons — roughly 125,000 liters, or 274,000 pounds of water sitting on your backyard slab. Pool volume drives every meaningful number that follows: chlorine and pH-adjuster doses, filter cycle times, hose fill hours, water-utility bills, and the structural load the deck and earth below have to carry. Getting the volume right is the difference between a balanced pool and one that swings green every July.
This guide covers the four common pool shapes (rectangle, circle, oval, kidney), how to convert between cubic feet, gallons, and liters, why average depth works for sloped floors but section-by-section is sometimes needed, and the everyday decisions — chemical dosing, fill time, fill cost — that the volume number ultimately decides.
Why pool volume matters
Pool chemistry is dose-by-volume. Liquid chlorine, calcium hypochlorite, sodium bicarbonate, cyanuric acid — every label gives the dose in ounces per 10,000 gallons or grams per cubic meter. If you guess the volume 20% low, you under-chlorinate and the pool turns. Guess 20% high and you over-dose, irritate swimmers' eyes, and waste money.
The U.S. Centers for Disease Control's Model Aquatic Health Code recommends a free chlorine residual of 1–3 ppm for residential pools (3–5 ppm for spas). At 2 ppm in a 20,000-gallon pool that is only 0.27 lb of chlorine in suspension — less than 5 oz dissolved in 167,000 lb of water. Tiny volumes of dosing chemical correspond to large pool volumes; the dose math has to be right.
One US gallon of water weighs 8.34 lb at 62 °F. A typical 20,000-gallon residential pool holds about 167,000 lb of water — the same mass as 40 mid-sized SUVs. That weight is what pool foundations, decks, and surrounding soil have to support without subsiding.
Pool volume formulas by shape
A rectangular pool is the simplest: length × width × depth. A 20 ft by 12 ft pool with 4.5 ft of water holds 1,080 ft³ or 8,080 gallons. Inground rectangular pools dominate U.S. residential builds; the National Association of Pool Builders' last industry survey put rectangles at roughly two-thirds of new installs.
A circular pool uses π r² D. The most common gotcha is using diameter instead of radius — halve the diameter first. A 24 ft round above-ground pool with 4 ft of water holds π × 12² × 4 = 1,810 ft³ or 13,530 gallons.
An oval pool is an ellipse: π × (L/2) × (W/2) × D, which factors to πLWD ÷ 4. Oval above-grounds are the second most common above-ground style after round, because they fit narrower yards better.
A kidney pool uses an empirical area factor: 0.45 × (W₁ + W₂) × L × D. The 0.45 multiplier matches the kidney profile to an ellipse with two unequal widths and was popularized by the National Spa and Pool Institute's design handbooks in the 1970s. Modern free-form pools with truly irregular outlines need section-by-section volume.
- Rectangle — V = L × W × D (simplest, most accurate)
- Circle — V = π × r² × D (use radius, not diameter)
- Oval — V = π × (L/2) × (W/2) × D (ellipse swept to depth)
- Kidney — V = 0.45 × (W₁ + W₂) × L × D (empirical)
- Truly irregular — divide into rectangles and circle segments; sum the parts
Handling sloped pool depths
Few residential pools have a flat floor. Most slope from a 3–3.5 ft shallow end to a 5–8 ft deep end. For a continuous, even slope — the most common build — the average depth (shallow + deep) ÷ 2 is mathematically exact, not an approximation. A pool with a 3 ft shallow and 8 ft deep has an average depth of 5.5 ft, and volume calculated with that figure is correct to within rounding.
The average breaks down for two cases: L-shaped or split-level floors where the deep end is a step instead of a slope, and pools with a flat shallow shelf followed by an angled drop. For those, split the pool into a shallow rectangle and a deep rectangle (or a wedge), compute each separately, and sum them.
Beach entries (a long, gradual zero-depth slope from waterline) and tanning shelves (a wide shallow platform around 9–12 inches deep) reduce the effective average depth significantly. Treat them as their own sections. A 20 ft by 40 ft pool with a 6 ft wide tanning shelf along one short edge actually holds about 5–8% less water than the rectangle formula suggests.
Pool volume unit conversion
U.S. pool chemistry uses gallons. European pool chemistry uses liters and cubic meters. The conversions are exact: 1 ft³ = 7.4805 US gallons = 28.3168 liters. 1 m³ = 264.172 US gallons = 1,000 liters. 1 US gallon = 3.7854 liters.
Watch out for the imperial vs US gallon trap: a UK gallon is 4.546 L, a US gallon is 3.785 L — about 20% larger. Most pool chemical labels sold in the U.S. assume US gallons; UK labels assume imperial. Dosing the wrong type creates a 20% error.
- Cubic feet to US gallons — multiply by 7.4805
- Cubic feet to liters — multiply by 28.3168
- Cubic meters to US gallons — multiply by 264.17
- Cubic meters to liters — multiply by 1,000 (exact)
- US to imperial gallons — multiply by 0.8327
- Gallons to pounds of water — multiply by 8.34 (at 62 °F)
Pool fill time and water cost
A standard 5/8 inch garden hose at typical residential water pressure flows 9–17 gallons per minute — call it 10 GPM as a planning number. At 10 GPM, a 15,000-gallon pool takes 1,500 minutes, or 25 hours, of continuous flow. A 30,000-gallon pool takes about 50 hours. Two hoses in parallel halve the time.
Bulk delivery is usually cheaper and far faster. U.S. water trucks carry 3,000–5,000 gallons per load and discharge in under an hour. The U.S. Environmental Protection Agency's WaterSense program reports average residential water rates of $4–$10 per 1,000 gallons depending on region; municipal sewer charges on top often double the effective rate for a one-time fill. Bulk truck pricing was $250–$450 per 5,000-gallon load in most metros as of 2024.
Many U.S. utilities will exempt a one-time pool fill from sewer charges if you request it in advance — the water never enters the sewer system. The savings can run $50–$150 on a 20,000-gallon fill. Call the utility before filling and ask about a "pool fill credit" or "deduct meter".
Common pool-volume mistakes
The pool shell is taller than the water column. Coping, tile band, and freeboard typically eat 4–8 inches of the wall. Measure from waterline to floor, not coping to floor. An 8 inch over-measurement on a 20 by 40 ft pool overstates volume by about 800 gallons.
The π r² formula uses radius (half the diameter). Plugging diameter in by mistake quadruples the answer. A 24 ft round pool's correct volume is π × 12² × D — not π × 24² × D. This is the single most common error in pool-volume math.
Length × width × depth in feet gives cubic feet, not gallons. Multiply by 7.48 to get US gallons. A 20 by 12 by 4.5 ft pool is 1,080 ft³ — not 1,080 gallons. The correct gallon figure is 8,080.
A UK gallon is 4.546 L; a US gallon is 3.785 L. UK pool owners reading a US chemical label and treating the dose as imperial gallons under-dose by 20%. Always check which gallon the label assumes.