A 32 x 16 ft pool that slopes from 3 ft to 8 ft deep holds about 21,065 gallons: 512 sq ft of water surface times a 5.5 ft average depth is 2,816 cu ft, and each cubic foot holds 7.48 gallons.
That is the whole method behind a swimming pool volume calculator: water surface area × average depth = cubic feet, then × 7.48, because a US gallon is 231 cubic inches and a cubic foot is 1,728.
Our pool volume calculator handles rectangle, round, and oval pools, gives gallons, cubic feet, and liters, and adds the fill time if you enter your hose's flow.
How to measure your pool
- Measure the water, not the deck. Take the inside length and width at the water line, wall to wall, in feet. For a round pool, measure the diameter straight across the middle. Convert inches to decimal feet (6 in is 0.5 ft); the square footage calculator guide has a conversion table.
- Measure the water depth at both ends. Lower a tape or a marked pole straight down from the water surface to the floor at the shallow end and at the deep end. If the bottom is flat, enter the same number twice.
- Take more readings if the floor isn't one straight slope. Measure at evenly spaced points along the length, average them, and enter that average as both depths.
- Pick the shape. Rectangle, round, oval (a true ellipse), or oval with straight sides and rounded ends. Kidney and freeform pools have their own section below.
- Optional: measure your hose flow with a bucket and a timer (see the fill time section).
The pool volume formula
- Water surface (sq ft): rectangle = length × width; round = 3.1416 × radius × radius; oval = 3.1416 × half the length × half the width; oval with straight sides = width × (length − width) + 3.1416 × (width ÷ 2) × (width ÷ 2)
- Average depth (ft) = (shallow end + deep end) ÷ 2
- Cubic feet = water surface × average depth
- Gallons = cubic feet × 7.4805; liters = gallons × 3.785
- Fill time (minutes) = gallons ÷ hose flow in gallons per minute (gpm)
NIST Handbook 44 defines the US gallon as 231 cubic inches exactly, so a cubic foot (1,728 cubic inches) holds 1,728 ÷ 231 = 7.4805 gallons; it also lists a gallon as 3.785411784 liters. Pool guides round the factor: the Tulsa Health Department's pool operator manual uses 7.48, and HTH's shock guide uses 7.5, about a quarter of a percent more.
Worked example (the calculator's default values)
A rectangular pool 32 ft long and 16 ft wide, 3 ft deep at the shallow end and 8 ft at the deep end:
- Water surface: 32 × 16 = 512 sq ft
- Average depth: (3 + 8) ÷ 2 = 5.5 ft
- Volume: 512 × 5.5 = 2,816 cu ft
- Gallons: 2,816 × 7.4805 = 21,065 gallons, or about 79,740 liters
The calculator shows: "Your pool holds about 21,065 gallons (2,816 cu ft of water)." and "Water surface: 512 sq ft (rectangle, 32 x 16 ft). Average depth: (3 + 8) / 2 = 5.5 ft. 512 x 5.5 = 2,816 cu ft x 7.4805 gallons per cu ft = 21,065 gallons, or 79,740 liters."
ProPools lists Latham rectangle pool kits from 12 x 24 to 20 x 40 ft, including 16 x 32, with "standard depths ranging from 3' to 8', depending upon pool length," and Pool Warehouse sells a 16 x 32 kit with an 8 ft deep end. The 5.5 ft average assumes an even slope.
Second example: a round pool 24 ft across
The radius is 12 ft, so the water surface is 3.1416 × 12 × 12 = 452.39 sq ft. With 4 ft of water at both ends: "Your pool holds about 13,536 gallons (1,809.56 cu ft of water)." The width field is ignored.
Third example: two kinds of oval
An oval 30 ft long and 15 ft wide with 4 ft of water holds about 10,575 gallons if it is a true ellipse, but about 12,020 gallons if it has straight sides and half-circle ends: 401.71 sq ft of water instead of 353.43, or 13.7% more. Check which shape you have.
Pool gallons by size
Computed with the calculator's math. For an above-ground pool measured in inches, or to top off, multiply the per-inch column by the inches of water. Rectangle sizes follow the Latham kit list on ProPools; round and oval sizes are examples.
| Pool | Water surface (sq ft) | Gallons per inch of depth | At 4 ft deep | At 5.5 ft average |
|---|---|---|---|---|
| Round, 12 ft across | 113.1 | 71 | 3,384 | 4,653 |
| Round, 15 ft across | 176.71 | 110 | 5,288 | 7,271 |
| Round, 18 ft across | 254.47 | 159 | 7,614 | 10,470 |
| Round, 24 ft across | 452.39 | 282 | 13,536 | 18,613 |
| Round, 30 ft across | 706.86 | 441 | 21,151 | 29,082 |
| Oval (ellipse), 15 x 30 ft | 353.43 | 220 | 10,575 | 14,541 |
| Oval with straight sides, 15 x 30 ft | 401.71 | 250 | 12,020 | 16,528 |
| Rectangle, 12 x 24 ft | 288 | 180 | 8,618 | 11,849 |
| Rectangle, 14 x 28 ft | 392 | 244 | 11,729 | 16,128 |
| Rectangle, 16 x 32 ft | 512 | 319 | 15,320 | 21,065 |
| Rectangle, 18 x 36 ft | 648 | 404 | 19,390 | 26,661 |
| Rectangle, 20 x 40 ft | 800 | 499 | 23,938 | 32,914 |
Kidney and freeform pools
There is no exact formula for a kidney shape, and two published shortcuts disagree:
- Tulsa Health Department (pool operator manual): (A + B) × L × 0.45 × average depth × 7.48, where A and B are the two widths across the pool and L is the length (Orenda describes A and B as the diameters across the main round areas). Its example, A 16 ft, B 18 ft, L 32 ft and 3 ft deep, comes to 10,987 gallons.
- Orenda (a pool chemical maker): length × the average of A and B. The same pool comes to 12,208 gallons, 11% more.
To run Tulsa's version through the calculator, choose Rectangle and enter 0.45 × (A + B) as the width: 0.45 × 34 = 15.3 ft, with 3 ft at both ends, gives "Your pool holds about 10,987 gallons (1,468.8 cu ft of water)." Treat either number as rough. For a freeform pool, split the water surface into rectangles, as the Tulsa manual does for multi-sided pools, plus circles or half circles for rounded parts; add the areas and multiply by the average depth.
Average depth, slopes, and hoppers
The calculator averages the two depths, the same "Deep End + Shallow End ÷ 2" the Tulsa manual uses. That is exact for a flat floor or an even slope. If the floor stays shallow for a long stretch and then drops into a deep hopper, the true average is lower, so the two-point average overestimates.
Orenda says average depth requires, "at a minimum," the deepest and shallowest readings. For a better number, average readings at evenly spaced points. Say five readings in the default pool are 3, 3.5, 4.5, 6.5, and 8 ft: they average 5.1 ft, and entering 5.1 as both depths gives "Your pool holds about 19,533 gallons (2,611.2 cu ft of water)," about 1,500 gallons less than the two-point estimate.
Above-ground pools
Use the water depth, not the wall height, because the water line sits below the top of the wall. Divide inches by 12 for feet: 48 in is 4 ft. Setting one up? The sand calculator sizes the sand base under the liner; choose Circle for a round pool.
When the maker lists a capacity for your model, use it. Intex lists its 18 ft x 9 ft x 52 in Ultra XTR rectangular frame pool at "4,545 gallon (90%)." If that means water 90 percent of the way up the 52 in wall, the depth is 46.8 in, or 3.9 ft, and the calculator gives 4,726 gallons for 18 x 9 ft, about 4% more. Intex gives 18 x 9 ft as the approximate set-up size, so the water inside can measure a little less.
How long does it take to fill a pool?
Divide the gallons by your hose's flow in gallons per minute. Measure the flow rather than guess. Iowa State University Extension's method: time how long the hose takes to fill a five-gallon bucket, then divide the bucket's capacity by that time in minutes.
If the bucket fills in 30 seconds, the flow is 5 ÷ 0.5 = 10 gpm. Enter 10 as the hose flow for the default pool and the result adds: "At 10 gpm, filling it from empty takes about 35.1 hours (1.5 days)." Measure with the hose and spigot you will actually use.
Why the gallons matter
Pool chemicals are dosed by volume. HTH's shock guide says to "Calculate your total pool volume first, then check the package of shock for dosing recommendations by volume." The CDC's advice for pool owners is to "Read and follow all directions on product labels," so take your gallons to the label; this guide doesn't give doses.
Building around the pool? The CDC recommends "four-sided fencing, which fully encloses the pool and separates it from the house, with self-closing and self-latching gates." The fence calculator counts posts, rails, and pickets for each straight run, and the paver calculator figures pavers, base gravel, and bedding sand for a patio. Check your local code for pool barrier rules.
Tips and common mistakes
- Measuring the deck or coping. Use the inside dimensions at the water line.
- Using the wall height of an above-ground pool. The water is lower than the wall.
- Mixing feet and inches. 3 ft 6 in is 3.5 ft, not 3.6.
- Picking the wrong oval. A straight-sided oval holds more than an ellipse of the same length and width.
- Guessing. Orenda warns that pool volume estimates made without measuring are often off by more than 2,000 to 3,000 gallons.
Frequently asked questions
How do I calculate how many gallons are in my pool?
Multiply the water surface in square feet by the average depth in feet, then by 7.48. A 32 x 16 ft pool from 3 to 8 ft deep is 512 × 5.5 = 2,816 cu ft, or about 21,065 gallons.
How many gallons is a 16 x 32 pool?
It depends on the depth. With 4 ft of water throughout it holds about 15,320 gallons; sloping evenly from 3 to 8 ft (a 5.5 ft average), about 21,065 gallons. Each inch of depth is about 319 gallons.
How do I find the volume of a round pool?
Multiply 3.1416 by the radius squared for the water surface, then by the depth and 7.48. A pool 24 ft across with 4 ft of water holds about 13,536 gallons.
How many gallons are in a cubic foot of water?
About 7.48 (7.4805), because NIST defines the US gallon as 231 cubic inches and a cubic foot is 1,728 cubic inches. That is about 28.3 liters.
How long does it take to fill a pool with a garden hose?
Gallons ÷ hose flow in gpm gives minutes. Time a five-gallon bucket to find the flow. At 10 gpm, a 21,065 gallon pool takes about 35.1 hours.
Sources
- NIST Handbook 44 (2026), Appendix C: General Tables of Units of Measurement (PDF)
- Tulsa Health Department: Pool Operators Permit Instruction Manual (PDF)
- Orenda: How to Calculate Swimming Pool Volume
- Intex: Rectangular Ultra XTR Frame 18 ft x 9 ft x 52 in pool
- ProPools: Latham Rectangle Inground Pool Kits
- Pool Warehouse: 16' x 32' Rectangle Pool Kit
- Iowa State University Extension and Outreach: How do I determine how much water to use when watering my garden plants?
- HTH: How much shock do I add to my swimming pool?
- CDC: Guidelines for Keeping Your Pool Safe and Healthy
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