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Pool volume math
How do you calculate a pool's volume in gallons?
Multiply the pool's surface area by its average depth to get cubic feet, then multiply by 7.48 to convert cubic feet to gallons. For a rectangle that is length times width times average depth times 7.48. Use the average depth, not the deep end.
Volume is the first number every other pool calculation depends on, so an error here quietly wrecks your turnover, flow, and dosing answers too. Below is the formula, a fully worked sloped-pool example, the one error that fails most people, and eight randomized problems to drill it. Check any answer against the pool chemistry calculator.
The error class that fails people · Confusing cubic feet with gallons The formula gives cubic feet until you multiply by 7.48. Stopping one step early reports cubic feet as if they were gallons, so the answer comes out about seven times too small. The second trap on a sloped pool is using the deep-end depth instead of the average depth, which inflates every downstream turnover, flow, and dosing figure.
Drill it: 8 randomized problems
Eight randomized volume problems across rectangular, sloped-bottom, and round pools. Each shows a full worked solution the moment you answer, and the wrong choices are the exact numbers a cubic-feet or deep-end slip produces, so a miss tells you which mistake you made. Free, no signup, progress saved on your device.
Sample problems
A preview of the kind of problem in the set, authored from the underlying pool-hydraulics facts. Open the answer for the full worked solution.
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A rectangular pool is 60 ft long by 30 ft wide with a constant depth of 5 ft. What is its volume in gallons?
- A 67,320 gallons
- B 9,000 gallons
- C 6,732 gallons
- D 673,200 gallons
Show the worked solution
Correct answer: A. 67,320 gallons
Volume = length x width x average depth x 7.48. First the cubic feet: 60 x 30 x 5 = 9,000 cu ft. Then convert to gallons: 9,000 x 7.48 = 67,320 gallons. The most common wrong answer, 9,000, stops at cubic feet and forgets the 7.48. The other two slip the decimal, reading 7.48 as 0.748 or 74.8.
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A rectangular pool is 75 ft by 35 ft. The bottom slopes from a 3 ft shallow end to a 9 ft deep end. What is its volume in gallons?
- A 176,715 gallons
- B 58,905 gallons
- C 117,810 gallons
- D 15,750 gallons
Show the worked solution
Correct answer: C. 117,810 gallons
With a sloping bottom you use the AVERAGE depth, not the deep end: (3 + 9) / 2 = 6 ft. Then 75 x 35 x 6 = 15,750 cu ft, and 15,750 x 7.48 = 117,810 gallons. Plugging in the 9 ft deep end gives 176,715 and overshoots; the 3 ft shallow end gives 58,905 and undershoots.
Questions people ask
What is the 7.48 in the pool volume formula?
It is the number of US gallons in one cubic foot of water. The dimensions give you cubic feet, and multiplying by 7.48 converts that to gallons. Some study guides round it to 7.5. Do not confuse it with 8.34, the weight in pounds of one gallon of water, which belongs in dosing math, not volume.
Do I use the average depth or the deep end?
Always the average depth. For a pool that slopes from a shallow end to a deep end, average depth is the shallow depth plus the deep depth divided by two. Using the deep-end number overstates the volume and cascades into wrong turnover, flow, and chemical-dose answers.
How do I find the volume of a round pool?
Use pi times the radius squared for the surface area, then multiply by the average depth and by 7.48. The radius is half the diameter, and squaring the diameter by mistake makes the answer four times too large, the most common round-pool error. The calculator can check your figure.