How to calculate the volume of a pool
A pool's volume is the area of its water surface multiplied by the average depth of the water, converted into US gallons. One cubic foot of water holds 7.48 gallons, and that is the number the rectangular formula is built on.
Rectangle: length × width × average depth × 7.48
Round: diameter × diameter × average depth × 5.9
Oval: length × width × average depth × 5.9
Average depth = (shallow end + deep end) ÷ 2
Every measurement is in feet and the result is in US gallons. The factor 5.9 for round and oval pools is 7.48 multiplied by 0.785 (π ÷ 4, the share of a square that a circle fills), rounded to one decimal. That makes the result a standard planning estimate rather than an exact figure.
Measure the water, not the pool shell: take the length and width at the water line, and the depth from the water surface to the floor at the shallowest and deepest points. If the floor slopes evenly between the two, their average is a good fit. A pool with a long flat shallow area and a short deep hole is less accurate, so if you have the manufacturer's stated capacity, trust that figure instead.
Worked example: a 20 × 10 ft rectangular pool
Take a rectangular pool that is 20 ft long and 10 ft wide, with water 3 ft deep at the shallow end and 5 ft deep at the deep end.
- Average depth: (3 + 5) ÷ 2 = 4 ft
- Volume in cubic feet: 20 × 10 × 4 = 800 cubic feet
- Volume in gallons: 800 × 7.48 = 5,984 gallons
- In liters: 5,984 × 3.785 = about 22,650 liters
The same method works for the other shapes. A round pool 15 ft across and 4 ft deep holds 15 × 15 × 4 × 5.9 = 5,310 gallons. An oval pool 24 ft long and 12 ft wide, 3 ft deep at one end and 6 ft at the other (average 4.5 ft), holds 24 × 12 × 4.5 × 5.9 = 7,646 gallons.
Typical pool sizes and their volume
These volumes assume an average depth of 4 ft. Your own depth scales the total in direct proportion, so a pool that averages 5 ft holds 25% more.
| Shape and size | US gallons |
|---|---|
| Rectangle, 12 × 24 ft | 8,617 |
| Rectangle, 16 × 32 ft | 15,319 |
| Rectangle, 20 × 40 ft | 23,936 |
| Round, 15 ft across | 5,310 |
| Round, 21 ft across | 10,408 |
| Round, 27 ft across | 17,204 |
| Oval, 12 × 24 ft | 6,797 |
| Oval, 16 × 32 ft | 12,083 |
What to do with your pool volume
The volume is the starting point for almost every pool chemical. Enter it in the pool salt calculator to find how many pounds of salt a saltwater pool needs, and in the pool chlorine calculator to see how much liquid chlorine or granular product raises the free chlorine by the amount you want.
For a hot tub or spa, look up the capacity in the owner's manual. Both calculators work in gallons, so the figure can be used there as it is. Planning the ground work for a new pool? The sand calculator and the gravel calculator cover the base layers.
Frequently asked questions
How do I calculate the gallons in a rectangular pool?
Multiply the length by the width by the average depth, all in feet, then multiply by 7.48. A 20 × 10 ft pool with an average depth of 4 ft holds 20 × 10 × 4 × 7.48 = 5,984 gallons.
How do I find the average depth of a pool?
Add the depth at the shallow end to the depth at the deep end and divide by two. A pool that is 3 ft deep at one end and 5 ft at the other has an average depth of 4 ft. For a pool with a flat floor, the average is simply its depth.
How many gallons are in a round pool?
Multiply the diameter by itself, then by the average depth, then by 5.9. A round pool 15 ft across and 4 ft deep holds 15 × 15 × 4 × 5.9 = 5,310 gallons.
Why does the formula use 5.9 for round and oval pools?
A circle fills about 78.5% of the square around it, so the rectangle factor of 7.48 is multiplied by 0.785 to get 5.87, which is rounded to 5.9. It is a standard approximation, so treat the result as an estimate.
How many gallons are in a cubic foot, and how many liters in a gallon?
One cubic foot of water is 7.48 US gallons. One US gallon is 3.785 liters, so a 5,984 gallon pool holds about 22,650 liters.
More free calculators
These calculators give estimates for planning only. Densities, bag sizes and coverage vary by supplier, so confirm quantities before you order.