The Basic Formula for Pool Volume

To find how many gallons your pool holds, you need three measurements: length, width, and average depth. Multiply length × width × average depth, then multiply that result by 7.5. That final number is your pool's volume in gallons.

The 7.5 is a conversion factor—it turns cubic feet into gallons, since one cubic foot equals 7.5 gallons. This works for rectangular pools. Oval and round pools use a slightly different formula, which we'll cover in the next section.

Measure in feet, not inches. If your pool is 20 feet long, 10 feet wide, and 5 feet deep at its deepest point, the math looks like this: 20 × 10 × 5 = 1,000 cubic feet. Then 1,000 × 7.5 = 7,500 gallons.

Key Takeaways

  • For a rectangular pool, multiply length × width × average depth, then multiply by 7.5 to convert cubic feet to gallons.
  • Average depth matters: measure the shallow end and deep end separately, then add them and divide by two.
  • Round pools use the formula: radius squared × 3.14 × average depth × 7.5.
  • Oval pools use: length × width × 3.14 ÷ 4 × average depth × 7.5.
  • Measure in feet from the ground up, and round your final answer to the nearest 100 gallons for practical purposes.

Why Average Depth Matters

Most pools don't have the same depth everywhere. The shallow end might be 3 feet, and the deep end 8 feet. Using one number would give you a wrong answer. Instead, measure the depth at the shallow end and the depth at the deep end, add those two numbers, and divide by two.

If your shallow end is 3 feet and your deep end is 8 feet, your average depth is (3 + 8) ÷ 2 = 5.5 feet. Use 5.5 in your calculation. This method works whether your pool slopes gradually or drops off sharply.

Calculating Volume for Round Pools

A round pool needs a different formula because it doesn't have length and width—it has a radius. The radius is the distance from the center of the pool to the edge. Measure across the widest part of the pool (the diameter), then divide by two to get the radius.

The formula is: radius squared × 3.14 × average depth × 7.5. If your pool is 20 feet across, the radius is 10 feet. Then: 10 × 10 = 100. Then 100 × 3.14 = 314. Then 314 × 5 (average depth) = 1,570. Then 1,570 × 7.5 = 11,775 gallons.

The 3.14 is pi, a constant used for circles. You can use 3.14159 if you want more precision, but 3.14 is close enough for pool math.

Calculating Volume for Oval Pools

An oval pool sits between a rectangle and a circle. Measure the length (the long way across) and the width (the short way across). The formula is: length × width × 3.14 ÷ 4 × average depth × 7.5.

If your oval pool is 30 feet long and 15 feet wide, with an average depth of 5 feet, the math is: 30 × 15 = 450. Then 450 × 3.14 = 1,413. Then 1,413 ÷ 4 = 353.25. Then 353.25 × 5 = 1,766.25. Then 1,766.25 × 7.5 = 13,246.875 gallons. Round to 13,250 gallons.

Common Mistakes to Avoid

The most common error is forgetting to convert to average depth. Many people measure only the deep end or only the shallow end, which throws off the entire calculation. Always measure both ends and average them.

Another mistake is measuring in inches instead of feet. If you measure in inches, you'll get a number that's way too large. Convert inches to feet first by dividing by 12. A pool that is 60 inches deep is 5 feet deep.

Don't forget the final multiplication by 7.5. Some people stop after calculating cubic feet and think they have gallons. Cubic feet and gallons are different units—you must multiply by 7.5 to convert.

When to Remeasure Your Pool

You only need to calculate pool volume once, unless the pool structure changes. If you drain and refill the pool, the volume stays the same. If you add or remove a section, or if the bottom settles and the depth changes noticeably, remeasure.

For most homeowners, one calculation done carefully is enough. Write down your result and keep it with your pool maintenance records. You'll need it when buying chemicals, estimating refill time, or troubleshooting equipment.

Frequently Asked Questions

Do I need to measure depth in multiple places?

For a straightforward sloped pool, measuring the shallow end and deep end is enough. If your pool has an unusual shape—a beach entry, a deep spot in the middle, or a shelf—measure three or four points and average all of them for better accuracy.

What if my pool is irregularly shaped?

Divide the pool into sections. If it's L-shaped, treat each arm as a separate rectangle, calculate each volume, and add them together. For very irregular shapes, break them into rectangles, circles, or ovals and sum the results.

Can I use centimeters or meters instead of feet?

Yes, but the conversion factor changes. If you measure in meters, multiply by 264.17 instead of 7.5. If you measure in centimeters, convert to feet first by dividing by 30.48, then use 7.5. Feet is simpler for pool math.

How accurate does my calculation need to be?

For chemical dosing and equipment sizing, within 5 percent is fine. Measure to the nearest half-foot and round your final answer to the nearest 100 gallons. Exact precision matters less than consistency—use the same method each time you check.

What if I don't know my pool's depth?

Use a long stick or a pool depth gauge. Mark where the water surface is, pull it out, and measure from the mark to the end. Do this at the shallow end and deep end. If you can't measure directly, ask the pool builder or check the original paperwork.