Free-Form Pool Volume Calculator: Irregular Shapes

Free-form, lagoon, and other curved pools have no simple formula. Measuring several widths along the length turns the curve into a series of strips, which gives a close answer with only a tape measure.

Calculator: length, widths, and depths

Enter the length along the longest line, up to 5 widths, and the water depth at the shallow and deep ends. With the example values:

Water surface: 504 sq ft, from 5 widths (length ÷ 5 × sum of the widths).
Average depth: 4.75 ft, or (shallow + deep) ÷ 2.
Volume: 17,908 gallons (67,791 L), using 7.48 gallons per cubic foot.

For chemical amounts near this size, the calculator links to the 20,000-gallon pool dose chart, the closest chart we have. For the most accurate doses, save your own gallons and use the calculators with them.

How to measure a free-form pool

Find the longest straight line across the water surface, end to end, and measure it. That is the length.
Divide the length into equal sections: 3 sections for a simple shape, 5 for a curvy one.
At the middle of each section, measure the width straight across the water, at a right angle to the length line.
Measure the water depth at the shallow end and at the deepest point. Measure water depth, not wall height.
Enter the length, the widths, and the two depths in the calculator.

Each section is the length ÷ the number of sections, and each width goes at the middle of its section, so the first width sits half a section from the end. Two people and a long tape make this easier. On a calm day, a string or a pool pole laid across the water helps keep the tape straight.

Why more widths give a better answer

The calculator uses the midpoint rule: each width stands for its whole strip of the pool. On a straight-sided pool, 3 widths are already exact. On a curvy pool, each strip is only close, so more strips mean a closer total. If the pool has a narrow neck or a wide bulge, place a measurement in it, because missing a bulge makes the total less accurate.

For a Grecian pool (a rectangle with cut corners), the corner cuts are near the ends, so the widths measured near each end catch them. For simple shapes, the pool volume calculator handles rectangles and L-shapes, and the kidney-shaped pool calculator handles kidneys.

Deep ends, hoppers, steps, and benches

Sloped floor. The calculator averages the shallow and deep depths. That is exact for a floor that slopes evenly from one end to the other.
Hopper or bowl-shaped deep end. The floor drops steeply in one area and is flat elsewhere, so a simple average of the two ends is only approximate. If most of the floor is shallow, the real volume is a bit less than the calculator shows. You can enter a lower "deep end" depth to reflect that.
Steps, benches, and sun shelves. They take up water space, so the real volume is a little less.
Water level. Measure depth to the actual water line, usually about halfway up the skimmer opening, not to the top of the wall.

An error of a few percent in volume is normal for any measured pool, and it only changes doses by the same few percent.

Check your answer

Two ways to get your pool's real volume:

Salt-rise check. Add a known amount of salt (in a salt pool, or one you plan to convert) and measure how much the salt level rises. The pool salt calculator works the math in reverse.
Water meter. Read your water meter before and after filling or topping up a measured number of inches. The pool fill cost calculator explains meter units.

Once you know your real gallons, use that number everywhere. For size charts of common pools, see pool gallons by size.

FAQ

How do you measure a free form pool?

Measure the longest line across the water, split it into 3 to 5 equal sections, and measure the width across the middle of each. Add the shallow and deep water depths. Area = length ÷ number of widths × sum of widths, and gallons = area × average depth × 7.48.

How many gallons is a Grecian pool?

It depends on its length, width, corner cuts, and depth. Measure the length and 3 to 5 widths, including widths near each end so the cut corners count, then enter them above with your depths.

How do I include the deep end in pool volume?

Measure the water depth at the shallow end and at the deepest point, and use their average. In the example, 3.5 and 6 ft average to 4.75 ft. If the deep end is a small hopper, the true average is a bit lower.

Sources

Gallons per cubic foot (7.48 US gallons) and the volume formulas our calculators use: methodology