How many gallons is your pond?

How do I calculate the gallons in a pond?

Multiply the surface area by the average depth to get cubic feet, then multiply by 7.48. A 10 × 8 ft pond that averages 2 ft deep holds 10 × 8 × 2 = 160 cubic feet, or 1,197 gallons. For a round pond, the area is 3.1416 × radius²; for an oval, 3.1416 × length × width ÷ 4.

What size pond liner do I need?

Add twice the deepest depth and twice the overlap to both the length and the width. For a 10 × 8 ft pond 3 ft deep with 1 ft of overlap on every side: 10 + 6 + 2 = 18 ft by 8 + 6 + 2 = 16 ft. Use the same size of underlayment beneath the liner.

How do I find the average depth?

Measure the depth at several spots — the shelves and the deepest part — and average them. A pond with 1 ft plant shelves around a 3 ft deep middle often averages about 2 ft. Use the deepest point for the liner, not the average.

What size pump do I need for my pond?

A common guideline is to move the whole pond's volume at least once an hour, so a 1,200 gallon pond needs a pump rated about 1,200 gallons per hour at the height it lifts the water. Waterfalls and long hoses need more.

How much water will it take to fill?

The gallons shown. With a hose running 10 gallons a minute, a 1,200 gallon pond takes about 2 hours. Use a dechlorinator if you keep fish, since tap water is usually treated with chlorine or chloramine.

How much does the water in a pond weigh?

Water weighs about 8.33 pounds per US gallon, so a 1,616 gallon pond holds roughly 13,460 pounds of water. Keep that in mind for raised ponds and for preformed shells set on a deck or patio.

The volume formulas

Pond volume is the surface area times the average depth. For a rectangle the area is length × width; for a circle it is π × (diameter ÷ 2)²; for an oval it is π ÷ 4 × length × width. The result is in cubic feet, and each cubic foot holds 7.48052 US gallons (1 US gallon is 231 cubic inches). In metric units the volume in cubic meters multiplied by 1,000 gives liters. Because a typical pond has shallow shelves and a deeper middle, the average depth is always less than the deepest point.

A worked example

A rectangular pond 12 ft by 9 ft with an average depth of 2 ft covers 108 ft² and holds 216 ft³, which is 1,616 gallons or 6,116 liters. If its deepest point is 3 ft and you want 1 ft of liner overlap at each edge, the liner is 20 ft by 17 ft: 12 + 2 × 3 + 2 × 1 and 9 + 2 × 3 + 2 × 1. A round pond 10 ft across averaging 2.5 ft deep holds 1,469 gallons, and a 14 × 8 ft oval at 2 ft holds 1,316 gallons.

Common mistakes

The most common error is using the deepest point as the average depth, which overstates the volume and therefore the pump and treatment amounts. The reverse mistake is sizing the liner from the average depth; the liner has to follow the bottom down to the deepest point, so use the maximum depth there. A third is measuring the pond to the outer edge of the coping or stones rather than to the water line. Finally, ponds that are only filled to part of their depth, or that lose water to evaporation, hold less than the design volume, so measure the depth of the actual water.

Sources: the cubic-foot-to-gallon conversion (7.48052) and the gallon definition (231 in³, 3.785411784 L) are from NIST Handbook 44 and NIST SP 811; the weight of water (8.33 lb per gallon) is from the USGS Water Science School; the liner formula is the usual maker’s rule (length or width plus twice the depth plus twice the overlap). Related: pool volume calculator, tank volume calculator, rainwater harvesting calculator and GPM calculator.