How the calculator works
Enter the pond's shape, length and width (or diameter), its average depth and its deepest point.
- Surface area: length × width for a rectangle, 0.785 × diameter² for a round pond, and 0.785 × length × width for an oval, which also works as an estimate for an irregular pond.
- Volume: area × average depth in feet = cubic feet. Gallons = cubic feet × 7.48, and liters = gallons × 3.785.
- Average depth: ponds with plant shelves are not as deep all over as at the deepest point. Estimate the depth across the whole pond, or work out the deep zone and the shelves separately and add them.
- Liner: width + 2 × deepest point + 2 × overlap, and the same for the length. The default overlap is 12 inches at each edge.
- Pump: gallons per hour equal to the pond's volume, one full turnover every hour.
- Weight: 8.34 pounds per gallon, or 1 kilogram per liter.
Pond volume chart (rectangular ponds)
| Pond size | 18 in deep | 24 in deep | 36 in deep |
|---|---|---|---|
| 4 × 6 ft | 269 gal | 359 gal | 539 gal |
| 6 × 8 ft | 539 gal | 718 gal | 1,077 gal |
| 8 × 10 ft | 898 gal | 1,197 gal | 1,795 gal |
| 10 × 12 ft | 1,346 gal | 1,795 gal | 2,693 gal |
| 10 × 15 ft | 1,683 gal | 2,244 gal | 3,366 gal |
| 12 × 16 ft | 2,154 gal | 2,873 gal | 4,309 gal |
Round ponds
| Diameter | 18 in deep | 24 in deep | 36 in deep |
|---|---|---|---|
| 4 ft | 141 gal | 188 gal | 282 gal |
| 6 ft | 317 gal | 423 gal | 635 gal |
| 8 ft | 564 gal | 752 gal | 1,128 gal |
| 10 ft | 881 gal | 1,175 gal | 1,763 gal |
| 12 ft | 1,269 gal | 1,692 gal | 2,538 gal |
Pond liner size chart
With 1 foot of overlap at each edge, by the pond's deepest point.
| Pond size | 18 in deep | 24 in deep | 36 in deep |
|---|---|---|---|
| 4 × 6 ft | 9 × 11 ft | 10 × 12 ft | 12 × 14 ft |
| 6 × 8 ft | 11 × 13 ft | 12 × 14 ft | 14 × 16 ft |
| 8 × 10 ft | 13 × 15 ft | 14 × 16 ft | 16 × 18 ft |
| 10 × 12 ft | 15 × 17 ft | 16 × 18 ft | 18 × 20 ft |
| 10 × 15 ft | 15 × 20 ft | 16 × 21 ft | 18 × 23 ft |
| 12 × 16 ft | 17 × 21 ft | 18 × 22 ft | 20 × 24 ft |
Liners are sold in set sizes, so round up to the next size on offer.
Example: an oval pond with a plant shelf
The pond is about 12 feet long and 8 feet wide, 2 ½ feet at its deepest, with a shelf around the edge, so it averages about 21 inches (1.75 feet) deep.
- Area: 0.785 × 12 × 8 = 75.4 square feet.
- Volume: 75.4 × 1.75 = 131.9 cubic feet, or about 987 gallons (3,736 liters).
- Liner: 8 + 2 × 2.5 + 2 × 1 = 15 feet wide, and 12 + 2 × 2.5 + 2 × 1 = 19 feet long. Buy a 15 × 20 ft liner if 15 × 19 is not a stock size.
- Pump: about 1,000 gallons per hour at the height of the waterfall or filter outlet.
- Weight: 987 × 8.34 = about 8,232 pounds of water.
Tips
- Lay the hose first. Mark the outline with a garden hose, then measure the length and width across it.
- Use underlayment. A felt or geotextile underlayment below the liner protects it from stones and roots. Buy it the same size as the liner.
- Treat tap water. City water usually contains chlorine or chloramine. Dose a dechlorinator for the full volume before adding fish.
- Top up gradually. Summer evaporation can take an inch or more a week. Add small amounts of treated water rather than a big refill.
- Check your local rules. Some places treat ponds over a certain depth, often 18 or 24 inches, like pools and require a fence.
Frequently asked questions
How many gallons is my pond?
Multiply the surface area in square feet by the average depth in feet, then by 7.48. An 8 × 6 ft pond with an average depth of 2 feet holds 8 × 6 × 2 = 96 cubic feet, or about 718 gallons. For a round pond the area is 0.785 × diameter², and for an oval about 0.785 × length × width.
What size pond liner do I need?
Add twice the deepest point and twice the overlap to both the length and the width. An 8 × 10 ft pond 2 feet deep with 1 foot of overlap at each edge needs a 14 × 16 ft liner. Plant shelves do not change the size, since the liner follows the sides down to the bottom either way.
What size pump do I need for my pond?
A common guide is a pump that moves the whole pond volume at least once an hour, so a 1,000-gallon pond needs about 1,000 gallons per hour. Koi ponds and ponds with a waterfall or a filter that needs more flow often use more. Check the pump's flow at the height it has to lift the water, which is lower than its maximum.
How deep should a fish pond be?
Many guides suggest at least 18 to 24 inches for goldfish and 3 feet or more for koi. Deeper water stays cooler in summer, gives fish somewhere to hide from herons and raccoons, and is less likely to freeze solid in cold winters. Shallow shelves 8 to 12 inches deep around the edge hold marginal plants.
How do I find the volume of an irregular pond?
Split the outline into rectangles and circles, work out each one and add them, or use the oval setting with the longest length and widest width as a rough estimate. For the exact number, read your water meter before and after filling the pond, or time a hose filling a 5-gallon bucket and then time the pond.
Why does my pond's volume matter?
Pond treatments, dechlorinator, salt and medication are all dosed per gallon, and pumps, filters and UV clarifiers are sized by it. Too little treatment does nothing and too much can harm fish, so measure rather than guess.
Last updated October 20, 2026.