How this calculator works
A pond's volume is its length times its width times its average depth, adjusted for the shape: 1 for a rectangle, π/4 (0.785) for an oval or a circle, and about 0.7 for a free-form shape. Cubic feet times 7.481 gives US gallons. The liner has to cover the bottom, rise up each side by the depth, and overlap the edge: length plus twice the depth plus a couple of feet.
Worked example: an oval pond 10 × 6 ft with an average depth of 2 ft holds 10 × 6 × 2 × 0.785 = 94 ft³, which is 705 gallons (2,668 liters) and weighs about 5,900 lb. With a deepest point of 3 ft the liner is 18 × 14 ft. A pump that turns the water over once an hour needs to move 705 gallons per hour.
Gallons in common ponds (oval, 2 ft average depth, 3 ft deepest)
With the liner to buy and the pump flow for one turnover an hour.
| Pond | Gallons | Liner | Pump GPH |
|---|---|---|---|
| 6 × 4 ft | 282 | 14 × 12 ft | 282 |
| 8 × 5 ft | 470 | 16 × 13 ft | 470 |
| 10 × 6 ft | 705 | 18 × 14 ft | 705 |
| 12 × 8 ft | 1,128 | 20 × 16 ft | 1,128 |
| 15 × 10 ft | 1,763 | 23 × 18 ft | 1,763 |
| 20 × 12 ft | 2,820 | 28 × 20 ft | 2,820 |
Tips before you buy
- A pond that is at least 2 feet deep over part of it stays cooler in summer and does not freeze solid in winter in most places. Fish need more, so ask about koi.
- Dig the shelves for plants about 9 to 12 inches deep, and keep the edge level all the way round, so that the water does not spill over one side.
- Put an underlay or sand under the liner to protect it, and leave slack so that it does not stretch when the pond fills.
- Plan the pump and filter at the same time as the pond. Too small a pump leaves the water green, and too big a one stresses the fish.
- Check local rules on fencing and depth: a pond can be a drowning hazard for young children, and some areas require a barrier.
Tools & materials for this project
As an Amazon Associate we earn from qualifying purchases. These links do not change what you pay.
Frequently asked questions
How do I calculate the gallons in my pond?
Multiply the length by the width by the average depth (all in feet) to get cubic feet, multiply by 0.785 for an oval or 0.7 for a free-form pond, and then by 7.481 to get gallons.
What size liner do I need for my pond?
Length plus twice the deepest point plus 2 feet, by width plus twice the deepest point plus 2 feet. A 10 × 6 ft pond that is 3 ft deep needs an 18 × 14 ft liner.
What size pump do I need for a pond?
Enough to move the pond's volume once an hour: a 700 gallon pond needs a pump of about 700 GPH at the height it lifts the water, and more for a waterfall or heavy fish load.
How much does the water in a pond weigh?
About 8.34 pounds a gallon, so a 700 gallon pond has about 5,800 lb of water: make sure the ground, a deck or a roof can carry it.
Add this calculator to your website
Running a blog, a contractor site or a class page? You can put this calculator on it for free. Copy the code below, and please leave the link underneath the calculator in place. The small script just resizes the frame to fit the results; if your site strips scripts, the calculator still works inside a scrolling frame.
Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.