Pond Calculator
How much water a garden pond holds, rectangular, round, oval or irregular, in litres, US gallons and UK gallons, the liner and underlay size it needs, and the pump flow to turn the water over in a time you choose.
Result…
How it's calculated
Surface: rectangle L × W · circle π × d² ÷ 4 · oval π × L × W ÷ 4 · irregular average L × average W Volume = surface × average depth (1 m³ = 1,000 L; 1 ft³ = 28.316846592 L; US gal = 3.785411784 L; UK gal = 4.54609 L) Liner = (L + 2 × deepest + 2 × overlap) × (W + 2 × deepest + 2 × overlap), underlay the same Pump flow = volume ÷ turnover time
A pond’s volume is its surface area times its average depth: shelves and sloping sides make the average shallower than the deepest point, so measure in several places. The liner is sized as if the deepest point ran the whole way across, plus an overlap on each edge to bury or hide under stones; the protective underlay below it is cut to the same size. A pump is sized by how often it should move the whole volume, a choice that depends on fish, plants and the filter, so the time here is yours to set.
Your numbers, step by step
About the Pond calculator
This pond calculator works out how much water a garden pond holds and how big a liner it needs. Choose a rectangle, circle, oval or irregular shape, enter the sizes in metres or feet with the average depth, and it gives the volume in litres, US gallons and UK gallons. The liner is sized so the deepest point could run all the way across, plus an overlap at every edge to bury or hide under stones, and the underlay beneath it is cut to the same size. Enter how often the whole volume should pass through the pump and it gives the flow rate to look for. Every step is shown with your numbers, and the factors between units are exact. The turnover time and overlap are yours to set, since fish, plants, filters and liner makers all differ.
How to use it
- Pick the “Units” and the “Shape”, then enter the “Length” and “Width” or the “Diameter”.
- Enter the “Average depth” and the “Deepest point”.
- Set the “Liner overlap at each edge” from the liner maker's advice.
- Enter how often to “Turn the water over every” in hours to size the pump.
- Read the volume, the liner and underlay size, and the pump flow.
Worked example
A rectangular pond 3.5 m × 2.5 m with an average depth of 0.6 m and a deepest point of 0.9 m. Surface = 3.5 × 2.5 = 8.75 m², and volume = 8.75 × 0.6 = 5.25 m³ = 5,250 litres, about 1,387 US gallons or 1,155 UK gallons. Liner = (3.5 + 2 × 0.9 + 2 × 0.3) × (2.5 + 2 × 0.9 + 2 × 0.3) = 5.9 m × 4.9 m = 28.91 m², with an underlay the same size. To turn the water over every 2 hours, the pump needs 5,250 ÷ 2 = 2,625 litres an hour at the height it lifts the water.
Updated
Questions people ask
How do I calculate the volume of a pond?
Multiply the surface area by the average depth. For a rectangle that is length × width × depth; for a circle π × diameter² ÷ 4 × depth; for an oval π × length × width ÷ 4 × depth. Multiply cubic metres by 1,000 for litres.
What size pond liner do I need?
Add twice the deepest point and twice the edge overlap to both the length and the width. A 3.5 m × 2.5 m pond 0.9 m deep with 0.3 m overlap needs a 5.9 m × 4.9 m liner.
How do I convert pond litres to gallons?
Divide litres by 3.785411784 for US gallons or by 4.54609 for UK (imperial) gallons. 5,250 litres is about 1,387 US gallons or 1,155 UK gallons; the volume converter handles other units.
What is a common mistake when measuring a pond?
Using the deepest point for the volume. Shelves and sloping sides make the average depth smaller, so measure in several places and average them; keep the deepest point for the liner.
How accurate is the pond volume estimate?
It is only as good as the average sizes, and real ponds are rarely regular. For water treatments that are dosed by volume, read a water meter while filling, and follow the product's own instructions. For tanks and containers, try the tank volume calculator.