Pond Calculator

About the Pond Calculator

Knowing how much water a garden pond holds is the basis for almost every decision about it. Pumps and filters are sized by gallons per hour, water treatments and fish medications are dosed by gallons, and the liner must be big enough to line the bottom and sides and still lap over the edge. Guessing any of these leads to cloudy water, sick fish or a liner that comes up short when the hole is already dug.

This pond calculator works out the water volume of a rectangular, oval or round pond from its size and average depth, in gallons and litres. It sizes a pump to circulate the water over your chosen turnover time, and gives the dimensions of a flexible liner from the pond's size and deepest point.

How to Use the Pond Calculator

Choose the shape and enter the length and width or the diameter in feet.

Enter the average depth and the deepest point in feet.

Enter the turnover time, how often the pump should move the whole volume.

The Formulas

  area      = length × width, or π × diameter² ÷ 4
  gallons   = area × average depth × 7.48
  pump GPH  = gallons ÷ turnover hours
  liner     = (length + 2 × max depth + 2 ft) × (width + 2 × max depth + 2 ft)

Step-by-Step Example

A 10 × 8 ft pond, 2 ft average depth, 3 ft at the deepest, one-hour turnover.

  area      10 × 8 = 80 ft²
  volume    80 × 2 = 160 ft³ × 7.48 = 1,197 gallons (4,531 litres)
  pump      1,197 ÷ 1 = 1,197 GPH
  liner     (10 + 6 + 2) × (8 + 6 + 2) = 18 × 16 ft

Average Depth

Few ponds are a simple box. Most have planting shelves around the edge, a sloping bottom and a deeper area in the middle. Using the deepest point would overstate the volume, sometimes by half. Estimate the average depth by measuring at several points across the pond and averaging them, or by picturing the pond as a shallow shelf around a deeper centre and weighting each by its area. For an existing pond, another accurate method is to read the water meter before and after filling or topping up.

Pump Sizing

A pond pump keeps water moving through the filter, adds oxygen and powers waterfalls and fountains. Fish ponds usually need the whole volume circulated at least once an hour, ponds with heavy fish loads or koi sometimes twice. Plant and wildlife ponds can manage with every two hours. Pump flow drops as the height it lifts water increases, so check the pump's curve and pick the flow at the head your waterfall or filter needs, not the maximum flow printed on the box. Friction in long hoses reduces flow too, as the friction loss calculator shows.

Liner Size

A flexible liner must cover the floor, run up every side and overlap the edge so it can be held down and hidden. The usual rule adds twice the maximum depth to both the length and the width, plus about a foot of overlap on each edge. Shelves do not change the calculation because the liner follows them. Liners are sold in set sizes, often in 5-foot steps, so round up. EPDM rubber is the most common liner; it is flexible, durable and fish-safe. An underlay beneath the liner protects it from stones and roots.

Water Treatments

Dechlorinators, beneficial bacteria, salt, algae treatments and fish medications all state their doses per gallon or per thousand gallons. Underestimating the volume risks overdosing, which can kill fish and plants; overestimating it makes the treatment ineffective. Write the calculated volume somewhere handy so the right figure is always used.

Evaporation and Top-Ups

Ponds lose water to evaporation and splashing, often an inch or more a week in hot weather. Top up with dechlorinated water, and if the level drops much faster than that, look for a leak at the edges, in the liner or in the waterfall.

Choosing a Filter

Pond filters are rated for a maximum pond volume, often with separate figures for ponds with and without fish. Choose a filter rated for at least the calculated gallons, and preferably for a larger pond if you keep koi or many goldfish, because fish waste is the main load a filter deals with. Ultraviolet clarifiers, which kill the algae that turn water green, are also sized by gallons and flow rate.

Irregular Shapes

Kidney, oval and free-form ponds are hard to measure exactly. A simple approach is to measure the longest length and the widest width, treat the pond as a rectangle, and multiply the area by about 0.8 for an oval or kidney shape. For complex shapes, split the pond into simpler parts and add them together.

Litres and Imperial Gallons

The calculator uses US gallons. For UK imperial gallons, divide by 1.2; the litre figure is shown as well.

Understanding Your Result

The headline gives the pond's volume in gallons.

The working line shows the area, depth, cubic feet and litres.

The pump line gives the pump flow for your turnover time.

The liner line gives the liner size.

When Should You Use This Calculator?

Use it when building a new pond, to order a liner and pump.

Use it to dose water treatments correctly.

Use it to check whether an existing pump is large enough.

Common Mistakes

Using the deepest point for volume. Use the average depth.

Choosing a pump by its maximum flow. Check the flow at your lift height.

Ordering a liner without overlap. Add the edge allowance.

Guessing the volume for medications. Dose by the calculated gallons.