How to Calculate a Pond Filter Size

A water garden or fish pond can turn your backyard into an exotic getaway. One of the most important parts of maintaining a healthy pond is ensuring that the water stays clean. Pond filters are used to trap and filter out decaying organic matter, fish waste and anything else that winds up in a pond. Before you purchase a filter, however, you need to first calculate the size of your pond so you can ensure you get the right pond filter size.

Tape measure
Pencil
Paper

Instructions

- 1
  Determine the shape of your pond. You will need this for calculating the size of your pond. Common pond sizes are circular, rectangular and oval or irregular.
- 2
  Measure the depth of your pond, along with the width and length in feet. For irregularly shaped ponds, take several measurements of the width, length and depth and average them together. For example, if your pond measured 2 feet in width in one part, 3 feet in another and 4 at the end, you would add these together to get 9, then divide them by 3 to get an average width of 3 feet. Write these numbers down.
- 3
  Measure the distance across the surface of a circular pond and the depth. Multiply the pond distance by itself, then by the depth. For a circular pond that was 4 feet in diameter and 2 feet in depth, for example, that would be 4 x 4 x 2 for a total of 32.
- 4
  Multiply your measurements by a figure of 6.25 to get the size of the pond in gallons.
- 5
  Purchase a filter rated for at least one-and-a-half times the gallons in your pond. If your pond size is 100 gallons, buy a filter rated for at least 150 gallons per hour. This will ensure that you have a big enough filter to do the job.

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