Pool Pump Size Calculator (GPM)

How pool pump sizing works

Turnover time is how long it takes your pump and filter to circulate a volume of water equal to your entire pool once. The formula is straightforward: GPM needed = pool gallons ÷ (turnover hours × 60). A 20,000 gallon pool targeting an 8-hour turnover needs about 41.7 GPM of flow.

Six to ten hours is the commonly cited residential turnover target, with 8 hours as a reasonable middle default -- faster turnover (6 hours) keeps water cleaner and better circulated but needs a bigger pump and more energy; slower turnover (10 hours) saves on pump running cost but circulates and filters less often per day.

This GPM figure tells you the flow rate your pump needs to deliver against your plumbing and filter's actual resistance (head), which is a separate engineering question from raw GPM alone -- a pump's rated GPM on the box assumes ideal conditions, so pipe size, fitting count and filter type all affect real-world flow. Use this number as a starting target when comparing pump models and consult your pump or plumbing professional for final sizing.

Where these numbers come from

The formulas and default values in this calculator are based on:

• standard residential pool turnover target of 6-10 hours (8 hours commonly used as a default), used widely by pool builders and pump manufacturers
• GPM = gallons / (turnover hours x 60)

These are planning estimates. Test your actual water, follow product labels, and check your equipment manufacturer's guidance before you buy or dose chemicals.