Pool Pump Size Calculator

Pool pump size calculator: the turnover formula

A pool pump size calculator tells you how fast water must move to cycle your pool’s full volume in a chosen number of hours. Convert the hours to minutes, and then divide the gallons by the minutes.

Minimum GPM = pool gallons ÷ (turnover hours × 60).

The answer is a flow rate in gallons per minute. It is a minimum target, not the name of a pump to buy.

The picture behind the math

Think of the pool as a tank holding 20,000 gallons. If you want the pump to move all of that through the filter in 8 hours, you need to move 20,000 gallons in 480 minutes. 20,000 ÷ 480 = 41.67 gallons every minute.

Worked examples

Example 1, the default values. 20,000 gallons, 8 hours.

• Minutes: 8 × 60 = 480
• Flow: 20,000 ÷ 480 = 41.67 GPM

Example 2, a smaller pool. 15,000 gallons, 8 hours.

• Flow: 15,000 ÷ 480 = 31.25 GPM

Example 3, a big pool with a longer cycle. 30,000 gallons, 10 hours.

• Minutes: 600
• Flow: 30,000 ÷ 600 = 50.00 GPM

What turnover time does to flow

All rows below use a 20,000 gallon pool.

Turnover	Minutes	Minimum flow
4 hours	240	83.33 GPM
6 hours	360	55.56 GPM
8 hours	480	41.67 GPM
10 hours	600	33.33 GPM
12 hours	720	27.78 GPM

Cutting the turnover time in half doubles the flow you need.

From GPM to an actual pump

The minimum GPM is only the start. A pump produces its rated flow only when nothing pushes back. Real plumbing adds resistance from pipe length, elbows, the filter, the heater, and any lift to the pump. That resistance is called head, and pump makers publish curves that show flow at each head.

So the question to ask is: which pump delivers at least your GPM at the head of my system? A pool supply store can help you read the curve. If you are unsure of your pool’s volume, the pool gallons calculator finds it first.

Checking the rest of the equipment

• Filter. Every filter has a maximum flow rating. A sand filter that tops out below your pump’s flow will channel water and clean poorly.
• Pipe size. Narrow pipes make high flow noisy and wasteful, because the friction rises quickly with speed.
• Heater and chlorinator. These have their own flow limits.
• Skimmer count. Large flow through one skimmer can pull in air.

Running costs

A bigger pump is not always better. Pump power goes up quickly with speed, so a smaller or variable-speed pump running longer can use far less electricity than a large fixed-speed one. Look at the efficiency data on the pump label and the utility’s rate before you decide.

Common mistakes

• Using the pool’s nominal volume instead of the real one.
• Reading the pump’s maximum flow as the flow in your system.
• Forgetting about the filter’s rating.
• Sizing from a one-hour turnover idea. That calls for a huge pump.

Safety and wiring

Pool pumps run on electricity near water. Wiring, bonding, and ground-fault protection are jobs for a licensed electrician, and your local code decides what is required. Pool chemicals should be added according to the product label.

An example of reading the curve

Say your pool needs 41.67 GPM and your plumbing has some resistance. You look at a pump’s curve and see it moves 60 GPM at a light load and 45 GPM at your system’s head. That pump clears the 41.67 GPM target with some room to spare. A second pump that moves 55 GPM free but only 38 GPM at your head does not clear it, even though its label number looks larger. The curve at your head is the number that matters.

Some owners run the pump longer on a smaller unit and some run a bigger one for fewer hours. Both can work, so long as the daily total of water moved reaches the volume you want and the filter is not pushed past its limit.

Related calculators

The pool sand calculator sizes sand for the filter, the pool chlorine calculator covers sanitizer, and the pool heat pump sizing calculator helps with heating.