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Pumps

How to Size a Replacement Pool Pump

Work out the flow you need, check it against your pipes, then match it to a real pump's flow range and curve. No horsepower-for-gallons charts.

As an Amazon Associate I earn from qualifying purchases. The "Search Amazon" buttons and the "Worth a look" links on this page are Amazon affiliate links; we show no prices or ratings. Facts recorded Oct 25, 2026 from the official sources linked on each section. Manufacturer pages and rules change, so check the current version and your model's manual.

Difficulty: easy math, plus reading a manual

Safety first

  • Power off first. Pentair says to turn off the breaker for the pool pump before maintenance and to confirm the system isn't running before you open a pump. Keep the breaker off while you work (lock or tag it if you can so nobody switches it back on). Pentair: 7 Servicing Tips
  • GFCI. Pentair's IntelliFlo3 guide says to connect the pump only to a GFCI-protected branch circuit and to contact a qualified electrician if you can't verify that the circuit is GFCI-protected. Pentair IntelliFlo3 VSF guide
  • Wiring, bonding and grounding are electrician work. We don't give instructions for them.
  • Don't let a pump run dry. Pentair's IntelliFlo3 guide says: "Do not allow your pump to run longer than 30 minutes without successfully priming." Pentair IntelliFlo3 VSF guide

The question

"What size and flow do I need, and will the new pump fit my pipes and filter?" Start from flow (gallons per minute, GPM), not horsepower.

Step 1: the flow you need (arithmetic)

Required GPM = pool gallons ÷ turnover hours ÷ 60

That's plain math. You need your pool's volume in gallons (from your builder's paperwork or your own measurement) and a turnover time: how many hours it should take to move the whole pool's volume through the filter once.

Retailer statement, not a manufacturer rule Leslie's pump-sizing guide says an 8 to 10 hour turnover is typical, and that flow drops with distance and head (resistance in the plumbing). Turnover requirements can be set by local code, so check your local code before you pick a number.

Source: Leslie's: Pool Pump Sizing 101 (retailer guide)

Worksheet. The 20,000-gallon / 8-hour row is an arithmetic example only, not a recommendation.
Item Example Your pool
Pool volume (gallons) 20,000  
Turnover target (hours), check local code 8  
Gallons ÷ hours = gallons per hour 2,500  
÷ 60 = required GPM about 41.7  

Step 2: check what your pipes can carry

Hayward's TriStar VS Technical Guide (a 2014 document) lists a "maximum recommended system flow rate by pipe size." For existing pools, it says "care should be taken to insure the maximum flow does not exceed the capacity of the pipe and equipment."

From Hayward's 2014 TriStar VS Technical Guide, p. 3. fps = feet per second of water velocity in the pipe.
Pipe size Max flow @ 6 fps Max flow @ 8 fps
1.5" 38 GPM 51 GPM
2" 63 GPM 84 GPM
2.5" 90 GPM 119 GPM
3" 138 GPM 184 GPM

The same page says the straight pipe between the pump's suction port and the first elbow "should be at least 5 times pipe size."

Source: Hayward TriStar VS Technical Guide (2014), p. 3

Your filter, heater and salt cell have their own flow limits too. Look them up in each model's manual. (For example, Pentair's IntelliChlor Plus/LT page lists a minimum and maximum flow for that cell; see How a salt chlorine generator works.)

Step 3: match a real pump's flow range and curve

Variable-speed pumps are set by speed or flow. As one real example, Pentair's IntelliFlo3 VSF / IntelliPro3 VSF guide says flow programs can be set from 20 to 90 GPM on 1.5 HP models or 20 to 140 GPM on 3 HP models, and the default Program 1 is 24 hours at 30 GPM.

Source: Pentair IntelliFlo3 VSF / IntelliPro3 VSF Installation and Maintenance Guide

Before you decide a pump "fits," open that exact model's manual and check its pump curve against your plumbing. We don't publish head numbers without a pump curve.

Step 4: how to run it

Pentair says variable-speed pumps "don't need to run at full speed or a high flow rate all the time": low flow for long periods circulates chemicals, and short periods of higher flow give the surface movement that helps skimming.

Source: Pentair: 7 Servicing Tips For Variable Speed Pumps

What we left out on purpose

  • No "horsepower for gallons" chart: horsepower alone doesn't tell you flow in your plumbing.
  • No residential turnover or minimum-flow code numbers: we haven't checked them against the code text.
  • No European sizing formulas: this site is for U.S. pools and U.S. units.

Should you DIY? / When to call a pro

Fine to DIY: measuring, the GPM math, reading rating plates and manuals, comparing pump curves.

Call a licensed electrician: wiring the pump, GFCI, bonding. Call a pool professional: re-plumbing, suction outlets and drain covers, or if your numbers don't line up with your pipe sizes.

See our full When to Call a Pro page. Follow your model's manual; this site isn't a substitute for it, and it isn't legal advice.

If your model isn't listed

The numbers on this page come from specific manufacturer documents and models. If your equipment isn't one of them, don't assume the same numbers apply: find the model number on the rating plate, open that model's manual on the manufacturer's site, or contact the manufacturer. You can also email us with the brand and model number.

Related guides: Pool pump won't prime or is loud · Variable-speed pump rules

Facts from the manufacturer (and where else they came from)

Where to look

Pump links are in the “Worth a look” box above; other retailers come after program approvals. These buttons cover small plumbing items. As an Amazon Associate I earn from qualifying purchases.