Easy Pool Calc

What size pump for a 20,000 gallon pool?

Short answer

A 20,000 gallon pool on an 8-hour turnover needs a pump that delivers about 41.7 gallons per minute through your filter and plumbing. The slowest the DOE Building America guide accepts is 13.9 gallons per minute (one turnover a day), and the fastest recommended is 55.6 (a 6-hour turnover).

How to work it out

  1. 20,000 gal ÷ (8 h × 60 min) = 41.7 gpm
  2. Slowest: 20,000 gal ÷ 1,440 min = 13.9 gpm (53 L/min)
  3. Fastest: the larger of 20,000 gal ÷ 360 min and 36 gpm = 55.6 gpm (210 L/min)
  4. Filter: rated for at least the pump’s 41.7 gpm (158 L/min); DOE’s efficiency advice adds 50%: 62.5 gpm (237 L/min). Sand bed 41.7 gpm ÷ 20 gpm per sq ft = 2.08 sq ft

Twenty thousand gallons is a typical family inground pool: close to a 16 × 32 with a diving end. It is also right at the edge of what 1-1/2 inch plumbing comfortably carries, which matters more than the pump’s horsepower.

Adjust for your project

gal
Don’t know it? Work it out with the pool volume calculator or the above-ground pool calculator.
hours
A turnover every 24 hours is the minimum (DOE Building America guide). The energy code that APSP adopted says don’t go faster than a 6-hour turnover (or 36 gpm, whichever is greater). 8 hours is a middle-of-the-road starting point.
Printed on the pipe. Above-ground pools often use hoses; check the size on the pump and filter ports.
On the filter’s label or spec sheet (“design flow” or “max flow”). Blank to skip.
Pump flow you need41.7 gpm (158 L/min)
Water moved per hour
2,500 gal
Slowest allowed (one turnover in 24 hours)
13.9 gpm (53 L/min)
Design limit where adopted (6-hour turnover or 36 gpm)
55.6 gpm (210 L/min)
Pipe check
OK: 1-1/2 in pipe carries up to 45 gpm
Filter rated for at least (your pump’s flow)
41.7 gpm (158 L/min)
DOE efficiency advice (not a minimum): rated 50% above
62.5 gpm (237 L/min)
Sand filter area at 20 gpm per sq ft
2.08 sq ft
Smallest listed sand filters that meet it
Hayward S210T (44 gpm) or Pentair Triton II TR50 (49 gpm)
  • Horsepower alone tells you little. DOE says a pump of 0.75 hp or smaller is generally enough for a home pool; a variable-speed pump lets you run the flow above for longer at low speed. See what a pump costs to run.
Before you buyThis gives the flow you need, not a model. A pump’s real flow depends on the resistance of your pipes, filter and fittings; check the pump’s published curve at your system’s head, and have plumbing and electrical work done to code.

For this job · affiliate link
Variable-speed pool pumpA variable-speed pump can deliver 42 gpm when you need it and far less the rest of the day, which is where nearly all of the electricity saving comes from.
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20,000 gallon pool by turnover time

20,000 gallon pool by turnover time
TurnoverPump flow you needPipe checkFilter rated for at least (your pump’s flow)
6 hours55.6 gpm (210 L/min)Too much for 1-1/2 in pipe (max 45 gpm): use bigger pipe or a longer turnover55.6 gpm (210 L/min)
8 hours41.7 gpm (158 L/min)OK: 1-1/2 in pipe carries up to 45 gpm41.7 gpm (158 L/min)
10 hours33.3 gpm (126 L/min)OK: 1-1/2 in pipe carries up to 45 gpm33.3 gpm (126 L/min)
12 hours27.8 gpm (105 L/min)OK: 1-1/2 in pipe carries up to 45 gpm27.8 gpm (105 L/min)
24 hours13.9 gpm (53 L/min)OK: 1-1/2 in pipe carries up to 45 gpm13.9 gpm (53 L/min)

Sizing the pump for 20,000 gallons

Moving 20,000 gallons in 8 hours means 2,500 gallons an hour, or roughly 42 gpm. That flow sits just under the 45 gpm Hayward lists as the maximum for 1-1/2 inch pipe, so on older 1-1/2 inch plumbing an 8-hour turnover is about as fast as you should go. With 2 inch pipe the ceiling rises to 80 gpm and the pipe stops being the limit.

  • Slower is cheaper. On a 12-hour turnover the same pool needs only about 28 gpm. A variable-speed pump set low and left running longer meets that easily and, by the affinity laws, draws a fraction of the power.
  • The fast limit. A 6-hour turnover is about 56 gpm, the design limit in California’s energy rules and ANSI/APSP/ICC-15 for this volume (binding only where adopted). That would overrun 1-1/2 inch pipe anyway.
  • The filter. Any filter rated for about 42 gpm or more handles this flow. DOE’s efficiency advice — 50% more, not a minimum — points to roughly 63 gpm: a Pentair TR60 (63 gpm) or Hayward S270T (74 gpm) among the sand filters listed here. A Hayward S244T is rated at 62 gpm, a whisker under.
  • Horsepower. Pump labels quote horsepower, but DOE’s point is that 0.75 hp or less is generally enough for a residential pool. Read the pump curve at your head instead.

For a smaller pool, compare what size pump for a 15,000 gallon pool; above-ground owners should see what size pump and filter for a 24 ft above-ground pool. To dose this pool, the worked page for how much chlorine for a 20,000 gallon pool uses the same volume.

Different size? Use the full Pool Pump Size Calculator (GPM & Turnover).

What you’ll need

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Frequently asked questions

What gpm does a 20,000 gallon pool need on a 12-hour turnover?

About 27.8 gpm (105 L/min), or 1,667 gal an hour. That is comfortably inside what 1-1/2 inch pipe carries.

Can 2-inch plumbing handle a 6-hour turnover on 20,000 gallons?

Yes. A 6-hour turnover needs about 55.6 gpm (210 L/min), and the pipe check says: OK: 2 in pipe carries up to 80 gpm.

How big a sand filter for a 20,000 gallon pool?

At 8 hours, look for a filter rated for at least 41.7 gpm (158 L/min), such as Hayward S210T (44 gpm) or Pentair Triton II TR50 (49 gpm) — a sand bed of at least 2.08 sq ft at 20 gpm per sq ft. DOE’s efficiency advice would be 62.5 gpm (237 L/min).

Other common sizes

Sources