Sump Pump Size Calculator

What size sump pump do I need? Get GPM, horsepower and pit size in 30 seconds.

Project Details
Enter your measurements and requirements
Results
Calculated estimates for your project
Inflow at peak6.6 GPM (397 GPH)
Pump must move (with 25% margin)8.3 GPM (496 GPH)
Total dynamic head10.4 ft (10 ft lift + 0.4 ft friction)
Pump size1/4 HP, about 24.3 GPM (1,457 GPH) at that head
Basin24 x 24 in (1.96 gal per inch)
Cycles per hour at peak18
Discharge velocity4.4 ft/s
Generator for this pump600 W running, 1,200 W starting
Battery backup runtime at peakabout 22 hours on 75 Ah

Turn these numbers into a build plan.

  • 9 GPM flow needed
  • 1/4 HP pump
  • 24 x 24 in basin
Materials List
Recommended materials and quantities
1/4 HP submersible sump pump1
1-1/2 in PVC discharge pipe4 x 10 ft
Check valve1
Sump basin24 x 24 in
Battery backup pump and alarm1 (recommended)
Typical gallons per hour by horsepower and total head; your pick is highlighted
Pump5 ft10 ft15 ft20 ft25 ft
1/4 HP2,1001,500900300-
1/3 HP2,6002,2001,600900200
1/2 HP3,3002,9002,4001,7001,000
3/4 HP4,4003,9003,3002,6001,800
1 HP5,2004,7004,1003,4002,600

How we got this: 1.1 gal per inch of a 18 in pit x 6 in / 1 min = 6.6 GPM, x 1.25 margin. Head is the lift plus Hazen-Williams friction over the pipe, elbows and check valve.

Track this pump: test every 3 months

Most sump pumps fail without warning in the first big storm after a dry spell. A bucket test every quarter catches a stuck float or a dead battery first.

The calendar file repeats every 3 months. In Blueprint the pump becomes a tracked item with quarterly tests, a storm-season check before spring and a replacement alert at year 7 to 10.

Cut the inflow first: send roof water away with the gutter calculator, and put the pump on its own circuit with the electrical load calculator.

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Project plan · built from your numbers

Install a 1/4 HP sump pump with 10 ft of lift

Water comes in at about 6.6 GPM at peak, so plan for a 1/4 HP pump moving at least 9 GPM at 10.4 ft of total head (10 ft lift plus pipe friction), in a 24 x 24 in basin.

Materials
$292
Time
1 afternoon
Level
Easy
8 steps 8 tools 8 materials
  1. 1

    Check the pit and circuit

    Measure the existing pit. It should be at least 24" across and deep enough for the pump and float. Confirm a GFCI-protected outlet sits within cord reach on its own circuit. If you need a new outlet or circuit, hire a licensed electrician.

    • Pit at least 24" across
    • Dedicated GFCI outlet tested
    • Electrician booked if wiring is needed
  2. 2

    Pick the pump from its curve

    Look at the pump curve, not just the horsepower. It should deliver at least 9 GPM at 10.4 ft of total head, not just 10 ft of lift. Choose a cast-iron body with a vertical or mechanical float for long life.

    • 9 GPM at 10.4 ft confirmed
    • Float type fits the pit
    • Backup system chosen
  3. 3

    Remove the old pump

    Unplug the pump, vacuum the pit dry, and cut the discharge pipe above the check valve. Lift the old pump out with its cord and float.

    • Pump unplugged first
    • Pit vacuumed out
    • Old pump and pipe removed
01 · Your numbers
Flow needed
9 GPM
Pump
1/4 HP
Basin
24 x 24 in
Est. cost
$292
02 · The plan

8 steps · 8 tools · 8 materials

5 more steps, the full tool list and a shopping list you can check off as you go.

03 · Finished

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Install a 1/4 HP sump pump with 10 ft of lift

Finished in 1 afternoon · $292 DIY

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What size sump pump do I need?

Most basements need a 1/3 HP pump (about 35 to 45 GPM at 10 ft). Use 1/2 HP for high water tables or lifts over 15 ft. Size by flow at your total head, not horsepower: the chart below shows how fast each size falls off as the lift rises.

Sump pump size chart (GPH by horsepower and head)

Typical submersible pump curves, gallons per hour
Pump5 ft10 ft15 ft20 ft25 ft
1/4 HP2,1001,500900300-
1/3 HP2,6002,2001,600900200
1/2 HP3,3002,9002,4001,7001,000
3/4 HP4,4003,9003,3002,6001,800
1 HP5,2004,7004,1003,4002,600

How to calculate sump pump GPM (flow rate)

  1. Measure the pit. Measure the inside diameter of the sump pit. Each inch of water holds pi x (diameter / 2)^2 / 231 gallons.
  2. Time the rise. In a heavy rain, unplug the pump and time how long the water takes to rise a few inches, then plug it back in.
  3. Compute the flow. GPM = gallons per inch x inches of rise / minutes. Add 25% for margin.
  4. Add up the head. Total head = vertical lift + pipe friction over the run, elbows and check valve.
  5. Pick the horsepower. Choose the smallest pump whose curve delivers your GPM at that head.

Example 1: a 24 in pit holds 1.96 gal per inch. If it rises 6 in in 45 seconds, inflow is 11.8 gal / 0.75 min = 15.7 GPM; with 25% margin the pump needs 19.6 GPM at 11.8 ft of head, a 1/4 HP pump.

Example 2: a 1,200 sq ft basement, a high water table and 2 in/hr of rain: 1,200 x 2 / 12 x 1.0 x 7.48 / 60 = 24.9 GPM, so 31.2 GPM with margin, a 1/2 HP pump at 14.2 ft.

Sump pit and basin sizes

BasinGallons per inchTotal gallons
18 x 22 in1.124
18 x 24 in1.126
18 x 30 in1.133
24 x 24 in1.9647
24 x 36 in1.9671

Total head explained

Total dynamic head is the vertical lift from the pit bottom to the highest point of the discharge, plus friction: every foot of pipe, each elbow (about 4 ft of 1-1/2 in pipe) and the check valve (about 10 ft). A pump rated 40 GPM at 10 ft may move only 25 GPM through a long, narrow run. Keep the outside run sloped 1 to 2% away from the house so it drains between cycles, and size the pipe with the plumbing cheat sheet and pipe sizing chart.

What size generator or battery backup for a sump pump?

PumpRunning wattsStarting watts
1/4 HP6001,200
1/3 HP8001,300
1/2 HP1,0502,150
3/4 HP1,5003,000
1 HP1,9004,000

A 75 Ah battery runs a typical 12 V backup pump about 6 hours of continuous pumping, much longer at normal duty. Put the primary pump on its own circuit (check capacity with the electrical load calculator), keep roof water away with the gutter calculator, and fix the source with the basement waterproofing cheat sheet. Test it as part of the home maintenance inspection; on a septic system, size the tank with the septic tank calculator.

Sump pump sizing FAQ

What size sump pump do I need?

Most basements need a 1/3 HP pump, about 35 to 45 GPM at 10 ft of head. Use 1/2 HP for a high water table, a big basement or lifts over 15 ft, and always choose from the pump curve at your head, not the horsepower alone.

How many GPM does a sump pump move?

A 1/4 HP pump moves about 25 GPM at 10 ft, 1/3 HP about 37, 1/2 HP about 48 and 3/4 HP about 65. Flow drops as the lift gets higher.

1/3 vs 1/2 HP sump pump?

A 1/3 HP pump handles most homes with lifts up to about 15 ft. Choose 1/2 HP if the pit fills in under a minute in a storm, the lift is over 15 ft, or the run to the outlet is long.

How big should a sump pit be?

18 in across and 24 in deep at minimum; 24 in pits hold about 2 gal per inch and cut pump cycling. The pit should hold a few minutes of peak inflow between cycles.

How deep should a sump pit be?

22 to 36 in deep, deep enough that the pump sits below the floor drain tile and the float has room to travel. Set the pump on a brick to keep it out of silt.

Pedestal vs submersible sump pump?

Submersible pumps sit in the water, run quieter and handle more flow; they last 7 to 10 years. Pedestal pumps keep the motor above the pit, are easier to service and fit narrow pits, but are louder.

Do I need a backup sump pump?

Yes, if the basement is finished or holds anything you care about. Storms knock out power exactly when the pump is needed; a battery backup or water-powered pump keeps going.

What size generator for a sump pump?

About 800 W running and 1,300 W to start a 1/3 HP pump, and 1,050 W running and 2,150 W starting for 1/2 HP. A 3,000 W generator covers either plus a few lights.

How long do sump pumps last?

7 to 10 years on average; cast iron outlasts plastic. Test every 3 months and replace before it fails, not during a storm.

Why does my sump pump run constantly?

A high water table, an undersized pump, a stuck float or a failed check valve. Check the float moves freely and that the discharge line is not blocked or frozen.

Where should sump pump water discharge?

At least 10 to 20 ft from the foundation onto ground that slopes away, never into the sanitary sewer. Some towns require a storm drain or dry well.

Is a sewage ejector sized the same way?

No. A sewage ejector is sized by the fixtures it serves (drainage fixture units) and must pass 2 in solids into a sealed, vented basin; it is a plumbing permit job. Septic homes also need the tank sized: see the septic tank calculator.

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