Sump Pump Size Calculator

Size a sump pump from how fast water rises in the pit, the vertical lift and the discharge run, giving inflow in gpm, total dynamic head and the pump horsepower you need.

By CalcBuilt Editorial TeamUpdated September 10, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: To size a sump pump, time how fast water rises in the pit during heavy rain, convert that to gallons per minute from the pit diameter, add a safety factor, then find the total head from the vertical lift, discharge run and fittings and pick a pump whose curve delivers that flow at that head. An 18 in pit rising 1 in per minute takes in 1.1 gpm, or 1.7 gpm with a 50% safety factor, against 14 ft of head, so a 1/3 HP pump has plenty of margin.

Your measurements

A column or tube: volume is the circular area (from the diameter) times the height.

Pit diameterinStandard basins are 18 in across; larger pits are 24 in. IRC P3303.1.1 sets a minimum of 18 in across and 24 in deep with a fitted lid, and a 24 in basin cuts short-cycling in a fast pit.Water rise per minute during heavy raininDuring a heavy rain, unplug the pump, time how many inches the water rises in one minute, then plug it back in.Vertical lift, pit bottom to dischargeftFrom the bottom of the pit to the highest point of the discharge pipe.Horizontal discharge runftPipe length from the top of the lift to the outlet outside. The check valve and two elbows are in the fittings allowance; add 4 ft of run for each extra 90.Discharge pipe sizeMost pumps have a 1-1/2 in NPT outlet; a 1-1/4 in outlet gets a bushing to 1-1/2 in pipe right at the pump. Schedule 40 PVC, glued, with a union above the check valve.Safety factor%50% covers a storm heavier than the one you measured.

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Inflow to pump1.7 gpmAll results

Results

Inflow to pump: 1.7 gpm; 1.1 gpm measured, plus a 50% safety factor
Total dynamic head: 14 ft; 10 ft of lift + 2 ft of friction for the 20 ft run + 2 ft for the check valve and fittings
Recommended pump: 1/3 HP; Delivers about 27 gpm at 14 ft of head on a typical curve
Pit capacity between float on and off: 6.6 gal; 6 in of float travel; the pump starts about every 4 minutes at this inflow
Battery backup: 1.7 gpm; Size a battery or water-powered backup pump for the same inflow and head; heavy rain and power outages arrive together
Planning estimate only: Check the pump curve; Match the manufacturer's flow at your head; a licensed plumber and local code govern where the discharge may go

Sump Pump Size formula

Inflow (gpm) = 3.1416 x (Pit diameter / 2)² x Rise per minute (in) / 231 x (1 + safety factor)
Total dynamic head (ft) = Vertical lift + Horizontal run x friction (0.1 for 1.5 in pipe) + 2 for the check valve and fittings
Pump = smallest pump whose curve delivers the inflow at that head (1/3 HP about 35 gpm at 10 ft, 1/2 HP about 50, 3/4 HP about 65, 1 HP about 80)
Pit capacity (gal) = 3.1416 x (Pit diameter / 2)² x 6 in of float travel / 231

Worked example

A 24 in pit in a high water table rises 8 in per minute in a storm, with a 12 ft lift and a 40 ft run of 1.5 in pipe. Inflow = 3.1416 x 12² x 8 / 231 = 15.7 gpm measured, times 1.5 for the safety factor = 23.5 gpm. Head = 12 + 40 x 0.1 + 2 = 18 ft.

At 18 ft a 1/3 HP pump delivers about 35 - 8 x 2 = 19 gpm, which is short, so step up to a 1/2 HP pump at about 50 - 8 x 2 = 34 gpm. The pit holds 3.1416 x 12² x 6 / 231 = 11.8 gal over 6 in of float travel, so the pump starts about every 0.5 minutes in that storm, which argues for a larger pit or a second pump.

Quick reference

Inflow by pit diameter and rise per minute (measured, and with a 50% safety factor)
Pit diameter	Rise per minute	Measured inflow	With 50% safety
18 in	1 in	1.1 gpm	1.7 gpm
18 in	2 in	2.2 gpm	3.3 gpm
18 in	4 in	4.4 gpm	6.6 gpm
18 in	8 in	8.8 gpm	13.2 gpm
24 in	1 in	2 gpm	2.9 gpm
24 in	2 in	3.9 gpm	5.9 gpm
24 in	4 in	7.8 gpm	11.8 gpm
24 in	8 in	15.7 gpm	23.5 gpm

Measuring inflow and head

The only inflow number worth having is one you measured in the pit during a hard rain. Unplug the pump, mark the water level, wait one minute, measure the rise, and plug the pump back in. The pit diameter turns inches of rise into gallons: an 18 in basin holds 1.1 gal per inch. Multiply by a safety factor because the next storm will be worse than the one you measured.

Head is what the pump works against. Measure the vertical distance from the bottom of the pit to the highest point in the discharge line, then add an allowance for friction in the horizontal run and for the check valve and elbows. In 1.5 in pipe each 90 adds about 4 ft of equivalent run and a swing check about 5 ft, and dropping to 1.25 in doubles the friction. A discharge that climbs to a basement window and then runs 40 ft to the curb puts close to 20 ft of head on a pump whose box says 43 gpm at zero.

Choosing the pump and a backup

Read the flow at your head off the manufacturer's curve, not the headline gpm on the box, which is measured at zero head. A 1/3 HP pump handles most homes with a normal water table and a 10 ft lift; step up to 1/2 HP for a high water table, a lift above about 12 ft or a long discharge run, and to 3/4 HP or 1 HP only when the measured inflow demands it. Oversizing is not free: a pump that empties the pit in seconds short-cycles, which wears out the switch and motor.

Heavy rain and power outages arrive together. A 12 V battery backup on a 75 to 100 Ah AGM battery moves 1,500 to 2,500 gph at 10 ft and runs 4 to 7 hours of continuous pumping, days of intermittent. A water-powered backup needs city water at 40 psi or more and a backflow preventer, and uses about a gallon of water for every two pumped, so it is out on a well. Set the check valve on a union 12 to 18 in above the pump and drill the 3/16 in vent hole below it. Run the discharge 10 ft from the foundation to a pop-up emitter or drywell, never into the sanitary sewer, and add a freeze-relief fitting at the wall where the line is buried. The basin gets a fitted lid (IRC P3303.1.1), sealed with grommets where there is a radon system. The pump gets a dedicated GFCI receptacle within cord reach and a high-water alarm.

What this calculator leaves out

The basin and lid, check valve and union, 1.5 in PVC and fittings, and the discharge to daylight with a pop-up emitter or a drywell. Freeze relief, the dedicated circuit, the alarm and backup pump, and the perimeter drain tile that feeds the pit. It does not size a sewage ejector, which handles solids and needs a sealed, vented basin (IRC P3007). It does not fix the water coming in either: grading, downspout extensions and exterior drain tile cut the inflow the pump has to handle.

Code limits that apply

IRC P3303.1.1checked
Sump pit at least 18 in across and 24 in deep with a fitted cover
If you miss it: A bucket-sized pit fills in seconds, the float cycles every few seconds and the switch burns out in its first wet season
IRC P3301
Storm water and groundwater not discharged into a sewer intended for sewage only
If you miss it: Sump water surcharges the sanitary sewer in a storm, backs up into basements down the street and the utility fines the owner
IRC P3007
Fixtures below the sewer drain to a sealed, vented sewage ejector basin, not a sump pump
If you miss it: A sump pump on a basement bath clogs on solids and the open basin vents sewer gas into the house
NEC 210.8(A)
GFCI protection on the basement receptacle that serves the pump
If you miss it: A pump plugged into an unprotected outlet beside a wet pit is a shock hazard the electrical inspector flags at final
IRC R401.3
Grade falls at least 6 in in the first 10 ft from the foundation; the discharge lands past that
If you miss it: Discharge dumped at the wall runs straight back to the drain tile and the pump recycles the same water all night

Provision	Limit	If you miss it
IRC P3303.1.1checked	Sump pit at least 18 in across and 24 in deep with a fitted cover	A bucket-sized pit fills in seconds, the float cycles every few seconds and the switch burns out in its first wet season
IRC P3301	Storm water and groundwater not discharged into a sewer intended for sewage only	Sump water surcharges the sanitary sewer in a storm, backs up into basements down the street and the utility fines the owner
IRC P3007	Fixtures below the sewer drain to a sealed, vented sewage ejector basin, not a sump pump	A sump pump on a basement bath clogs on solids and the open basin vents sewer gas into the house
NEC 210.8(A)	GFCI protection on the basement receptacle that serves the pump	A pump plugged into an unprotected outlet beside a wet pit is a shock hazard the electrical inspector flags at final
IRC R401.3	Grade falls at least 6 in in the first 10 ft from the foundation; the discharge lands past that	Discharge dumped at the wall runs straight back to the drain tile and the pump recycles the same water all night

Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.

Mistakes that cost money

Check valve glued in with no union: the pump cannot be pulled without a saw; use a rubber-coupled check 12 in above the pump.
No 3/16 in vent hole in the discharge below the check valve: the pump air-locks and runs dry; the Zoeller manual calls for it.
Discharge dumped at the foundation or tied to the sewer: it comes back into the pit; sewer tie-ins draw fines; run it 10 ft out.
Buried discharge with no freeze relief: the line freezes in January, the pump deadheads and burns up; add an ice guard fitting at the wall.
An open pit: radon, humidity and debris; IRC P3303.1.1 wants a fitted lid, and a sealed lid with grommets is part of any radon system.
Pump on an extension cord or shared circuit: a tripped breaker in a storm floods the basement; give it a dedicated GFCI receptacle and alarm.

Key facts

A gallon is 231 cubic inches, so an 18 in pit holds about 1.1 gal per inch of depth and a 24 in pit about 2 gal per inch.
A typical 1/3 HP sump pump moves about 35 gpm at 10 ft of head and a 1/2 HP about 50 gpm; 1/3 HP covers most homes, and 1/2 HP is for high water tables or lifts over 10 to 15 ft.
Every foot of vertical lift is a foot of head, and a check valve plus elbows adds the equivalent of a couple more feet; pumps lose flow steadily as head rises.
Sump pumps use 1.5 in discharge pipe; reducing to 1.25 in roughly doubles the friction loss and cuts the flow.
Discharge water should exit at least 10 ft from the foundation, and most codes prohibit connecting a sump pump to the sanitary sewer.

Frequently asked questions

What size sump pump do I need?

Measure the inflow in a storm and the total head. An 18 in pit rising 1 in per minute takes in about 1.1 gpm, or 1.7 gpm with a 50% safety factor, against 14 ft of head with a 10 ft lift, so a 1/3 HP pump has plenty of margin. Rises of several inches per minute or lifts over 12 ft call for 1/2 HP or more.

Is a 1/2 HP sump pump better than 1/3 HP?

Only when the inflow or head needs it. A 1/3 HP pump moves about 35 gpm at 10 ft of head and a 1/2 HP about 50 gpm. In a pit that fills slowly the bigger pump just short-cycles, which wears it out faster.

How do I calculate sump pump gpm?

Multiply the pit's cross-section in square inches by the rise in inches per minute and divide by 231 (cubic inches per gallon). An 18 in pit is 254 sq in, so a 1 in rise per minute is 254 / 231 = 1.1 gpm; add a safety factor of 50% for 1.7 gpm.

Do I need a battery backup sump pump?

If the basement is finished or the pit runs in every storm, yes. Outages and heavy rain come together, and a primary pump also fails on a stuck float or a worn switch. A 12 V backup on a 75 to 100 Ah AGM battery moves 1,500 to 2,500 gph at 10 ft for 4 to 7 hours of continuous pumping. Figure $400 to $900 installed with the battery, and a new battery every 3 to 5 years. On city water a water-powered backup runs as long as the main holds pressure; on a well it is not an option.

Sources and references

Model 53 Sump Pump product page (3/10 HP, 43 gpm max, 19.25 ft max head) (Zoeller Pump Company)
Technical Data Sheet FM2778, Mighty-Mate Models 53, 55, 57 and 59, with pump performance curve (Zoeller Pump Company)
CDU980E 3/4 HP and CDU1000 1 HP stainless steel sump pump specification sheet (gpm at 5, 10, 15 and 20 ft of head) (WAYNE Pumps)
Chapter 33 Storm Drainage, Section P3303 Sumps and Pumping Systems, 2021 IRC (ICC)

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