Sump, Sewage & Dewatering Pump Sizing Calculator

Engineering-Led Sizing · Watermain Supply · Houston, TX

Sump, Sewage & Dewatering Pump Sizing Calculator

Work out the flow and total dynamic head your pump has to meet, from a basement sump pit to a municipal lift station. You get the duty point, the code checks that apply, the pump class that typically covers it, and where to look next.

9Jobs covered
2 ft/sScouring check built in
3/4–24 inPipe and hose sizes
1 pagePrintable sizing sheet

The short version

  1. Pick the job. Sump, sewage ejector, septic effluent, grinder or utility for homes and light commercial; dewatering or a lift station for bigger work.
  2. Enter how much water. Time the rise in your pit, count bedrooms or orifices, or enter the flow you already know.
  3. Enter the piping. How high the water goes, how far it travels, the pipe size and the fittings.
  4. Read the duty point, then the curve. Any pump that shows at least your TDH at your GPM on its published curve will do the job.

Size your pump

Every result is a duty point: the flow the pump must move against the total head of your piping. Figures marked as estimates are estimates; pump figures come from published curves.

1. The job

Pick the scale first, then the job.

2. How much water comes in

Best: let the pit fill with the pump unplugged and time it. An 18 in basin holds about 1.1 gallons per inch.

inches
minutes, during the wettest weather you can
sq ft
GPM
1.5 to 2 is usual
inches between pump-on and pump-off

2. The sewage basin

Code sets the minimum flow: 2 ft/s in the discharge pipe. Enter a higher flow only if the building needs it.

GPM; leave blank for homes

2. The septic system

Design flow is bedrooms times gallons per day; your state or county sets the figure, 150 is common.

gallons per day
sets the pump rate

2. The pressure distribution zone

Flow per orifice = 11.79 × diameter² × √head. Your designer's drawings list the orifice count, size and distal head.

ft of squirt height at the far end, often 2 to 5

2. The grinder station

The main has to run at 2 ft/s to stay clean, so pipe size sets the minimum flow.

GPM; leave blank for one home

2. The transfer job

GPM

2. How much water

Enter a target flow, or the size of the area to drain and the time you have. Groundwater inflow into an excavation is a geotechnical calculation: enter it if your engineer has it.

GPM
ft
ft
ft
hours
GPM

3. The suction side

A self-priming pump sits above the water and lifts it. Lift, hose losses, elevation and warm water all eat into the head available at the suction.

ft, lowest water level to pump centerline
ft
ft above sea level
°F
in (optional)

3. What you are pumping

2. Flow and wet well

Peak flow comes from average daily flow times the Harmon peaking factor, unless you enter a peak flow directly.

gallons per day
for the peaking factor
gallons per day
GPM per pump
ft, pump-off to lead-pump-on
ft
minutes; 10 means 6 starts an hour

Discharge piping

Count the full run to where the water leaves the pipe. Elbows, valves and the outlet are added as fitting losses.

ft
ft

Power

Shown as a check, never a reason to rule a pump out

Print your sizing sheet

One letter page on our letterhead, with what you entered, the duty point and the chart. Use "Save as PDF" to keep a file.

Send us this sizing

We check the duty against published curves and reply with pumps that fit, usually the next business day.

Sizing receivedThanks. We will reply by email, usually the next business day.

How the duty point is worked out

A pump has to do two things at once: move enough water, and push it against the total head of your piping. The calculator works out both.

Flow: how much water

Each job has its own rule. A sump pump has to beat the inflow with a safety margin. A sewage ejector has to keep the discharge pipe scoured at 2 ft/s. An effluent pump doses the drain field, or feeds every orifice in a pressure zone. A lift station has to carry the peak hour, not the average.

Head: how hard it pushes

Total dynamic head (TDH) is the vertical lift, plus friction in the pipe, plus losses in the fittings and outlet, plus any pressure needed at the far end. Friction uses the Hazen-Williams formula; fittings use standard loss factors.

Worked example: sump pitAn 18 in basin rises 4 in in 1 minute with the pump off. Inflow = 1.10 gal per inch × 4 in ÷ 1 min = 4.4 GPM. With a 1.5 safety factor the pump must move 6.6 GPM. Lifting 10 ft through 25 ft of 1-1/2 in PVC with two elbows and a check valve adds only about 0.2 ft, so the duty is 6.6 GPM at about 10 ft.
Worked example: pressure-dosed septic zone20 orifices of 3/16 in at 5 ft of distal head. Each passes 11.79 × 0.1875² × √5 = 0.93 GPM, so the zone needs 20 × 0.93 × 1.10 = 20.4 GPM. TDH = 15 ft lift + 3.9 ft friction in 150 ft of 1-1/2 in pipe + 0.8 ft fittings + 5 ft distal head = 24.7 ft.
Rule: read the curve at your flowA pump's shut-off head is the most it can push with nothing flowing. What matters is the head it still makes at your GPM. On any published curve, find your flow and check the head is at least your TDH, with some to spare.
Common mistake: buying by horsepowerTwo 1/2 HP pumps can differ by 50 ft of head. One may move 80 GPM at 10 ft and nothing at 26 ft; another moves 40 GPM at 30 ft. Horsepower tells you the power draw, not whether the pump fits your duty.

Code minimums the calculator checks

A summary of the rules most inspectors look for. Your local code and inspector govern; some jurisdictions add to these.

Rule Applies to What the calculator does
Pass 2 in spherical solids Sewage pumps with a toilet draining to the basin (IPC) Only suggests sewage classes that pass 2 in solids
2 in minimum discharge Sewage pumps with a toilet (IPC) Stops the result if the pipe is smaller
2 ft/s in the discharge Sewage ejectors (IPC: about 21 GPM in 2 in, 30 in 2-1/2 in, 46 in 3 in) Sets the minimum flow from your pipe size
2 ft/s minimum, 2 to 8 ft/s window Grinder mains and lift station force mains (Ten States Standards minimum) Flags low and high velocity
Sealed, vented basin and high-water alarm Sewage basins Reminds you on every sewage result
Effluent filter ahead of the pump tank Most septic codes Reminds you on every effluent result
Control panel with alarm Grinder pumps Flags it on every grinder result

IPC: International Plumbing Code. Ten States Standards: Recommended Standards for Wastewater Facilities.

Dewatering and lift stations

The commercial tier adds the checks that decide large jobs. It returns a duty, a pump class and an estimated power, then hands you to an engineered quote.

Suction lift and NPSH

A self-priming pump lifts water by vacuum, and the atmosphere does the pushing. Elevation, warm water and suction hose losses all reduce the net positive suction head available (NPSHa). Past about 25 ft of lift at sea level, a submersible is the reliable answer.

Drain-down flow

Volume in gallons is length × width × depth × 7.48. Divide by the minutes you have, then add any water still flowing in. A 200 × 100 × 6 ft pond holds about 898,000 gallons: 624 GPM to empty it in 24 hours.

Peak flow at a lift station

Stations are sized for the peak hour. The calculator multiplies average daily flow by the Harmon peaking factor, 1 + 14 ÷ (4 + √P) with P in thousands of people. 200 homes at 250 gpd gives 50,000 gpd and a factor of 3.97: about 138 GPM peak.

Wet well and cycling

Working volume = minimum cycle time × flow ÷ 4. At 138 GPM and a 10-minute cycle that is 345 gallons, about 1.6 ft of depth in a 6 ft wet well. Too little volume means too many starts and short motor life.

A band, not a line

Static head changes as the wet well draws down, and friction grows as pipe ages. The calculator sizes at high static and aged pipe, and shows the low end so the pump is not pushed off its curve when the well is full.

Power estimate

Water horsepower = GPM × ft ÷ 3,960. Pumps run at roughly 50 to 70 % efficiency, so the calculator shows that range and a likely motor size. The exact figure comes from the selected pump's curve.

Where the calculator stopsGroundwater inflow into an excavation depends on soil permeability, drawdown and wellpoint spacing. That is a geotechnical calculation. Enter the flow your engineer gives you, and we will size the pumps to it.

Frequently asked questions

What size sump pump do I need?

Time how fast the pit fills, multiply by 1.5, and find a pump whose curve shows that flow at your lift. Most homes land between 1/4 and 1/2 HP. Run the calculator.

What is the difference between sump, effluent and sewage pumps?

A sump pump moves groundwater with small solids. An effluent pump moves settled septic water with up to about 3/4 in solids. A sewage pump passes 2 in solids, which the code requires when a toilet drains to the basin.

When do I need a grinder pump?

When sewage has to travel far or uphill through a small main, as in low-pressure sewer systems. A grinder cuts the solids so they flow through 1-1/4 in or 2 in pipe at higher head than a standard ejector reaches.

Why does the calculator ask for pipe size on a sewage pump?

Code requires 2 ft/s in the discharge so solids keep moving. A bigger pipe needs more flow to reach that speed: about 21 GPM in 2 in, 46 GPM in 3 in.

Is a bigger pump always safer?

No. An oversized sump pump empties the pit in seconds and starts too often, which shortens its life. The calculator shows the busiest-hour start count so you can widen the float span or use a bigger basin.

Will a battery back-up keep up?

Often not in a heavy storm. The calculator compares a typical 12 V back-up pump at your head with your inflow and tells you which way it falls.

How high can a self-priming pump lift water?

Practically about 25 ft at sea level, less at elevation or with warm water. Beyond that, set the pump lower or use a submersible. See dewatering and lift stations.

Does the calculator pick a model?

No. It gives the duty and the pump class that typically covers it, so you can compare any brand's published curve. Send us the result and we will match it to specific pumps.

Other calculators

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Send us the duty

Print the sheet or send the result. We check it against published curves and come back with pumps that fit.

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Watermain Supply · Houston, Texas · 1-888-510-4036 · sales@watermainsupply.com
Watermain Supply is a DBA of E4 Industrial LLC, an independent industrial supplier based in Houston, Texas. Brand names are used for product identification only. Results are estimates for selection; confirm the final pump on the manufacturer's published curve.