Sump Pump Sizing Calculator — GPM, Head & Pump Capacity
Measure the pit fill rate when you can, add a safety margin, then size the pump by the GPM it must deliver at the discharge total dynamic head — not by horsepower alone.
Sump pump calculator
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What size sump pump do I need?
Size a sump pump by the water inflow it must handle and the total dynamic head of the discharge. Measure the pit fill rate when you can, apply a safety margin, calculate TDH from lift plus pipe and fitting losses, then choose a pump whose published curve meets that GPM at that head. Horsepower alone is not a size.
How to Use the Sump Pump Calculator
- Prefer Measured inflow. Time a water-rise test in the pit.
- Enter pit diameter, rise, and duration. Apply a safety factor (1.5 is a common planning margin).
- Enter vertical lift, pipe run, and the actual discharge pipe — the list shows inside diameter, not just the trade size.
- Add elbows and a check valve in Advanced if you know them.
- Read required GPM, GPH, TDH, and the duty point, then match a manufacturer curve.
How to Calculate Sump Pump GPM
For a cylindrical pit, gallons = π × (diameter ÷ 2)² × rise ÷ 231, with diameter and rise in inches. Inflow GPM is that volume divided by the test time in minutes. Required GPM is inflow times the safety factor. That is a measured fill rate — not “10 GPM per 100 square feet of basement.”
How to Calculate Total Dynamic Head
TDH is the head the pump works against: static lift plus friction in the pipe plus friction from fittings. This calculator uses Hazen–Williams for water, with published C values (PVC 150, copper 140, new steel 120) and the actual inside diameter. Fitting loss uses Crane-style L/D equivalent lengths so a 90° elbow, 45° elbow, swing check, or open gate/ball valve adds length instead of a guessed “head penalty.”
Sump Pump Sizing Formula
- Pit gallons =
π × (D ÷ 2)² × rise ÷ 231 - Inflow GPM =
pit gallons ÷ minutes - Required GPM =
inflow × safety factor - Rainfall estimate GPM ≈
C × i × A × 0.0103(rational method; labeled an estimate) - Hazen–Williams:
hf = 10.44 × L × Q^1.852 ÷ (C^1.852 × d^4.87) - TDH =
lift + pipe friction + fitting friction
Sump Pump GPM vs GPH
GPH is GPM × 60. A duty point of 20 GPM is 1,200 GPH. Manufacturers usually publish GPM at a listed head. Use GPM at TDH to shop; GPH is the same flow on another clock.
Sump Pump Sizing Examples
These rows use the same function as the calculator.
| Example | Required GPM | TDH |
|---|---|---|
| Measured 18 in pit, 8 in rise in 1 min | 13.2 GPM | 8.5 ft |
| Same inflow, 16 ft lift | 13.2 GPM | 16.5 ft |
| Same inflow, 60 ft of pipe | 13.2 GPM | 8.9 ft |
| Safety factor 2.0 | 17.6 GPM | 8.8 ft |
| Rainfall estimate, 1,500 sq ft | 18.5 GPM | 8.9 ft |
How Horsepower Relates to Sump Pump Size
A 1/2 HP pump is not automatically “better” than a 1/3 HP pump. Two pumps with the same nameplate horsepower can deliver very different GPM at 10 or 20 feet of head. Use the duty point, then read the curve. The retail 1/3 vs 1/2 HP label is only a shopping aisle — it is not this calculator’s answer.
Common Sump Pump Sizing Mistakes
- Buying from zero-head GPM on the box. Flow falls as head rises.
- Using basement square footage as if it were a measured pit inflow.
- Counting only vertical lift and ignoring pipe friction and the check valve.
- Treating a 1-1/2 in trade size as a 1.500 in bore. Sch 40 PVC is 1.610 in ID in the table.
- Using a fixture-unit tool to pick the discharge. The pipe sizing calculator is for plumbing fixture demand, not sump GPM.
How Pipe Size and Fittings Affect Sump Pump Performance
Smaller inside diameter and more fittings raise friction, so TDH goes up and the pump delivers less GPM. A long 1-1/4 in line with several elbows can need a much stronger curve than a short 2 in run. If you need the water volume sitting in that discharge, use the pipe volume calculator. A French drain calculator estimates trench gravel if you are adding drainage to the pit. Gravity fall is a different job — the drain slope calculator.
Sump Pump Calculator FAQ
- What size sump pump do I need?
- You need a pump whose published curve meets the required GPM at your total dynamic head. Measure pit inflow, apply a safety factor, add lift and friction, then shop the duty point — not the horsepower label.
- How many GPM does a sump pump need?
- As many gallons per minute as the pit inflow, times your safety factor. An 18 in pit that rises 8 in in one minute is about 8.8 GPM inflow; at 1.5× that is about 13 GPM before you still check TDH on a curve.
- How do I calculate sump pump GPM?
- Gallons in the rise = π × (pit diameter ÷ 2)² × rise ÷ 231. Divide by the test time in minutes. That is measured inflow. Required GPM multiplies by the safety factor.
- What is total dynamic head?
- TDH is static lift plus friction in the discharge pipe plus friction from fittings and valves. It is the head the pump must work against, in feet.
- How does head height affect sump pump flow?
- As TDH rises, a given pump delivers less GPM. A box rating at 0 ft of head overstates flow at a real basement discharge.
- What is the difference between GPM and GPH?
- GPH is GPM × 60. 20 GPM is 1,200 GPH. Pump curves are usually plotted in GPM versus feet of head.
- Is a 1/2 HP sump pump better than a 1/3 HP?
- Not automatically. The two classes overlap. Compare published GPM at your TDH. This tool does not claim you need a specific horsepower.
- Does pipe size affect sump pump performance?
- Yes. Smaller actual inside diameter and more fittings raise friction, so TDH goes up and delivered GPM falls. Use the listed ID, not the nominal trade size.
- How do I calculate sump pump friction loss?
- This planner uses Hazen–Williams: hf = 10.44 × L × Q^1.852 ÷ (C^1.852 × d^4.87), with L including Crane-style equivalent lengths for elbows and valves.
- Can I size a sump pump from basement square footage?
- Square footage is not a measured inflow. Use the pit rise test when you can. Rainfall mode is a labeled estimate from area, intensity, and a runoff coefficient — not a guarantee of groundwater.
- How do I calculate sump pump cycle time?
- Usable gallons = the cylinder between the on and off float levels. Minutes to refill ≈ usable gallons ÷ measured inflow GPM. That is a basin timing check, not a pump-curve substitute.
- Should I use a battery backup pump?
- A backup pump is a reliability choice for power loss, not something this calculator sizes. Size the primary duty point first, then consider a second pump if outages or high water are a concern.
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