A sump pump is sized by two numbers: the flow it has to move (GPM) and the total dynamic head (TDH) it has to push against. TDH is the vertical lift from the pump to the discharge plus the friction the water fights on the way — through the pipe and every elbow, check valve, and fitting. The number on the box (“max 60 GPM”) is measured at zero head and is not your operating point. The right way to size is to find your duty point — say, 30 GPM at 14 feet of head — and confirm it lands on or below the model's pump-performance curve. Horsepower (⅓ vs ½ HP) is a shortcut for that curve, not the spec itself.
Sizing a sump or effluent pump? The sump pump size calculator adds up your static lift and friction head — including fitting equivalents — to give the total dynamic head at your design flow, so you can match it to a pump curve. Free, no signup. This guide explains what TDH is, why the box rating misleads, and the extras (check valve, backup) that keep a basement dry.
💧 Start with the flow (GPM)
The first number is how much water the pump has to move, in gallons per minute. In a basement sump that's driven by how fast water flows into the pit during a heavy rain or a high water table — the inflow rate — with a safety factor on top. A common field method is to let the pit fill with the pump unplugged, time how fast it rises, and convert that to GPM, then add margin.
Get the flow honestly. Undersize it and the pump can't keep up during the storm that actually matters; wildly oversize it and the pump short-cycles, switching on and off so often it wears out early. The goal is a pump that empties the pit at a sensible pace and rests between cycles.
⬆️ Total dynamic head, explained
A pump doesn't just lift water — it fights resistance the whole way up and out. Total dynamic head is the sum of two things:
- Static lift — the plain vertical rise, from the pump's discharge up to where the water leaves the pipe at grade or the outlet.
- Friction head — the resistance of the water rubbing through the pipe and turning through every fitting. Longer runs, smaller pipe, and more elbows all add friction.
Friction is figured from the equivalent length of the system: the straight pipe run plus a “length penalty” for each fitting (a 90° elbow in 1½-inch pipe acts like about 8 extra feet of pipe; a check valve like 13). Multiply the total equivalent length by the friction loss per 100 feet at your flow, and you get the friction head in feet.
Worked example. 8 feet of static lift + a 30-foot run of 1½-inch pipe + three 90° elbows (24 ft) + one check valve (13 ft) = 67 feet of equivalent length. At 40 GPM that pipe loses about 9.4 feet per 100 feet, so friction ≈ 6.3 feet. TDH = 8 + 6.3 ≈ 14.3 feet at 40 GPM — that's the duty point.
📈 Read the curve, not the box
Here's the mistake that undersizes half the sump pumps out there. The headline “max GPM” on the box is measured at zero head — the pump discharging straight up with no lift and no pipe. Your pump never operates there. As head goes up, flow comes down, and every pump has a performance curve that plots exactly how much it delivers at each height.
So sizing is a graph check: plot your duty point (your GPM and your TDH) on the manufacturer's curve. If the point sits on or below the curve, the pump can do the job with margin. If it sits above the curve, the pump is too weak for your lift — no matter how big its zero-head “max” number looks. Horsepower (⅓, ½, ¾ HP) roughly ranks pumps along these curves, but the curve is the real answer.
🔧 Pipe size and velocity
Discharge pipe size matters because friction rises steeply as pipe shrinks. Most ⅓–½ HP pumps use 1½-inch discharge; higher-flow or sewage pumps step up to 2 inches. Bumping the pipe up a size can dramatically cut friction head — the same 40 GPM that loses 9.4 feet per 100 feet in 1½-inch pipe loses under 3 feet per 100 in 2-inch.
A good rule of thumb is to keep water velocity at or below about 5 feet per second. Faster than that and you add friction and water hammer for no benefit — a sign to step up a pipe size.
🛡️ Check valve and backup
Two pieces aren't optional in a real install. A check valve on the discharge keeps the column of water in the vertical pipe from draining back into the pit every time the pump shuts off — without it, the pump re-pumps the same water and short-cycles. (It also adds meaningful friction head, which is why it's in the TDH math.)
And because a sump pump fails exactly when you need it — during the storm that knocks out the power — a battery backup or water-powered backup pump is the difference between a dry basement and a flooded one. Size the backup for the same duty point as the primary, and test both before the wet season.
✅ The bottom line
Size a sump pump on its duty point — your design GPM at your total dynamic head — and confirm that point falls on or below the model's pump curve. Add up static lift plus friction (pipe + fittings), don't trust the zero-head “max GPM” on the box, keep pipe velocity under about 5 ft/s, and never skip the check valve or the backup pump.
Friction values and fitting equivalents are engineering estimates and vary with pipe material and condition, so treat the result as a design target and confirm against the specific pump's published curve. Start with the sump pump calculator to get your TDH.
Estimate your Sump Pump materials
Free sump pump sizing calculator — Total Dynamic Head from static lift, pipe run, and fittings, so you can match the duty point to a pump curve.
Estimate with the Sump Pump Calculator →