What Size Sewage Pump Do I Need? Flow & Head Explained

“What size do I need?” is the most common question we get, and it usually comes with the least information. That’s okay — this article shows you how to figure it out yourself, and what to send us so we can double-check it.

Sizing is really just two numbers

Every pump is sized on two things: flow (how much water per hour) and head (how hard it has to push). Get those two right and everything else follows.

Flow — how much

If a pit fills up and you want to empty it in a set time, that’s your flow. Example: a 5 m³ pit that fills over an hour and you want emptied in 15 minutes means the pump needs to move roughly 5 m³ in 15 minutes — about 20 m³/h. If you know daily volume instead, divide by the hours the pump will actually run.

Don’t have any numbers? Tell us the application (how many people, how big the site) and we’ll estimate a realistic flow for you.

Head — how hard

Add up:

• the vertical lift from pump to discharge, in metres, plus
• a friction allowance for the pipe length and fittings.

For short, simple runs the friction is small. For long pipe runs or lots of bends, it adds up fast and you shouldn’t ignore it. When in doubt, tell us the pipe length and number of elbows and we’ll calculate it properly.

A worked example

Say you’re draining a basement sump: pit fills at about 15 m³/h, you’re lifting 8 m up to the sewer, with maybe 20 m of horizontal pipe and a couple of bends. You’d look for a submersible sewage pump rated around 15 m³/h at roughly 10–12 m of total head (the extra covers friction). Pick a pump whose curve sits comfortably above that point, not right on the edge.

If the pump can’t sit in the pit — say you need it on dry ground drawing from a tank — you’d size a self-priming sewage pump to the same flow and head instead.

The one mistake to avoid

Bigger is not safer. An oversized pump empties the pit too fast, switches on and off constantly (“short-cycling”), and that on/off hammering is what actually kills pumps and float switches. Size to the job, not “just in case.”

Just send us this

If maths isn’t your thing, send us four lines:

1. How much water per hour (or pit size + how fast it fills)
2. How high you’re lifting it
3. Pipe length and rough number of bends
4. What’s in the water

We’ll size it and quote it. No charge for the sizing.

📧 sales@haquanpump.com