Sump pump sizing: the calculation most quotes skip
A pump is specified by how much it can lift, how high, against how much pipe. Quotes usually specify it by horsepower, which tells you almost nothing. Here is the arithmetic.
Why horsepower is the wrong question
Two ⅓ hp pumps from different manufacturers can differ by 40% in delivered flow at 10 feet of head. What matters is the published flow at your total dynamic head — vertical lift plus friction loss in the pipe and fittings. Every reputable pump has a performance curve. Ask for it. A contractor who cannot produce the curve for the pump they are installing is telling you something.
The three things that actually decide whether you flood
- Backup power. The storm that causes the inflow is the storm that takes out the power. One pump with no backup is a pump that fails at exactly the moment it is needed. A battery backup runs $750–$1,900; a water-powered backup runs $550–$1,500 and never needs charging, but needs municipal water pressure.
- Where the discharge ends. Pumping water two metres from the wall onto clay means pumping the same water again in twenty minutes. The line needs to reach somewhere it will actually leave — $850–$3,050 buried to daylight.
- The check valve and the freeze path. In the northern counties we cover, a discharge line that freezes turns a working pump into a sealed system. Ask specifically how the installer handles winter discharge.
Get quotes with this specification in hand
Contractors respond differently to a homeowner who already knows the required flow at a stated head.
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