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Sump Pump Size Calculator

An undersized sump pump means a flooded basement. This calculator estimates the required pumping capacity based on your drainage area, expected rainfall, and discharge height.

Enter Your Measurements

Results

Enter your measurements to see results

How to Use This Calculator

  1. Enter the drainage area (typically your basement/crawlspace footprint).
  2. Enter the design rainfall rate (2 in/hr is standard; use 3–4 for heavy storm areas).
  3. Enter the vertical lift from pit bottom to discharge outlet.
  4. Enter the total discharge pipe length.

How Sump Pump Calculation Works

This calculator estimates water inflow based on your drainage area and rainfall rate, then adds a friction factor for the discharge pipe to determine total dynamic head (TDH). The recommended pump capacity includes a 1.5× safety factor to handle peak events. HP recommendation is based on typical pump performance at the calculated TDH: 1/3 HP handles most residential basements, but high water tables or large areas may need 1/2 HP or more.

Formula

Inflow GPM = (Drainage Area × Rainfall Rate ÷ 12 × 7.48) ÷ 60. TDH = Vertical Lift + (Pipe Length × 0.10 friction factor). Required GPM = Inflow × 1.5 safety factor.

Frequently Asked Questions

What size sump pump for an average basement?
Most residential basements (1,000–1,500 sq ft) need a 1/3 HP pump rated for 35–50 GPM at 10–15 feet of head. High water table areas should go 1/2 HP.
Should I get a battery backup sump pump?
Yes, if your basement is finished or flooding risk is high. Power outages often accompany the storms that cause the worst flooding. A battery backup rated at 1,500+ GPH provides critical redundancy.
How often should a sump pump cycle?
During heavy rain, every 2–4 minutes is normal. Cycling more than once per minute indicates the pump is undersized or groundwater pressure is very high. Running continuously means the pump can't keep up.

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