Find the right pump capacity (GPH/GPM) for your pit dimensions, water inflow, and discharge head height.
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Enter your sump pit's shape and dimensions (diameter or length/width, plus depth), then set how fast water rises in your pit during a storm event. Add the total head height — that's the vertical distance from your pump to where water exits, plus friction losses — and choose a safety factor. Hit Calculate Pump Size and you'll instantly see the minimum GPH, GPM, and recommended horsepower for your situation.
An undersized sump pump is one of the leading causes of flooded basements. During a heavy rainstorm, groundwater can pour into a sump pit at 10–20 gallons per minute or more. If your pump can only move 8 GPM, it will run continuously, overheat, burn out — and your basement still floods.
Conversely, an oversized pump cycles on and off too quickly ("short cycling"), which wears out the motor and float switch prematurely. A 1/3 HP pump typically handles 35–40 GPM at 10 feet of head, while a 1/2 HP pump moves 45–60 GPM. Most average homes in moderate rainfall zones need a 1/3 HP unit rated at 2,000–3,000 GPH. Homes in flood-prone areas, with large drainage footprints (over 2,000 sq ft), or with long discharge runs exceeding 20 feet often need 1/2 HP or larger with a battery backup system. Getting this right the first time saves hundreds on cleanup and pump replacement.
The calculation follows this methodology:
1. Pit Volume per Inch: For a round pit: V = π × (d/2)² × 1 (cubic inches per inch of depth). For rectangular: V = L × W × 1. Convert to gallons: 1 gallon = 231 cubic inches.
2. Raw Inflow (GPM): = (Pit cross-section area in²) × (rise rate in/min) ÷ 231
3. Required Pump Output (GPM): = Raw Inflow × Safety Factor ÷ Pump Efficiency
4. GPH: = Required GPM × 60
5. Horsepower Recommendation: Based on GPM and total head height using standard pump curves: <20 GPM → 1/4 HP; 20–40 GPM → 1/3 HP; 40–60 GPM → 1/2 HP; 60–80 GPM → 3/4 HP; >80 GPM → 1 HP+.
Most single-family homes in moderate climates need a 1/3 HP pump rated at 2,000–3,000 GPH at 10 feet of head. Homes with large drainage areas, high water tables, or frequent heavy rainfall often need 1/2 HP (3,000–4,500 GPH). Use this calculator to get a number specific to your pit and conditions.
During or just after a heavy rain, unplug the pump and watch how many inches the water rises per minute. Mark the pit wall with tape and use a stopwatch. Even a 2–3 minute observation gives a good estimate. During extreme storms, rise rates of 8–15 in/min are common in high-water-table areas.
Total head is the total resistance the pump must overcome. It equals: vertical lift from pump to discharge outlet + friction loss from pipe length (≈1 ft per 10 ft horizontal) + any fittings. For a typical basement with an 8-foot vertical run and 20 feet of horizontal pipe, total head ≈ 10 feet.
If this calculator recommends under 40 GPM, a quality 1/3 HP pump is usually sufficient and costs less to run. If your required GPM is 40–60, go with 1/2 HP. The key is buying a pump with a published performance curve that meets your GPM at your specific head height — not just the maximum rated GPH.