Sump Pump Sizing Calculator
Free sump pump sizing calculator. Determine the gallons-per-hour capacity needed to keep a basement dry from pit fill rate, pipe size, and head height.
Quick answer
Required GPH = pit fill rate × safety margin (1.5-2×). For a typical basement, 2,500-4,000 GPH at 10-foot head is standard. Submersible pumps with cast iron housings outlast plastic. Backup battery sump pumps are essential — primary failure during storms is exactly when you need them.
Sump Pump Sizing Calculator
How it works
Pump capacity is rated in gallons per hour (GPH) at zero lift. Real-world capacity drops about 10% per 5 feet of lift, so a pump rated 4,000 GPH at zero feet might only deliver 2,500 GPH at 10 feet of head. The calculator accounts for the lift loss and adds a 30% safety margin so the pump isn't running 100% of the time during a storm.
When to use it
Before buying a primary or backup sump pump, when sizing a battery backup, or when your existing pump is running constantly during storms (sign of being undersized).
Common mistakes
Sizing for "average" inflow rather than worst-case-storm inflow. The pump only matters when it's overwhelmed; size for the day you'll really need it. Also: skipping battery backup. Storms knock out power exactly when you need the pump.
How the sump pump sizing calculator works
Step 1: measure pit fill rate. Plug or unplug the pump and time how fast water fills the pit (gallons in 1 minute). Step 2: multiply by 60 for gallons per hour. Step 3: apply a safety margin of 1.5-2× to handle peak rain events. Step 4: account for head loss — pump capacity decreases as it pushes water higher. Most residential pumps lose 30-50% of rated GPH at 10-foot head. The calculator handles all four steps from your inputs.
When to use it
Replacing a failed sump pump. Sizing a primary pump for a new finished basement. Sizing a battery backup pump (typically 1/3 to 1/2 the capacity of the primary). Evaluating an existing pump's adequacy after a major rain event flooded the basement. Planning for finished-basement insurance requirements.
Common mistakes
- Sizing on rated GPH instead of GPH-at-actual-head. A pump's rated capacity is at zero head. Real installations always have some lift — check the pump curve at your specific head height.
- Skipping the battery backup. Primary sump pumps fail during storms (power outages) — exactly when they're needed. A battery backup pays for itself the first time it's used.
- Ignoring discharge pipe sizing. A 1-1/2 inch discharge restricts flow significantly. Use 2 inch discharge for pumps over 50 GPM rated capacity.
Frequently asked questions
How do I size a sump pump?
Time how fast your pit fills (gallons per minute), multiply by 60 for GPH, apply a 1.5-2x safety margin, and check pump curves at your actual head height. The calculator above takes those inputs and outputs the minimum recommended capacity.
Do I need a battery backup sump pump?
Strongly recommended. Primary pumps fail during storms — power outages happen exactly when you need the pump most. A battery backup at 1/3 to 1/2 the primary's capacity provides several hours of pumping during outages.
How long should a sump pump last?
5-10 years typical. Cast iron housings last longer than plastic. Pumps that run frequently (50+ cycles per day) wear out faster. Plan to replace at the 7-year mark proactively rather than wait for failure during a storm.