How to use this calculator
- Enter contributing area. Use the basement, footing-drain or exterior area that can feed this sump.
- Set inflow assumptions. Enter an area-based water-entry rate, capture coefficient, measured inflow and safety factor.
- Use pump curve flow. Enter selected pump capacity at the installed total dynamic head, not the zero-head catalog number.
- Set basin depths. Enter basin diameter, pump-on depth, pump-off depth and high-water alarm depth.
- Check capacity and cycling. Compare required capacity, selected-pump utilization, run time, starts per hour and high-water reserve time.
How it works
Area inflow starts with
Q = C i A
where C is the capture coefficient, i is the
water-entry rate and A is the contributing area. Measured or
direct inflow is added before the safety factor is applied.
The basin drawdown volume is a cylinder volume between the pump-on and pump-off depths: V = pi D^2 (y_on - y_off) / 4. Required pump capacity at head is then Q_req = Q_design + V / t_pumpdown.
Use nearby drainage tools for the upstream and downstream checks: French drain sizing, catch basin sizing, swale capacity and pipe pressure drop.
Worked example
Verified against the live calculator
With the defaults, a 1,500 ft^2 drainage area, 1 in/h
water-entry rate, C = 0.5, 5 gpm measured inflow
and a 50% allowance produce about 19 gpm of
design inflow. A 24 in basin with a 10 in drawdown
stores about 19.6 gal; pumping that volume down in
2 min requires about 29 gpm at the installed
head.
Frequently asked questions
How do you size sump pump capacity?
Estimate the design inflow, apply a safety factor, then choose a pump that can deliver at least the required capacity at the installed total dynamic head.
Can I use the pump zero-head rating?
No. Use the manufacturer pump curve at the installed head, including discharge pipe, fittings, check valve and elevation losses.
Why does basin volume matter?
The drawdown volume between pump-on and pump-off depths controls pump run time and cycling rate. Too little volume can make the pump short-cycle.
Is this a waterproofing or plumbing-code design?
No. It is a capacity and cycling screen. Final work still needs pump curves, discharge details, power, alarm, backup pump and local plumbing/electrical requirements.
Method & assumptions
- Area-based inflow is a screening estimate; measured inflow is better when available.
- Selected pump capacity must come from the pump curve at installed total dynamic head.
- The basin is treated as a simple cylinder between the entered water levels.
- Starts per hour is only compared with the user-entered target; manufacturer switch and motor limits control final cycling.
- Final design still needs pump curves, discharge pipe and check-valve losses, solids handling, basin geometry, controller limits, power, alarm, backup pump, waterproofing details and local plumbing/electrical review.