Construction · basement water
Sump pump size & cycle calculator.
Time how fast water rises in your basin, then see inflow, the flow you should look for in a pump, and how often it will cycle. Runs in your browser; nothing is sent anywhere.
Flood-risk note: a wrong pump size, a failed float or a power outage can flood a basement. Treat these results as a planning aid and have a licensed plumber or drainage professional confirm the pump, discharge, check valve, backup and alarm.
Basin test and pump
Results
Formulas, sources, and limits
Formulas: basin area = π × (diameter / 2)² in square inches. Gallons per inch = area / 231, because one U.S. gallon is 231 cubic inches. Inflow (GPM) = gallons per inch × rise / minutes. Required GPM = inflow × your safety factor. Cycle volume = gallons per inch × switch differential. Fill time = cycle volume / inflow. Pump run time = cycle volume / (pump GPM − inflow). Cycles per hour = 60 / (fill time + run time). This is plain geometry and arithmetic on your own measurements; the safety factor and switch differential are your assumptions.
Pump flow and head: a pump delivers less flow as total dynamic head rises. Zoeller's 150-series chart is an example of a manufacturer table of flow per minute at each foot of total dynamic head, and Zoeller's sizing worksheet (FM0551) shows total dynamic head as friction head plus static head plus any pressure head. Enter the flow your chosen pump's own curve gives at your head. Neither Zoeller document supplies this tool's safety factor or cycle formulas, and no pump model is recommended here.
Limits: assumes a round basin, steady inflow during the test and a pump that does not run during the fill. It does not cover discharge code, check valves, power backup, groundwater causes or sewage pumps. Confirm with a licensed professional.
Frequently asked questions
What size sump pump do I need?
Match a pump's published flow at your head to the required GPM above. The result is a floor to compare against, not a model choice.
What is a normal cycle rate?
This page does not claim a standard. Very frequent cycling means a small switch differential or a pump barely above inflow; ask the pump's manufacturer about its duty limits.
Does this prevent flooding?
No. Pumps, floats and power can fail. Get professional advice and consider a backup and alarm.