Well Pump Cycle Rate Calculator
Calculate how many times per hour your well pump cycles on and off, and determine if your pressure tank is properly sized to protect your pump from short-cycling damage.
Total rated volume of the pressure tank (not drawdown volume)
Gallons per minute your pump delivers
Average household demand in gallons per minute
Pressure at which pump turns ON
Pressure at which pump turns OFF
Air pre-charge pressure (typically 2 PSI below cut-in)
Formulas Used
Drawdown Volume (Boyle's Law for bladder/diaphragm tanks):
Vdrawdown = Vtank × (Pprecharge + 14.696) × [1/(Pcut-in + 14.696) − 1/(Pcut-out + 14.696)]
All pressures converted to absolute PSI (gauge + 14.696).
Pump ON Time per Cycle:
Ton = Vdrawdown / (Qpump − Qdemand) (minutes)
Pump OFF Time per Cycle:
Toff = Vdrawdown / Qdemand (minutes)
Cycle Rate:
Cycles/Hour = 60 / (Ton + Toff)
Minimum Required Drawdown:
Vdrawdown,min = Qpump × 60 / (4 × Maxcycles/hr)
Derived from the worst-case condition where demand = Qpump/2, which maximizes cycle rate.
Assumptions & References
- Assumes a bladder or diaphragm pressure tank (not a galvanized air-over-water tank).
- Boyle's Law (P&sub1;V&sub1; = P&sub2;V&sub2;) governs air volume changes in the tank at constant temperature.
- Atmospheric pressure is taken as 14.696 PSI for absolute pressure conversion.
- Industry standard maximum cycle rate is 6 cycles per hour for most residential submersible and jet pumps (Goulds, Grundfos, and Pentair pump guidelines).
- Pre-charge pressure should be set to 2 PSI below cut-in pressure per pressure tank manufacturer recommendations (Amtrol, Well-X-Trol).
- Demand flow must be less than pump flow for the pump to cycle; if demand ≥ pump flow, the pump runs continuously.
- Worst-case (maximum) cycle rate occurs when system demand equals exactly half the pump flow rate.
- References: Hydraulic Institute Standards; Amtrol Well-X-Trol Sizing Guide; Grundfos Pump Engineering Manual; AWWA Well Design Standards.