Well Pressure Tank Sizing Calculator
Pressure tanks do not create water pressure—they function as a mechanical hydraulic battery. Sizing a pressure tank correctly ensures your submersible pump motor runs for the manufacturer-mandated minimum runtime, protecting motor windings from short-cycling burnout.
Min 10 Gal Drawdown (provides safe 60s motor cycle)
Cross-sectional engineering diagram: Well casing, dynamic water level, submersible pump, pitless adapter, and residential pressure tank assembly.
The Critical Function: Motor Cooling Run-Time Cycles
Submersible motors rely on water flowing past the exterior motor housing to dissipate intense rotor heat. Franklin Electric and National Ground Water Association (NGWA) standards specify that a motor must run for a minimum of:
- 60 seconds minimum runtime: For motors rated 0.5 HP, 0.75 HP, 1.0 HP, and 1.5 HP.
- 120 seconds minimum runtime: For motors rated 2.0 HP, 3.0 HP, and 5.0 HP.
If your pump delivers 12 GPM and your pressure tank only provides 4 gallons of drawdown, the motor will shut off after only 20 seconds of pumping. The motor shuts down before cooling water can circulate across the stator jacket, causing thermal fatigue.
Pressure Tank Sizing Reference Matrix
| Pump Flow Rate (GPM) | Motor HP | Min Run Cycle | Required Drawdown | Recommended Gross Tank Size (40/60 PSI) |
|---|---|---|---|---|
| 5 GPM | 0.5 HP | 60 seconds | 5.0 Gallons | 20 Gallon Tank (e.g., WX-202) |
| 8 GPM | 0.75 HP | 60 seconds | 8.0 Gallons | 32 Gallon Tank (e.g., WX-203) |
| 10 GPM | 1.0 HP | 60 seconds | 10.0 Gallons | 34 – 36 Gallon Tank (e.g., WX-203) |
| 12 GPM | 1.5 HP | 60 seconds | 12.0 Gallons | 44 Gallon Tank (e.g., WX-250) |
| 15 GPM | 2.0 HP | 120 seconds | 30.0 Gallons | 86 – 119 Gallon Tank (e.g., WX-302 / Dual Tanks) |
| 20 GPM | 3.0 HP | 120 seconds | 40.0 Gallons | 119 Gallon Tank (or 2 × 86 Gallon Tanks) |
Boyle's Law Acceptance Factor Formula
Pre-charged bladder tanks utilize Boyle's ideal gas law ($P_1 V_1 = P_2 V_2$) to compress an internal air cushion as water enters the diaphragm:
F_a = 1 - [(P_cutin + 14.7) / (P_cutout + 14.7)]
For a standard 40/60 PSI switch (Atmospheric pressure = 14.7 PSI absolute):
F_a = 1 - [(40 + 14.7) / (60 + 14.7)] = 1 - (54.7 / 74.7) = 1 - 0.732 = 0.268
Sizing Calculation: If your pump produces 10 GPM with a 60-second runtime target, your system requires 10 gallons of active drawdown:
Gross Tank Volume = Required Drawdown / Acceptance Factor = 10 / 0.268 = 37.3 Gallons Because tanks are sold in standardized commercial volumes, you round up to the next standard residential unit: a 32 to 36-gallon diaphragm tank.
Frequently Asked Questions About Pressure Tank Sizing
What happens if a well pressure tank is undersized?
An undersized pressure tank causes the pump to short-cycle (turn on and off rapidly every few seconds when water is running). Submersible motors draw up to 500% of their rated running current during startup. Rapid cycling causes severe heat buildup in the motor windings, boiling the start capacitor and burning out the stator within 1 to 2 years.
How much usable water is actually inside a 32-gallon pressure tank?
A 32-gallon pre-charged diaphragm pressure tank does not hold 32 gallons of water. Operating on a standard 40/60 PSI switch, the tank's Boyle's Law acceptance factor is roughly 27% to 30%, yielding only 8.5 to 9.5 gallons of actual usable water drawdown before the pump must turn on.
What should the air pre-charge pressure be in my well tank?
The air pre-charge pressure inside an empty bladder tank must be set exactly 2 PSI below your pressure switch cut-in pressure (with the pump powered off and all water drained from the system). For a 40/60 PSI system, pre-charge to 38 PSI. For a 30/50 PSI system, pre-charge to 28 PSI.
System Cross-References:
Review vertical well lift calculations with our Total Dynamic Head Tool or size your electrical drop cable in the Wire Size Calculator.