🔨

Well Pressure Tank Drawdown & Sizing Calculator

Drawdown of a bladder or diaphragm well pressure tank from its total volume and the pump cut-in/cut-out settings, the tank size for a minimum pump run time at your pump’s flow, pump cycles per hour, pre-charge pressure and a check of a chosen tank.

Pump flow at the cut-out pressure

Pump cut-in pressure

Pump cut-out pressure

Minimum pump run time wanted

Candidate tank total volume (optional check)

Typical draw during a low-use hour (for cycle count)

Loading calculator…

How it works

Drawdown = tank volume × (1 − P_cut-in(abs) ÷ P_cut-out(abs)); tank volume needed = pump gpm × minimum run time ÷ drawdown factor; pre-charge = cut-in − 2 psi; cycles/hour = hourly draw ÷ drawdown

A well pump should not start every time a tap opens, so a pressure tank stores water under a cushion of compressed air and lets the pump run in useful bursts. The tank’s “drawdown” — the water it actually delivers between the pump switching off at cut-out and on again at cut-in — is only a fraction of its total volume, set by Boyle’s law: with the usual 40/60 psi switch about 27% of the tank is usable, so a 20-gallon tank yields under 6 gallons. Sizing works backwards from the pump: it should run at least a minute (longer for larger motors) each cycle to cool and to avoid hammering the switch and contacts, so the needed drawdown is pump flow times run time, and the tank is that divided by the drawdown factor. The air pre-charge must sit 2 psi below cut-in or part of the tank is wasted.

Worked example

A 10 gpm pump on a 40/60 switch: the usable share is 1 − 54.7/74.7 = 26.8%. A 1.5-minute run needs 15 gallons of drawdown, so the tank must hold at least 56 gallons total — a standard 62-gallon tank; a 20-gallon tank would give only 5.4 gallons and 32-second runs, cycling about 11 times an hour at 60 gal/h.

Frequently asked questions

Can I just widen the pressure switch?

Going from 40/60 to 40/70 raises the drawdown factor from 27% to 35%, but the pump must be able to reach 70 psi and fixtures see more pressure swing; a constant-pressure (VFD) system is the alternative for small tanks.

What about old-style galvanized tanks without a bladder?

They waterlog as air dissolves, and their usable volume is much lower — roughly a third of a bladder tank of the same size; replace with a diaphragm tank when they start short-cycling.

Related calculators

Well Pump Sizing Calculator

Estimate the total dynamic head and horsepower a well pump needs from the water depth, desired house pressure and peak demand.

Thermal Expansion Tank Sizing Calculator

Size a potable-water or hydronic expansion tank in gallons or litres: water or glycol expansion between fill and maximum temperature (°F or °C), required acceptance volume, acceptance factor from the fill pressure and maximum pressure (psig, bar or kPa), wrong pre-charge effects, the next standard tank, peak pressure for each tank size, a candidate tank check and a suggested hydronic fill pressure from building height.

Total Dynamic Head (TDH) Pump Calculator

Total dynamic head for pump selection: static lift and discharge elevation, outlet pressure (psi, bar or kPa) minus any source pressure, Hazen-Williams pipe friction for Schedule 40, Schedule 80 or a custom pipe, fittings as a percentage or counted one by one, velocity head, specific gravity, NPSH available, a system curve, water and brake horsepower, motor size and running cost.

Hydraulic estimate with standard formulas and typical fluid properties (water unless stated); verify against pump curves, pipe data and the applicable code before specifying equipment.