ToolnozaNominal tank size is not usable drawdown
A pressure tank’s label gives its nominal internal volume. Drawdown is the amount of water the tank can deliver between the pressure switch’s cut-out and cut-in points. Only a fraction of the nominal volume is usable, and that fraction depends on precharge and both switch pressures.
For a well system, required drawdown is usually based on the pump’s delivered flow and the minimum run time specified for that pump or motor. A larger tank generally provides more drawdown and can reduce frequent starts, but it does not increase the well’s sustainable yield.
Inputs to collect before sizing
Use the pump’s flow at the actual operating duty, not only its maximum advertised flow. Find the minimum run time from the pump or motor manufacturer. Record the switch cut-in and cut-out pressures, and measure tank precharge with the pump off and the water side drained to zero pressure.
Keep pressure readings in one unit system. The pressures entered here are gauge pressures. Confirm the tank is a bladder or diaphragm type and that its maximum working pressure suits the system.
Pressure tank drawdown formula
Required drawdown = pump flow × minimum pump run time. For flow in gallons per minute and time in minutes, the result is US gallons; for liters per minute, it is liters.
For a bladder tank, estimated acceptance fraction f = P₀(abs) × [1/Pᵢ(abs) − 1/Pₒ(abs)]. P₀ is precharge, Pᵢ is cut-in and Pₒ is cut-out. Convert each gauge pressure to absolute pressure first: add about 14.7 psi or 1.013 bar. Estimated nominal tank volume = required drawdown ÷ f. This ideal-gas estimate assumes the tank and pressure switch operate as described; manufacturer drawdown tables can differ.
Worked example: 10.57 GPM and a 30/50 switch
Assume a pump delivers 10.57 GPM and needs a one-minute minimum run time. Required drawdown = 10.57 × 1 = 10.57 US gallons. Use a 28 psi precharge, 30 psi cut-in and 50 psi cut-out.
Absolute pressures are 42.7, 44.7 and 64.7 psia. The estimated acceptance fraction is 42.7 × (1/44.7 − 1/64.7) = 0.2953, or 29.53%. Nominal tank volume = 10.57 ÷ 0.2953 ≈ 35.8 gallons. Toolnoza’s calculator rounds up to the next size in its generic list, 44 gallons. Compare that estimate with the drawdown published for the exact tank model and duty point.
Use the Toolnoza calculator
Open the Well Pressure Tank Sizing Calculator. Choose US or metric units, enter pump flow and minimum run time, then add precharge, cut-in and cut-out. The result shows required drawdown, estimated acceptance fraction, nominal tank volume and the next size in Toolnoza’s generic list.
The listed size is a screening estimate, not a product selection. If you do not know the pump’s flow at the operating point or its minimum run time, check the pump curve and manufacturer documentation before relying on the result.
Check the manufacturer chart and installation limits
Tank brands and models have different nominal sizes and published drawdown values. A manufacturer chart may use pump discharge rate, motor horsepower, pressure-switch range and a stated run time together. Choose a model whose published drawdown meets or exceeds the requirement, and check its pressure rating and connection details.
Amtrol advises setting its well-tank precharge 2 psi below pressure-switch cut-in and publishes a model sizing chart. Follow the instructions for the actual tank you buy; do not assume one brand’s chart applies to another. A tank-size estimate does not check well yield, pump-curve suitability, water quality, plumbing code or electrical protection.