Where the number comes from

A pressure tank is mostly air by design: the air cushion is what pushes water to your taps between pump runs. The math is Boyle's law — air pressure times air volume stays constant — applied between your switch's cut-in and cut-out pressures. The standard precharge is 2 psi below cut-in, and the tool assumes your tank is a bladder or diaphragm (captive-air) type, which is what nearly every residential well tank sold today is.

The sanity check: at 40/60, drawdown lands around a quarter of the tank's nominal size. A 44-gallon tank gives you roughly 11 gallons per cycle — not 44. That surprises everyone once, and then it makes sense.

Reading your result

Runtime per cycle is drawdown divided by pump flow. The pro rule of thumb: the pump should run at least a minute per cycle. Under a minute is short cycling — hard on the motor and the usual sign of a waterlogged tank or lost precharge.
Sizing rule: one gallon of drawdown per gpm of pump capacity. A 10 gpm pump wants at least 10 gallons of drawdown, which at 40/60 means roughly a 44-gallon tank.
Pressure vs. capacity trade-off: cranking the switch from 30/50 to 40/60 buys you more pressure but costs you drawdown from the same tank.

Honest limits

Captive-air tanks only. Old galvanized (air-over-water) tanks don't follow this math — their air dissolves into the water and has to be replenished.
Estimate, not a spec sheet. Manufacturer tables differ a few percent by convention; this uses the physics directly. If your tank's published drawdown and this tool disagree by more than ~10%, trust the published table for that exact model.
Assumes correct precharge. Garbage in, garbage out: if the precharge hasn't been checked in years, measure it with the tank drained (system pressure at zero, pump power off) before trusting any number.

Frequently asked questions

Why is drawdown so much smaller than the tank?

Because most of the tank is the air cushion — that's the part doing the work. Around 25% usable at 40/60 is normal. If you need more usable water, you need a bigger tank, not a different brand.

Will a bigger tank give me more water pressure?

No. The pressure switch sets your pressure. A bigger tank only stretches the time between pump cycles, which is what saves the pump.

What's the correct precharge?

Two psi below the pump's cut-in pressure: 18 for a 20/40 switch, 28 for 30/50, 38 for 40/60. Check it with the tank drained of water and the system at zero pressure, using a tire gauge on the air valve — with the pump's power off.

My pump runs every 30 seconds. Is the tank the problem?

Almost certainly the prime suspect. Run this calculator first: if the math says your setup should give multi-minute cycles and you're getting seconds, the tank has likely lost its air cushion (waterlogged) or the bladder has failed. Check the precharge with the tank drained; if water comes out of the air valve instead of air, the bladder is ruptured and the tank needs replacing. The full diagnostic walkthrough is in well pump and pressure tank troubleshooting.

Does this work with a variable-speed (constant-pressure) system?

The drawdown math still describes the tank, but constant-pressure systems run the pump nearly continuously at variable speed — the one-minute-runtime rule doesn't apply the same way. Size those per the manufacturer's guidance.

Want the full picture? Read the pressure tank guide: types, sizing, and precharge.