How to Size a Pressure Tank | Match Pump Run Time

Sizing a pressure tank comes down to matching pump flow, pressure switch settings, and runtime so the pump avoids short-cycling.

An undersized pressure tank makes the pump cycle on and off rapidly, which wears out the pump, motor, and pressure switch far sooner than normal use would. Sizing a pressure tank correctly prevents that damage while keeping water pressure steady through the house, and the process takes about five minutes with a few numbers from your system.

What You Need Before You Size a Tank

Gather three numbers first: the pump’s usable flow rate in gallons per minute (GPM), the pressure switch cut-in and cut-out settings, and the target run time for the pump. The Wellcare/Water Systems Council guidance says to collect system demand, pump capacity, and well capacity, then match those against a tank selection table.

Most residential pressure switches are set to 30/50 or 40/60, meaning the pump starts at 30 or 40 psi and stops at 50 or 60 psi. The pump’s flow rate is usually printed on the pump label or in the owner’s manual.

The Runtime Rule and the Numbers That Matter

The most reliable sizing method targets a minimum pump run time. Common guidance calls for at least 1 minute of runtime for pumps under 2 HP, and about 2 minutes for pumps rated 2 HP and above. A pump that runs at 10 GPM with a 1-minute target needs 10 gallons of usable drawdown.

That drawdown number is not the tank’s total volume.

Drawdown Math and the Selection Check

For a more precise fit, use the pressure-factor method described by the Water Well Journal. The acceptance factor (AF) is the ratio of pre-charge pressure to cut-out pressure, and the pressure factor (PF) equals 1 minus the AF. Total tank volume equals required drawdown divided by the pressure factor.

The confirmation checklist looks like this:

• Flow rate. Usable pump output in GPM.
• Switch settings. Cut-in and cut-out pressures.
• Target runtime. 1 minute for pumps under 2 HP, 2 minutes for 2 HP and above.
• Required drawdown. Flow rate × runtime.
• Tank rating. Rated drawdown at your actual pressure settings must meet or exceed the requirement.
• Pre-charge.

When the tank is too small, the pump short-cycles and the pressure switch contacts wear rapidly. When the pump can exceed the tank’s maximum working pressure, install a relief valve per safety guidance. And if your system sits at the smaller end of the range — a low-flow pump serving a cabin or a single fixture — a compact tank can be the right call; our best 2-gallon pressure tank roundup covers the options that fit tight spaces without starving the pump.

Verify the tank’s maximum working pressure against the pump’s capability, and check the manufacturer’s drawdown chart rather than relying on nominal size alone. Those two checks prevent the most common installation failures.

FAQs

What happens if the pressure tank is too small?

The pump cycles on and off too frequently, a condition called short-cycling. Each start drives current spikes and mechanical stress through the pump motor and pressure switch, which shortens their service life. A correctly sized tank keeps runtime within the 1–2 minute window and reduces starts per hour to a safe level.

Does a bigger pressure tank always mean better performance?

Not exactly. A larger tank provides more drawdown and fewer pump cycles, but it also costs more and takes up more space. Oversizing beyond the pump’s recovery rate adds expense without meaningful benefit. The right size matches the pump flow, switch settings, and a reasonable runtime target.

Should the tank pre-charge match the cut-in pressure?

A pre-charge set too high or too low reduces usable drawdown.