What size pressure tank for a 15 GPM pump at 30/50 psi?

15 GPM pump at 30/50 psi

62 gal tank

19.2 gal of drawdown — 1.3 minutes of run time per cycle

Tank sizenominal size on the label	62gal
Usable drawdown31% of the tank at 30/50 psi — the rest is air	19.2gal
Drawdown needed15 GPM × 1 minute minimum run	15gal
Run time per cycleunder one minute and the motor never sheds its start heat	1.3min
One size down (44 gal)below the one-minute rule — this is the size that kills pumps	0.9min
Pre-charge pressure2 psi below cut-in, measured with the tank empty and the pump off	28psi

Notes

A 62 gallon tank does not hold 62 gallons of water. Most of it is compressed air. At 30/50 psi you get 19.2 gallons of usable drawdown, and that is the only number that matters when sizing.
Higher pressure gives you LESS drawdown, not more. 30/50 psi yields 31% of the tank against 31% at 30/50. Turning the pressure switch up for better showers shrinks the water buffer and makes the pump start more often — the opposite of what most people expect when they make that change.
The one-minute rule is about the motor, not the water. A submersible pump has to run at least a minute per start to shed the heat of starting. Pumps die from cycle count, not run hours. A 44 gallon tank here gives only 0.9 minutes — that is the choice that quietly destroys pumps, and it saves about a hundred dollars once.
Set the pre-charge to 28 psi, with the tank empty. Two psi below cut-in, measured at the schrader valve with the pump off and a tap open. Wrong pre-charge is the most common failure in a well system: too high and the tank waterlogs, too low and drawdown collapses. Check it yearly — it is a five-minute job with a tyre gauge.
Constant-pressure (variable speed) pumps change this arithmetic completely: they modulate instead of cycling, so they need only a small tank. If the system has one, size by the manufacturer's table rather than by the rule above.

A 15 GPM pump at 30/50 psi

62 gallon tank — giving 19.2 gallons of usable drawdown and 1.3 minutes of run time per cycle.

The tank does not hold what the label says

Most of a pressure tank is compressed air. The water you actually get between pump starts is called drawdown, and at 30/50 psi it is only 19.2 gallons of a 62 gallon tank.

Drawdown is the number to size by. Tank volume is the number on the box.

Higher pressure gives you LESS drawdown

This is the part that catches everyone.

Switch	Usable share of tank
30/50	~31%
40/60	~26%
50/70	~22%

Turning the switch up for better showers shrinks the buffer and makes the pump start more often — the opposite of what most people expect when they make that change.

The one-minute rule is about the motor

A submersible pump has to run at least a minute per start to shed the heat of starting.

Pumps die from cycle count, not run hours. Undersizing the tank saves a hundred dollars once and costs a pump.

Pre-charge is the annual five-minute job

Set it to 28 psi — two below cut-in — measured at the schrader valve with the pump off and a tap open until the tank is empty.

Wrong pre-charge is the most common failure in a well system. Too high and the tank waterlogs; too low and drawdown collapses. A tyre gauge checks it.

Constant-pressure pumps change all of this

A variable-speed pump modulates instead of cycling, so it needs only a small tank. If the system has one, size by the manufacturer's table rather than the rule above.