Well Pump Pressure Tank Sizing
Size on drawdown, not tank volume. Penn State Extension puts usable water at about 20 percent of tank capacity — a 42-gallon tank discharges roughly 8 gallons before the pump restarts. The goal is enough drawdown that your pump meets its manufacturer's minimum run time per cycle, which is a figure only their tables can give you.
Sources last checked 2026-08-31 · next review due 2027-08-31 · reviewed by HomesteadingGear Editorial
Tank volume is not water volume
This is the misconception the whole topic turns on. A pressure tank is mostly air, and the air is the working part: it is what pushes water back out and holds pressure while the pump is off. The gallon figure on the label describes the vessel, not its contents.
| Tank size | Water it actually discharges | Share |
|---|---|---|
| 42 gallons | about 8 gallons | ~19% |
| 82 gallons | about 16 gallons | ~20% |
| 120 gallons | about 24 gallons | ~20% |
Penn State Extension. The share column is arithmetic on their figures, and their own summary is that about 20 percent of tank capacity contains usable water.
The pressure switch sets the drawdown
Penn State describes the common arrangement: the switch stops the pump at about 40 psi and restarts it when pressure falls to about 20 psi. Everything the tank gives you between those two points is your drawdown.
That has a consequence people miss. The switch span and the tank are one system. Narrowing the span — say to hold higher minimum pressure at a shower — reduces drawdown from the same tank, and the pump then cycles more often. Changing the switch without reconsidering the tank is how a system that used to be fine starts short-cycling.
Why minimum run time is the real constraint
A submersible pump motor is hardest hit at the moment it starts. Manufacturers therefore publish a minimum run time per cycle and a maximum number of starts per hour. Those two figures, together with the pump's flow rate in gallons per minute, are what decide the drawdown you need.
The relationship is simple enough to state: a pump delivering more gallons per minute empties a given drawdown faster, so a higher-flow pump needs a larger drawdown to run for the same length of time. A tank that suited a modest pump can be badly undersized after the pump is replaced with a stronger one — which is exactly when people discover the problem.
We are not publishing a formula here that turns your flow rate into a tank size. The published minimum run times differ by manufacturer and model, and a general equation applied to a specific pump is precisely the kind of confident answer that damages equipment.
Common mistakes
- Buying on tank gallons. Two tanks with the same label can have different drawdowns. The drawdown figure is the specification that matters and reputable manufacturers publish it.
- Ignoring air charge. A bladder tank with the wrong pre-charge, or a waterlogged tank that has lost its air, gives almost no drawdown regardless of its size. A tank that suddenly short-cycles usually has an air problem rather than a sizing problem.
- Sizing for peak demand only. Drawdown protects the pump from cycling; it is not household storage. If the well itself cannot keep up with demand, a bigger pressure tank is not the fix.
- Changing the switch alone. Covered above, and it is the most common self-inflicted version of this problem.
When to call a professional
Before you buy a tank, and always after a pump change
Penn State Extension's own advice is that a local water well contractor can provide guidance and a cost estimate. That is the right call here, not a hedge.
The failure mode is not an inconvenience. An undersized tank makes a pump start far more often than it was designed to, and pump replacement — particularly a submersible one — costs many times what getting the sizing right costs. Bring your pump's model number, its flow rate, and your current switch settings to that conversation and it will be a short one.
The equipment decision this leads to
- Well pump guide — how the pump, tank and switch fit together, and where the professional boundary sits
Related reading
Where these numbers come from
- Penn State Extension — Using Low-Yielding Wells
Primary source · read 2026-08-31
- About 20 percent of the capacity of a pressure tank contains usable water
- A 42-gallon tank discharges about 8 gallons before the pump starts; an 82-gallon tank about 16 gallons; a 120-gallon tank about 24 gallons
- A pressure-activated switch stops the pump at about 40 psi and restarts it at about 20 psi
- A local water well contractor can provide guidance and a cost estimate
Common questions
How do you size a well pressure tank?
By drawdown, not by tank volume. Penn State Extension states that about 20 percent of a pressure tank's capacity contains usable water: a 42-gallon tank discharges about 8 gallons before the pump restarts, an 82-gallon tank about 16, and a 120-gallon tank about 24. Sizing means choosing a drawdown large enough that the pump runs long enough per cycle, which is a calculation your pump manufacturer's tables govern.
Why is a 42-gallon tank only 8 gallons of water?
Because a pressure tank is mostly air. The air is what pushes water out and maintains pressure between pump cycles. Penn State Extension puts the usable share at about 20 percent of tank capacity, so the number on the tank is a physical volume rather than a water volume.
What are typical pressure switch settings?
Penn State Extension describes a common arrangement where a pressure-activated switch stops the pump at about 40 psi and restarts it when pressure drops to about 20 psi. That 20 psi span is what determines how much water leaves the tank between cycles, so changing the switch changes your drawdown.
What is short-cycling and why does it matter?
Short-cycling is a pump starting and stopping rapidly because the tank releases too little water per cycle. Motor starts are the hard part of a pump's life, so a system that cycles frequently wears out far sooner than one with adequate drawdown. It is the single most common consequence of an undersized or waterlogged tank.
Can I size a pressure tank myself?
You can understand the arithmetic, but the binding constraint is your pump manufacturer's minimum run time and flow rate, which come from their tables rather than a general rule. Penn State Extension advises that a local water well contractor can provide guidance. Getting this wrong damages equipment rather than merely disappointing you.