Well Pressure Tank Calculator
A twenty gallon pressure tank holds about six gallons of usable water. The rest of the volume is air that has to stay compressed to hold system pressure, and it never comes out of the tap. That gap between the label and the delivery is why so many wells short cycle: the tank was chosen by volume when the number that matters is drawdown, and the requirement that sets it is not household demand but the minimum time the pump has to run to survive.
- Usable water between pump starts
- 5.9 gal From a 20 gallon tank on a 30/50 switch. Only 30% of the tank volume is usable — the rest is the air charge that has to stay compressed to hold pressure. This is the number that matters, and it is not the number on the tank.
- Drawdown needed for a 1 minute run
- 10.0 gal 10 gpm × 1 minute. Every start heats the pump motor, and a pump that starts and stops every twenty seconds fails years early. Minimum run time is what sizes the tank — not the household size, and not the number of bathrooms.
- Actual run time per cycle
- 0.6 min Below the 1 minute minimum. At a continuous 10 gpm draw the pump will start roughly every 35 seconds, which is the classic cause of short pump life.
- Tank size that would work
- 40 gal To get 10.0 gallons of drawdown at 30/50 you need roughly 34 gallons of tank, rounded up to the next common size. Drawdown scales linearly with tank volume, so doubling the tank doubles the run time.
- Pre-charge is correct
- 28 psi (should be 28) Two psi below cut-in, which is the standard setting. Check it with the tank drained and no water pressure on it — measuring against a pressurised tank gives the water pressure, not the air charge.
- If you assumed the full tank volume
- 20 gal vs 5.9 gal Sizing on the tank label overstates the buffer by a factor of about 3.4. It is the reason so many wells are fitted with a tank a third of the size they needed.
- Raising the pressure setting costs drawdown
- 5.2 gal at 40/60 The same tank gives 13% less at the higher setting. Compressed air resists further compression, so a narrower proportional band at higher pressure stores less water. People raise the pressure for better showers and shorten their pump life doing it.
- Widening the band instead
- 7.7 gal at 30/60 Raising only the cut-out and leaving cut-in alone widens the band and increases drawdown substantially, at the cost of a larger pressure swing at the tap. It is the cheapest way to buy run time on an existing system, within the pressure rating of the tank and plumbing.
Drawdown is a fraction of the tank
A bladder pressure tank is a sealed vessel with air on one side of a diaphragm and water on the other. The air is charged to just under the cut-in pressure, so at the moment the pump starts the tank is nearly empty of water and the air fills almost all of it.
As the pump runs, water enters and compresses the air. When pressure reaches cut-out the pump stops. The water that entered during that stroke is all the water the tank can deliver, and it is governed by Boyle's law: how much a fixed mass of air compresses between two pressures.
For a thirty/fifty system that works out to about thirty percent of the tank volume. For a forty/sixty system it is closer to twenty-six percent, because the same ten psi band represents a smaller proportional change at higher pressure.
Manufacturers publish drawdown tables and they agree with this arithmetic closely. What almost nobody does is put the drawdown figure on the front of the tank, where the volume goes.
Run time, not demand, sizes the tank
The instinct is to size a pressure tank against household demand — more people, more bathrooms, bigger tank. That is not what the tank is for. Sustained demand is met by the pump, which can run all day; the tank only covers the gap between starts.
What the tank actually protects is the pump motor. Each start draws locked-rotor current for a moment and dumps heat into the windings, and the motor sheds that heat while it runs. Start it again before it has cooled and the heat accumulates. A submersible pump rated for a hundred starts a day and given a thousand does not last a quarter as long — it fails much faster than that.
So the sizing rule is a minimum run time, commonly one minute for small pumps and two for larger ones. Multiply the pump flow rate by that minimum and you have the drawdown required. Divide by the fraction of tank volume that is usable and you have the tank.
A ten gallon per minute pump on a one minute minimum needs ten gallons of drawdown, which at thirty percent is a thirty-five gallon tank or larger — well beyond what a homeowner replacing a leaking tank would typically pick off the shelf.
Pre-charge is the free repair
The air charge should sit two psi below the cut-in pressure. Set higher, the tank runs out of water before the pump starts, contributing nothing and letting the pump cycle on every tap. Set lower, the tank holds a permanent reserve of water it never gives back, and the usable drawdown falls.
Air migrates through the diaphragm slowly, so pre-charge drops over years. A tank that used to give a minute of run time and now gives fifteen seconds usually has nothing wrong with it except air pressure.
Checking it takes a tyre gauge and five minutes. The critical detail is that the tank must be drained and isolated first — with water pressure in it, the schrader valve reads the water side and gives a number that looks correct no matter what the air charge is.
If the tank will not hold pressure at all, the diaphragm has failed and the tank is waterlogged. That one does need replacing, and it is worth sizing the replacement on drawdown rather than matching the volume of the one coming out.
What this is based on
- Boyle’s law applied between cut-in and cut-out at absolute pressure
- Standard pre-charge practice — 2 psi below cut-in
- Pump manufacturer minimum run time and maximum starts per day guidance
A drawdown and sizing estimate for a bladder or diaphragm pressure tank. It assumes sea level atmospheric pressure and a tank in good condition. It does not size the pump itself, the well yield, or constant pressure systems with variable speed drives, which use a much smaller tank on a different principle.
Frequently asked questions
How much water does a 20 gallon pressure tank hold?
About six gallons of usable water on a 30/50 pressure switch, and around five on 40/60. The rest of the volume is the compressed air charge that holds system pressure and never leaves the tank.
What size pressure tank do I need?
Enough drawdown to give the pump its minimum run time — pump flow rate multiplied by one minute for most small pumps. A ten gallon per minute pump needs about ten gallons of drawdown, which means a thirty-five gallon tank or larger.
What should the pre-charge be?
Two psi below the cut-in pressure, so 28 psi on a 30/50 switch. Check it with the tank drained and isolated from system pressure — measuring a pressurised tank reads the water side and tells you nothing about the air charge.
Why does my pump cycle rapidly?
Almost always low drawdown: a tank that is too small, or one whose pre-charge has bled away over the years. Both produce the same symptom, and the pre-charge is worth checking first because it costs nothing to fix.
Does raising the pressure give me more water?
Higher pressure at the tap, less water in the tank. Compressed air resists further compression, so the same ten psi band stores less water at 40/60 than at 30/50. Raising only the cut-out, and leaving cut-in where it is, widens the band and increases drawdown instead.