What Size Pressure Tank Do I Need for My Well?

Pressure tank size calculator

Pump flow (GPM)From the pump's curve, or measured with the bucket test.

Motor size Under 2 HP — 1 minute minimum run2 HP or more — 2 minutes minimum runCustom run time

Switch setting 20/40 psi30/50 psi40/60 psiOther

Tank precharge (psi)Leave blank to use cut-in − 2 psi.

Elevation (feet above sea level, optional)

Your current tank's total volume (gallons, optional)From the tank label. We'll estimate its drawdown and pump run time.

Calculate

This tool is for educational guidance and does not replace an onsite diagnosis. Method: Water Well Journal (minimum run time and drawdown) and The Driller (Boyle's law with absolute pressures). Assumes constant pump flow and ignores temperature — round up, and check the tank maker's drawdown chart.

Why size by the pump

If you're not sure how a tank stores water, start with how a pressure tank works.

A pressure tank's job is to limit how often the pump starts. Franklin Electric, which makes most U.S. submersible pump motors, asks for at least one minute of running per start and caps single-phase motors at 300 starts a day up to ¾ HP and 100 starts a day from 1 to 5½ HP.^[2] Whether your tank meets that depends on how fast the pump fills it, which is why a tank sized by household size (a common rule of thumb in cost guides) can be too small for a larger pump.

The method, step by step

Step 1: Find the pump's flow rate

• From the pump: the model number (on the pump's paperwork, the control box label, or the installer's invoice) leads to the manufacturer's performance curve, which shows GPM at your pressure and pumping depth.
• By measurement: with no water in use, time how long the pump takes to go from cut-in to cut-out, then run the bucket drawdown test. Gallons collected ÷ run time in minutes ≈ pump flow in that pressure range. It's an estimate — pump output falls as pressure rises — but it's usually close enough for tank sizing.

Step 2: Choose the minimum run time

Water Well Journal's figures: 1 minute for motors under 2 HP, 2 minutes for 2 HP and above.^[1] Some installers size for longer runs; more is gentler on the pump.

Step 3: Calculate the drawdown you need

Pump GPM × run time. A 10 GPM, ¾ HP pump needs 10 gallons of drawdown.

Step 4: Divide by the drawdown factor

The drawdown factor is the share of the tank's volume that becomes usable water between cut-out and cut-in. Water Well Journal gives it as 1 − (cut-in + 14.7) ÷ (cut-out + 14.7), using absolute pressure (gauge psi + 14.7).^[1] That version assumes the precharge equals cut-in. With the recommended precharge of 2 psi below cut-in, the factor is a little smaller; The Driller's form of Boyle's law handles that case: (P1 ÷ P2 − P1 ÷ P3), with P1 = precharge, P2 = cut-in, P3 = cut-out, all absolute.^[3]

Our calculator uses the second column, because that's how tanks are actually set up. It gives a slightly larger (more conservative) tank.

Worked examples

Round up to the next tank size you can buy, then check the manufacturer's drawdown chart for that model — published figures can differ slightly from the formula.

What to know when you shop

• Gallon ratings are total volume. A "40-gallon" tank does not deliver 40 gallons; at 30/50 it delivers about 12. See the real drawdown by size.
• Raising the pressure shrinks drawdown. Moving from 30/50 to 40/60 costs about 13% of the drawdown from the same tank (factor 0.295 → 0.258). If you're changing settings, re-check the size.
• Two tanks can work as one. Two tanks with the same precharge on the same tee add their drawdowns together, which is a common way to add capacity where headroom is short.
• Check the maximum acceptance. Each tank model has a limit on how much of its volume can be water; Amtrol's sizing handbook says to confirm the calculated factor doesn't exceed the model's maximum.^[4] Residential settings are normally well inside it.
• Wider differentials need the maker's OK. The Driller advises checking with the tank manufacturer before widening the differential beyond 20 psi.^[3]

Sizing for constant-pressure systems

Variable-speed systems need much smaller tanks because the pump slows down instead of cycling. Franklin Electric's generic guidance for variable-frequency-drive systems is a tank of 10–20% of the pump's rated output in gallons (a 100 GPM system → 10–20 gallon tank minimum), with the drive maker's sizing taking priority.^[2] Grundfos's SQE controller manual is written around a small diaphragm tank of 8 liters (about 2 gallons).^[5] See constant-pressure well systems.

Assumptions and limits

• Pump flow is treated as constant between cut-in and cut-out; real pumps deliver a little less as pressure rises.
• Sea-level atmospheric pressure (14.7 psi) unless you enter an elevation in the calculator; altitude slightly increases drawdown.
• Temperature effects and gauge error are ignored — reasons to round up, not down.
• This is educational guidance, not a substitute for an installer's sizing or the tank maker's chart.