Well Water Pressure Tank Size Calculator

For a 20 GPM pump with a standard 30/50 psi pressure switch, the recommended cycle time is about 1–2 minutes to prevent short cycling. Using the middle value of 1.5 minutes: drawdown = 20 × 1.5 = 30 gallons. Precharge = cut-in minus 5 psi = 25 psi. Then Vtank = 30 ÷ (1 − 25/50) = 60 gallons raw. With the 0.75 acceptance factor for a bladder tank, you need 60 ÷ 0.75 = 80 gallons. That is a large tank — most residential 20 GPM installations pair with a 44–60 gallon tank and accept a slightly faster cycle. The calculator above lets you tune the cycle time to find the sweet spot.

Precharge (P1) is the air pressure inside the bladder or diaphragm when the tank is empty of water. When you open a faucet, water flows out until the tank pressure drops to the cut-in pressure (Pmin = 30 psi), then the pump kicks on and refills to cut-out (P2 = 50 psi). The precharge must be below the cut-in pressure so the bladder has room to expand — if precharge equals or exceeds cut-in, no water can enter the tank. The standard rule is P1 = PcutIn − 5 psi, so for 30/50 that is 25 psi. This gives a 5 psi 'window' that ensures the tank is nearly empty before the pump starts, maximizing drawdown.

Because pressure tanks are not water tanks — they are air tanks with water squeezed in. The rated '20 gallon' tank means the shell holds 20 gallons total, but the bladder only compresses to about 75% of that (the acceptance factor). For a 20 gal tank at 30/50 psi with 25 psi precharge: Vdraw = 20 × (1 − 25/50) × 0.75 = 20 × 0.5 × 0.75 = 7.5 gallons of usable water. That is why you need a surprisingly large tank to get a modest drawdown.

A bladder tank has a flexible rubber bladder inside a steel shell. The bladder expands as water enters, compressing air trapped between the bladder and the shell wall. These are the most common residential type. A diaphragm tank uses a flat rubber diaphragm clamped at the rim — less flexible, so acceptance factor is lower (0.70 vs 0.75), which means you need a physically larger tank for the same drawdown. Diaphragm tanks are usually smaller and used for point-of-use applications. Both types follow the same sizing formula; only the acceptance factor changes.

Yes — a gravity tank (elevated cistern) stores water above the plumbing line, and the water column creates pressure without a pump running continuously. A 40-foot elevation gives about 17 psi (0.433 psi/ft). Gravity tanks follow different physics — the pressure depends on height, not compressed air — so the sizing formula is just Vdraw = πr²h for the tank, and you size by the peak flow rate you need to cover before the level drops below your minimum pressure. The gravity tank's disadvantage is the installation height requirement; the pressure tank's disadvantage is the compressed-air complexity. Many rural homes use a hybrid: a well pump fills a gravity tank, which then feeds the house.

Short cycling. Every time you open a faucet, the pump kicks on and off rapidly — maybe 10–20 times per minute instead of 2–3. This wears out the pressure switch contacts, the pump motor bearings, and the check valve, and also delivers water at fluctuating pressure (the classic 'pulsing' faucet). A properly sized tank lets the pump run for at least 60 seconds on each cycle and stay off for at least 30 seconds. Short cycling is the #1 cause of premature well pump failure — a $1,500–$3,000 mistake that a $300 bigger tank prevents.

First, drain all water from the tank — open the nearest faucet and let it run until the tank is completely empty. Then check the pressure with a tire valve gauge at the Schrader valve on top of the tank. For a 30/50 psi switch it should read about 25 psi. If it is too high, press the valve stem to release air. If it is too low, add air with a compressor or tire inflator. Refill the tank by closing the faucet and letting the pump run until it cuts off at 50 psi. The precharge check should be done with the tank empty — that is the only time the air pressure is unopposed by water.

A booster pump pressurizes water from a lower-pressure source (city water at 40 psi, or a gravity tank at 15 psi) up to a higher system pressure (say 60–80 psi). The pressure tank on the outlet side of the booster pump is sized exactly like a well-water tank — using the booster's GPM, the target pressure switch range, and the same Vdraw = Vtank × (1 − P1/P2) formula. The only difference is that the inlet side has its own pressure; some booster systems also need a small inlet-side tank to prevent the main pump from cycling with each demand fluctuation.