Pressure Tank Size Calculator
A pressure tank's job is to let your pump rest. The rule that protects the pump: once it starts, it should run at least one minute — ideally 1.5 to 2. That minimum run time plus your pump's flow rate sets the drawdown you need, and drawdown (not the number on the tank) is what you're actually buying.
Enter three things below and get the nominal tank size that keeps your pump from short-cycling itself to death. Undersizing is the expensive mistake here — a too-small tank quietly wears out a $2,000 pump.
How this works
The calculation is the standard two-step tank-sizing method used across the industry. Step one, drawdown: multiply your pump's flow rate by the minimum run time you want per cycle (drawdown = GPM × minutes). A 10 GPM pump targeting a 1.5-minute run needs 15 gallons of usable water between pump starts. One minute is the accepted floor; 1.5–2 minutes measurably extends motor life because starting current — not running — is what wears a pump out.
Step two, converting drawdown to a nominal tank size: a diaphragm tank only yields part of its rated volume as usable water, and that share shrinks as system pressure rises. The standard drawdown factors are 0.31 at a 30/50 switch, 0.28 at 40/60, and 0.25 at 50/70 — so required nominal size = drawdown ÷ factor (roughly four times the drawdown). That 15-gallon drawdown at 40/60 means 15 ÷ 0.28 ≈ 54 gallons, which rounds up to the next standard nominal size: 85 gallons. We round up through the standard lineup (20, 30, 50, 85, 120 gal) because between sizes, bigger only costs a little more while smaller costs pump life; needs beyond 120 gallons call for twin tanks or a constant-pressure system.
Real installs add details this calculator can't see: the tank's air precharge must be set 2 PSI below the switch cut-on, high-flow pumps may pair better with a variable-speed controller than a huge tank, and a short-cycling system may already have a damaged pump. Replacement tanks run $400–$1,500 installed — the independent local pros we connect you with will confirm the size against your actual pump before quoting.
Estimates only — independent local providers quote their own pricing. Data last reviewed 2026-07.
Frequently Asked Questions
What size pressure tank do I need?
Multiply your pump's GPM by a minimum run time of 1–2 minutes to get required drawdown, then divide by the drawdown factor for your switch setting (0.31 at 30/50, 0.28 at 40/60, 0.25 at 50/70). A typical 10 GPM pump at 40/60 needs about 15 gallons of drawdown — an 85-gallon nominal tank.
Is a bigger pressure tank better?
Generally yes, within reason. A larger tank means longer run cycles and fewer starts, and starts are what kill pump motors. The tradeoffs are only price and floor space — oversizing doesn't hurt the system. When you're between standard sizes, go up.
Why does a 30-gallon tank only hold about 8 gallons of water?
Nominal size is the tank's total volume, but most of it holds the compressed-air cushion behind the diaphragm. Only 25–31% is usable water (drawdown), depending on your pressure switch setting — higher pressure ranges leave less room for water. That's why sizing works from drawdown, not the sticker number.
How much does a new pressure tank cost installed?
Most replacements run $400–$1,500 installed, driven by tank size and any related parts (pressure switch, relief valve, fittings). It's a same-day job. If the old tank failed by waterlogging, have the installer check the pump — short cycling may have already shortened its life.
Prefer to just talk to someone?
Call or send the short form — we'll route you to an independent local pro.