What size expansion tank do you need?
Enter the water, the temperatures and the pressures, get the tank volume and what it must accept.
Add the water in hot pipes and any storage tank that heats with the heater; the pipe volume calculator gives it by length.
Amtrol's factors assume 40 °F water. Watts tells installers to use 90 °F unless the whole heater is drained at each use. The table below the answer shows 40, 60 and 90 °F.
The IRC caps static pressure at 80 psi. Amtrol's tables use maximums of 100, 125 or 150 psi; Watts says to stay under 85% of the relief valve rating.
Boiler, pipes, radiators and fittings; see the pipe volume calculator for the pipes.
A relief valve starts to open about 10% below its set point, so design for 27 psig under a 30 psig valve (Bell & Gossett).
For example 0.0179 for 40 to 150 °F in Amtrol's table. Leave empty to use the density of water, which gives a slightly larger tank.
How do I size a thermal expansion tank for a water heater?
Work out how much the water expands, then divide by the share of the tank that can fill. Water expands by its cold density divided by its hot density, minus one: heating 50 gallons from 60 °F to 140 °F makes it 1.6% bigger, or 0.80 gallon. A precharged diaphragm tank set to the supply pressure can only take the part of its volume between the supply pressure and the highest pressure you allow, with both counted as absolute pressure (gauge plus 14.7 psi): 1 − 74.7 ÷ 114.7 = 0.349 for a 60 psi supply and a 100 psig limit. The tank volume is 0.804 ÷ 0.349 = 2.31 gallons. Bell & Gossett, Watts and Amtrol all use this formula; Amtrol writes the second step as a Design Pressure Factor, (maximum + 14.7) ÷ (maximum − supply), here 2.87.
Do I need an expansion tank?
Often, yes. The International Residential Code (2021 and 2024, P2903.4) asks for a device that controls the pressure from thermal expansion where a backflow preventer, check valve or similar device on a supply with storage water heating keeps expanded water from flowing back (P2903.4.2), or where the pressure past a pressure-reducing valve rises above its setting (P2903.4.1). A tank is the usual device, though the code does not name it. A.O. Smith's tank manual says that without one the relief valve will discharge on every heating cycle. The IRC also limits static pressure to 80 psi and requires a pressure-reducing valve above that (P2903.3.2). Local rules differ, so have a licensed plumber confirm what your house needs.
Why is my answer different from the maker's chart?
This calculator uses the expansion of the water alone, from its density (Kell's 1975 formula, checked against NIST data). Amtrol's and Watts' published factors are lower: for water warmed from 40 °F to 150 °F, Amtrol's table gives 0.0179 where water alone gives 0.0201, about 12% more. The gap grows by almost exactly 2.0 × 10⁻⁵ for every degree of warming, as if the expansion of a steel pipe or tank had been taken off; Bell & Gossett's formula subtracts the pipe's expansion explicitly. Neither maker says how its table was built, so ours is the safer figure. To match a maker's method, type its factor in the optional field: Amtrol's own example (240 gallons, 40 to 150 °F, 60 psi supply, 125 psi maximum) accepts 4.3 gallons and calls for a 9.0 gallon tank with its rounded pressure factor of 2.1, or 9.2 gallons with the exact 2.15. The starting temperature matters too: Amtrol's factors assume 40 °F water, while Watts tells installers to use 90 °F unless the whole heater is drained at each use.
How do I size an expansion tank for a heating system?
Use the same formula with the loop's water volume, the fill temperature, the average water temperature at full heat, the fill pressure at the tank and the highest pressure you will allow. Amtrol's boiler charts assume a 12 psig fill, a 30 psig relief valve and 200 °F average water. A relief valve starts to open about 10% below its set point, so Bell & Gossett's representative advises designing for 27 psig under a 30 psig valve. Example: 100 gallons filled at 60 °F and heated to an average of 180 °F expand 2.95%, or 2.95 gallons; with a 12 psig fill and a 27 psig maximum the pressure factor is 2.78 and the tank is 8.2 gallons. A plain compression tank, with air at atmospheric pressure, has to be much bigger: for a 30 psig fill and 50 psig maximum the same expansion needs about 3.0 times the diaphragm tank's volume.
What air pre-charge should the tank have?
The same as the water pressure at the tank: the supply pressure for a water heater, the fill pressure for a heating loop. Set it with the tank empty of water, before it goes on the line (Amtrol, Watts, Zilmet and Bell & Gossett all say so). Amtrol's tanks ship at 40 psi, and with a 70 psi supply the water would squeeze the diaphragm before it even warms up, so the tank would take less than you sized it for. A tank that is too small or poorly charged lets the pressure climb until the relief valve opens. On a water heater the tank goes on the cold line after the backflow preventer or pressure-reducing valve, supported if it hangs horizontally (Zilmet). When in doubt, take the next larger size.
Sources: Amtrol, “Sizing the Therm-X-Trol” (MC10007) and Therm-X-Trol catalog (MC 4401); Watts, Expansion Tank Sizing Calculator (Potable) and ET series installation sheet; R. L. Deppmann, “Monday Morning Minutes” on Bell & Gossett expansion tank sizing (2011–2012, 2018, 2024); A.O. Smith, thermal expansion tank manual; Zilmet USA, Thermal Expansion Tank Guide; International Residential Code 2021 and 2024, P2903.3.2 and P2903.4 (ICC, codes.iccsafe.org, read October 7, 2026); G. S. Kell, J. Chem. Eng. Data 20 (1975) 97, checked against the NIST Chemistry WebBook (IAPWS-95). Water in pipes: pipe volume calculator; the heater itself: water heater size calculator; running costs: heating cost calculator. How we check our calculators.