Expansion tank calculator: what size expansion tank do I need for my water heater?
0.9 gallons of tank volume
0.5 gal of expanded water × design pressure factor 1.8 · air charge 60 psi · for a 50 gal water heater at 120°F, 150 psi maximum
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Enter it the way you measured it: 4.5, 4 1/2,
3/8 or 10' 6".
0.9 gallons of tank volume
0.5 gal of expanded water × design pressure factor 1.8 · air charge 60 psi
- Tanks ship at 40 psi: set the air charge to 60 psi, your static line pressure, before you install it (AMTROL)
Source: AMTROL, Sizing the Therm-X-Trol, MC 10007 (03/19): the expansion factor table, the design pressure factor and the tank sizing equation, read November 7, 2026. Data sheet (PDF)
A 50 gallon water heater set at 120°F, on 60 psi line pressure with a 150 psi maximum, needs at least 0.9 gallons of expansion tank volume by AMTROL sizing: 0.5 gallons of expanded water times a 1.8 pressure factor.
A worked example
A 75 gallon heater set at 140°F on 72 psi city water, with a 125 psi maximum needs 2.9 gallons of tank volume. 75 × 0.0150 = 1.125, so 1.1 gallons of expanded water. (125 + 14.7) ÷ (125 − 72) = 2.64, a factor of 2.6. 1.1 × 2.6 = 2.86: 2.9 gallons, with the air charge set to 72 psi.
What size expansion tank for a 50 gallon water heater?
At 120°F with a 150 psi maximum: 0.9 gallons of tank volume on 60 psi line pressure, 1.1 on 70 psi and 1.2 on 80 psi. At 140°F the expanded water rises to 0.8 gallons, and the tank to 1.4 gallons on 60 psi.
What pressure should an expansion tank be set to?
The same as your static line pressure, AMTROL says. Its tanks ship at 40 psi: on 60 psi water, raise the air charge to 60 psi before the tank goes in, or the incoming water takes up part of the tank before any heating starts.
What else it is called
Also searched as expansion tank sizing, an expansion tank size chart, a thermal expansion tank calculator or what size expansion tank for a 50 gallon water heater. AMTROL puts the tank on the cold water supply after the backflow preventer, check valve or pressure reducing valve.
Tank volume by heater size and line pressure
| Water heater | 40 psi line | 50 psi line | 60 psi line | 70 psi line | 80 psi line |
|---|---|---|---|---|---|
| 30 gal | 0.5 gal | 0.5 gal | 0.5 gal | 0.6 gal | 0.7 gal |
| 40 gal | 0.6 gal | 0.6 gal | 0.7 gal | 0.8 gal | 1.0 gal |
| 50 gal | 0.8 gal | 0.8 gal | 0.9 gal | 1.1 gal | 1.2 gal |
| 65 gal | 1.1 gal | 1.1 gal | 1.3 gal | 1.5 gal | 1.7 gal |
| 75 gal | 1.2 gal | 1.3 gal | 1.4 gal | 1.7 gal | 1.9 gal |
| 80 gal | 1.2 gal | 1.3 gal | 1.4 gal | 1.7 gal | 1.9 gal |
| 100 gal | 1.5 gal | 1.6 gal | 1.8 gal | 2.1 gal | 2.4 gal |
AMTROL, Sizing the Therm-X-Trol (MC 10007): tank volume = expanded water × design pressure factor. Heater at 120°F (expansion factor .0100 from 40°F water), 150 psi maximum allowable pressure, factor (150 + 14.7) ÷ (150 − line psi) to the tenth, as in its Table 2. Air charge = line pressure.
Where the numbers come from
- AMTROL, Sizing the Therm-X-Trol, Table 1 Expansion Factor, based on 40°F incoming water: 100°F .0062, 120°F .0100, 130°F .0124, 140°F .0150, 150°F .0179, 160°F .0209, 170°F .0242, 180°F .0276. “Tv = Design Pressure Factor x expanded water.” Read November 7, 2026. AMTROL MC 10007
- AMTROL, Table 2: DPF = (Max. Allow. Pressure + 14.7) ÷ (Max. Allow. Pressure − Line Pressure), “For conditions not shown in table, use equation.” “The Therm-X-Trol air pressure should be equal to static line pressure.” “Standard Therm-X-Trol tanks are shipped at 40 psi.” AMTROL MC 10007
- As on the AMTROL worksheet, the expanded water and the pressure factor are rounded to the tenth before multiplying; its own example, 240 gallons at 150°F with 125 and 60 psi, gives 4.3 × 2.1 = 9.0 gallons here too. Only the Table 1 temperatures are offered: they are not interpolated.
AMTROL's factors start from 40°F incoming water, so warmer water expands less than shown. A tank has to meet both figures, the total volume and the expanded water it accepts, so check both on the model's data sheet.
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