EXPANSION TANK SIZE
EXPANSION TANK SIZE
V_tank = V_water × (Vexp ÷ Pa) × (Pa − Pp) ÷ (Ph − Pp)
gallons
PSI
PSI
°F
RESULT
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FILL IN ABOVE
Required by code (UPC 608.3, IPC 607.3.1) on any closed system. Check for: backflow preventer, PRV, or check valve at the meter. Estimate only — verify with a licensed plumber and local plumbing code/inspector before purchase or installation. Not a substitute for engineered drawings.
About this calculator
Closed plumbing systems (those with a check valve, pressure-reducing valve, or backflow preventer between the meter and the water heater) require a thermal expansion tank to absorb the volume increase as the heater warms cold water. Without it, every heating cycle pressurizes the system above safe limits, eventually rupturing the tank, blowing fittings, or triggering the T&P relief valve. This calculator sizes the expansion tank based on water heater capacity, supply pressure, and target heating range using the standard ASME formula. ESTIMATE ONLY — verify with a licensed plumber and local plumbing code before installation.
Common questions
Do I need an expansion tank with my water heater?
Yes if your system is closed. A closed system has a check valve, pressure-reducing valve (PRV), or backflow preventer between the water meter and the water heater — meaning expanding hot water has nowhere to go back to. Most modern homes have a PRV at the meter (incoming municipal pressure exceeds 80 PSI in many areas), which makes the system closed by code definition. UPC 608.3 and IPC 607.3.1 both require an expansion tank in this case.
What size expansion tank for a 50-gallon water heater?
For typical residential setup (60 PSI supply, 90°F temperature rise, 80 PSI max), a 50-gallon water heater needs about 1.85 gallons of expansion volume → standard 2-gallon expansion tank. Higher supply pressure or larger water heater pushes you to a 4.4-gallon tank. Always go up to the next standard size; under-sized tanks fill completely and stop accepting expansion, which is the same as having no tank.
Where does the expansion tank install?
On the cold water inlet side of the water heater, between the heater and the shut-off valve. Hanging vertically (tank below pipe) is preferred so debris drains out of the bladder area. Pre-charge the tank air pressure to match the supply pressure (typically 60 PSI) before installing — most tanks ship at 40 PSI from the factory and need to be adjusted. Use a Schrader-valve gauge with the tank disconnected from water pressure.