Expansion Tank Sizing Calculator

Size thermal expansion tanks for hot water systems based on system volume, temperature range, and pressure settings.

Inputs

System Water Volume

gallons

Cold Fill Temperature

°F

Maximum Hot Temperature

°F

Fill Pressure

psi

Relief Valve Pressure

psi

ASME Section IV: residential T&P typically 150 psi; commercial boilers 125–150 psi; tank must stay below setpoint

Tab to move between fields

Results

Minimum Tank Size

1.49 gallons

≈ 16 soda cans

Pre-Charge Pressure

40 psi

Expansion Volume0.99 gallons

Acceptance Factor0.5

How to Use This Calculator

Enter the system water volume in gallons (sum the water heater tank plus all pipe and radiator volume).
Set the cold fill temperature and maximum hot temperature (typically 40°F and 180°F).
Enter the fill pressure (system pressure when cold) and the relief valve setting.
Review Minimum Tank Size in gallons — select the next standard size up from your supplier.
Set the Pre-Charge Pressure on the tank to match your fill pressure before installing.

How the result changes with Maximum Hot Temperature

Maximum Hot Temperature	Minimum Tank Size	Pre-Charge Pressure
115	0.75 gallons	40 psi
153	1.56 gallons	40 psi
198	2.79 gallons	40 psi
235	4.02 gallons	40 psi

What each input means

System Water Volume: Total water volume in the system (water heater, piping, and all connected equipment).
Cold Fill Temperature: Temperature of the cold water fill supply when the system is cold.
Maximum Hot Temperature: Maximum water temperature setting of the water heater or boiler.
Fill Pressure: Static water pressure at the expansion tank location when cold.
Relief Valve Pressure: Pressure setting of the temperature & pressure (T&P) relief valve per ASME Section IV and ASHRAE 90.1. Standard residential water heater T&P valve: 150 psi; commercial: 125–150 psi. Expansion tank must prevent pressure from reaching relief valve setpoint.

How this is calculated

Worked example, using the default values

Identify Input Parameters
4 parameters
System Water Volume = 50, Cold Fill Temperature = 50, Maximum Hot Temperature = 150, Fill Pressure = 40 = 5 input(s) provided
Calculate Minimum Tank Size
Minimum Tank Size
1.49 = 1.49
Calculate Pre-Charge Pressure
Pre-Charge Pressure
40 = 40
Calculate Expansion Volume
Expansion Volume
0.99 = 0.99
Calculate Acceptance Factor
Acceptance Factor
0.497 = 0.497

Engine last updated July 28, 2026. Checked against 2 independently-derived tests — how we verify calculators.

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