Expansion Tank Sizing Calculator

Water expands between cold fill and hot running, and a sealed loop has no way to release that volume - so the tank absorbs it. Size it against the system volume, the temperature swing and the pressure window allowed by the relief valve.

Expansion Tank Sizing Calculator gives the tank volume in gallons and litres for closed hydronic loops.

• Fluid factors: Water, 30% or 50% glycol.
• Total volume: System volume feeds the math.
• Pressure window: Precharge, fill and relief.
• Accept volume: Real tank capacity used.
• Live update: Results while you type.
• Export: Text, CSV or JSON report.

1. Enter total system volume (pipes, radiators, boiler).
2. Pick fluid and the high temperature.
3. Set precharge, fill pressure and relief.
4. Read tank size in gallons and litres.
5. Export the result if needed.

Example 1. A 50-gallon loop of water heating to 190 F running at 12 psi with a 30 psi relief and a 12 psi precharge needs roughly a 3-gallon tank.

Example 2. Glycol at 50% expands faster - the same 50-gallon loop wants closer to a 4-gallon tank, which is why manufacturers ask for the fluid type.

• Closed-loop correct: Standard accept-volume method.
• Glycol aware: 30% / 50% mix factors.
• Dual units: Gallons and litres.
• Free and private: No uploads, no logging.