How to Use the Expansion Tank Sizing Calculator
In a closed hydronic piping system, water expands as its temperature rises. Without somewhere for that extra volume to go, pressure can climb until the relief valve discharges, or worse, stresses piping and equipment. An expansion tank absorbs that growth safely — too small and pressure spikes quickly, too large and you pay for unused capacity, so sizing it correctly matters.
This calculator uses the physical relationship between water temperature and density (based on the Kell density equation) to find the expansion fraction between your fill temperature and max design temperature, then applies an acceptance factor set by your fill pressure and relief valve setting (max allowable pressure) to size a diaphragm-type expansion tank. The formula is tank volume = expansion volume ÷ (1 − fill pressure(abs) ÷ max pressure(abs)), a standard approach in hydronic system design.
When selecting an actual product, it's common practice to round up from the calculated minimum, so this calculator also shows a recommended size with a 10% margin. Always confirm actual system water volume, relief valve setting, and final tank selection with a mechanical engineer before finalizing your design.
Frequently Asked Questions
Fill pressure is the gauge pressure right after filling the system with water, and max allowable pressure is the relief valve setting. The larger the gap between them, the smaller the tank needs to be.
Not recommended. An undersized tank lets pressure rise above the relief valve setting as temperature climbs, causing frequent relief valve discharge or stress on the piping system.