Thermal Expansion Tank Sizing Calculator
Size a potable-water or hydronic expansion tank in gallons or litres: water or glycol expansion between fill and maximum temperature (°F or °C), required acceptance volume, acceptance factor from the fill pressure and maximum pressure (psig, bar or kPa), wrong pre-charge effects, the next standard tank, peak pressure for each tank size, a candidate tank check and a suggested hydronic fill pressure from building height.
How it works
Expansion = V × (v_hot ÷ v_cold − 1) × fluid factor, using the specific volume of water; acceptance factor = 1 − P_fill(abs) ÷ P_max(abs) (with a different pre-charge P₀: P₀ × (1 ÷ max(P_fill, P₀) − 1 ÷ P_max), all absolute); tank volume = expansion ÷ acceptance factor; peak pressure = V_tank × P₀ ÷ (air volume at fill − expansion) − 14.7; pre-charge = static fill pressure; hydronic fill ≈ 0.433 psi/ft × height + 4 psi
Water expands about 2% between cold fill and a 140 °F water heater set-point, and in a closed system — one with a check valve, pressure-reducing valve or backflow preventer — that extra volume has nowhere to go, so pressure climbs until the relief valve lifts. An expansion tank holds a cushion of air behind a diaphragm; as the water expands it compresses the air, and Boyle’s law says how much of the tank’s volume that cushion can absorb between the fill pressure and the maximum you will allow. Pre-charging the air side to the fill pressure keeps the diaphragm neutral until heating starts, so the full acceptance volume is available. A tank left at its factory 40 psi charge in a 60 psi house is partly full of water before the heater even fires; enter the actual pre-charge under More options to see how much capacity that costs. Hydronic systems use the same arithmetic on the whole loop volume and a lower pressure range, and glycol mixes expand more than plain water. The peak-pressure table shows what each standard tank size would do, so you can pick one with margin rather than the bare minimum.
Worked example
A 50-gallon water heater filled at 50 °F and heating to 140 °F expands about 0.84 gallons (1.7%). With 60 psig fill and 130 psig allowed, 48.4% of a tank’s volume is usable, so it needs at least 1.74 gallons — a standard 2-gallon tank pre-charged to 60 psig works and peaks near 114 psig; a 4.4-gallon tank peaks near 78 psig. If the same 2-gallon tank were left at a 40 psig factory charge, only about 35% of it would be usable and it would be too small.
Sources: Watts — expansion tank sizing calculator (potable) · Watts — expansion tank sizing calculator (non-potable / hydronic)
Frequently asked questions
Where does it go?
On the cold-water inlet between the check/PRV and the heater (any orientation the maker allows), or on the return side near the boiler in hydronics; keep the pre-charge gauge port accessible.
How do I know it has failed?
Tap it — a waterlogged tank sounds solid; pressing the Schrader valve releases water instead of air. Check the charge yearly with the water side depressurised.
What should the pre-charge be?
The same as the static cold-water pressure at the tank: for a house with 60 psi after the PRV, set the air side to 60 psi before connecting. Enter a different actual charge under More options to see the effect of a mismatch.
What size expansion tank for a 50 or 80 gallon water heater?
At 60 psig house pressure and a 140 °F setting, a 50-gallon heater needs about 1.7 gallons of tank (a 2-gallon model) and an 80-gallon heater about 2.8 gallons (a 4.4-gallon model). Higher house pressure or hotter water needs a bigger tank.
Does glycol need a bigger tank?
Yes. Propylene and ethylene glycol mixes expand roughly 15–35% more than water over the same temperature range, so choose the fluid and the calculator scales the expansion. Use the glycol maker’s data for a final design.
Related calculators
Hydraulic estimate with standard formulas and typical fluid properties (water unless stated); verify against pump curves, pipe data and the applicable code before specifying equipment.