SettingsSettings for this calculationUS
The rated storage capacity of your water heater, in the unit shown.
Check the tank's data plate.
The percentage volume increase as water heats up.
About 2% is a commonly used estimate for water heated from typical cold-supply temperature to a standard water heater setpoint.
Minimum expansion tank acceptance volume
1 gal
This estimates the minimum acceptance volume only — actual expansion tank selection also depends on your system's static water pressure and the tank's precharge setting, which should match your incoming water pressure. Consult the tank manufacturer's sizing chart or a licensed plumber.
They open the calculator with your figures already in it
Thermal Expansion Tank Sizing Calculator: 1 gal — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Water expands roughly 2% in volume when heated from typical cold-supply temperature to a standard 120-140°F water heater setpoint; a closed system (with a check valve, backflow preventer, or pressure-reducing valve) needs an expansion tank sized to accept this volume
Inputs used
- Water Heater Tank Size
- 50 gal
- Thermal Expansion (%)
- 2
Confidence note: This estimates the minimum acceptance volume only — actual expansion tank selection also depends on your system's static water pressure and the tank's precharge setting, which should match your incoming water pressure. Consult the tank manufacturer's sizing chart or a licensed plumber.
What this calculation does not cover
- The only water in the model is what the heater stores. Pipework, a recirculation loop and any downstream storage hold water that expands as well, so on a long distribution run or a recirculating system the true expansion volume is larger than the figure shown.
- This is the volume of water the tank must accept, not the size of tank to buy. How much of a bladder tank's nominal capacity is usable acceptance depends on the gap between its precharge and the highest pressure the system is allowed to reach, so the tank you order is normally larger than this number — match it against the manufacturer's acceptance rating rather than its nameplate capacity.
- No pressure is entered anywhere in this calculation. It cannot confirm that the tank precharge matches your static supply pressure, and it does not check that system pressure stays below the water heater's temperature and pressure relief setting — a correctly sized tank charged to the wrong pressure still lets pressure spike, and nothing here would show it.
- The expansion percentage is a figure you type, not one derived from your incoming water temperature and heater setpoint. A colder cold-water supply, a higher setpoint, or a solar or heat pump cylinder swinging over a wider temperature range all expand more than the domestic default assumes.
- It does not establish whether you need a tank or whether the installation complies. Nothing tests for the check valve, backflow preventer or pressure-reducing valve that makes a system closed, and the heater size field stops at about 380 L (100 US gallons), so commercial storage and boiler or hydronic loops fall outside it. Treat the result as an estimate to check against local code and a licensed plumber, not as a sign-off.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.1.1
Regulatory standards & verification citations1
- Water expands roughly 2% in volume when heated from typical cold-supply temperature to a standard 120-140°F water heater setpoint; a closed system (with a check valve, backflow preventer, or pressure-reducing valve) needs an expansion tank sized to accept this volume
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