Expansion tank sizing calculator
Size a thermal expansion tank for a water heater on a closed system: enter the heater size, temperature and supply pressure, and get the minimum tank volume and acceptance volume using Amtrol's published method.
Precharge the tank to your static supply pressure before it is filled. If your heater, pressure or temperature is outside these ranges, follow the tank manufacturer's sizing instructions — that is what IPC 607.3 requires.
When the code requires one
Water expands as it heats. On an open system the extra volume pushes back into the street main; once a check valve, pressure-reducing valve or backflow preventer closes that path, every heating cycle raises the pressure until the relief valve drips. The International Plumbing Code puts it this way:
“Where a storage water heater is supplied with cold water that passes through a check valve, pressure reducing valve or backflow preventer, a thermal expansion control device shall be connected to the water heater cold water supply pipe at a point that is downstream of all check valves, pressure reducing valves and backflow preventers. Thermal expansion tanks shall be sized in accordance with the tank manufacturer's instructions…” — IPC 2018, Section 607.3
How the size is worked out
EW = heater gallons × expansion factorDPF = (Pmax + 14.7) ÷ (Pmax − Pline)Tv = EW × DPFThe tank must pass both checks: its total volume at least Tv, and its maximum acceptance volume at least the expanded water. Pressures are gauge (psig); adding 14.7 converts them to absolute, which is what Boyle's law needs.
Amtrol's worked example. A 240-gallon heater at 150 °F, 125 psi maximum, 60 psi supply: 240 × 0.0179 = 4.30 gal expanded water; DPF = 139.7 ÷ 65 = 2.149; Tv = 9.23 gal. Amtrol rounds to 2.1 and 9.0 and selects an ST-25V (10.3 gal, 10.3 gal acceptance). Enter it above to check.
| Heater temperature | Factor | Expansion | On a 50 gal heater |
|---|---|---|---|
| 100 °F | 0.0062 | 0.62 % | 0.31 gal |
| 120 °F | 0.0100 | 1.00 % | 0.50 gal |
| 130 °F | 0.0124 | 1.24 % | 0.62 gal |
| 140 °F | 0.0150 | 1.50 % | 0.75 gal |
| 150 °F | 0.0179 | 1.79 % | 0.90 gal |
| 160 °F | 0.0209 | 2.09 % | 1.04 gal |
| 170 °F | 0.0242 | 2.42 % | 1.21 gal |
| 180 °F | 0.0276 | 2.76 % | 1.38 gal |
| Max allowable | 40 psi | 50 psi | 60 psi | 70 psi | 80 psi |
|---|---|---|---|---|---|
| 100 psi | 1.91 | 2.29 | 2.87 | 3.82 | 5.74 |
| 125 psi | 1.64 | 1.86 | 2.15 | 2.54 | 3.10 |
| 150 psi | 1.50 | 1.65 | 1.83 | 2.06 | 2.35 |
| Model | Tank volume | Max acceptance |
|---|---|---|
| ST-5 | 2.0 gal | 0.9 gal |
| ST-12 | 4.4 gal | 3.2 gal |
| ST-25V | 10.3 gal | 10.3 gal |
| ST-30V | 14.0 gal | 11.3 gal |
| ST-42V | 20.0 gal | 11.4 gal |
| ST-60V | 34.0 gal | 34.0 gal |
| ST-80V | 44.0 gal | 33.9 gal |
| ST-180V | 62.0 gal | 34.1 gal |
| ST-200V | 81.0 gal | 33.2 gal |
| ST-210V | 86.0 gal | 46.4 gal |
Amtrol's own quick sizing chart does not always match its published formula — in some cells it picks a smaller tank than the formula needs (for example 50 gal at 160 °F and 40 psi, where 1.05 gal of expanded water exceeds the ST-5's 0.9 gal acceptance), in others a larger one. Where your inputs fall on the chart, the calculator shows both and recommends the larger.
Start temperature matters. These factors assume the heater refills with 40 °F water — conservative, because winter supply water in much of the US is that cold. Watts' calculator assumes a 90 °F start, which gives less expansion and a smaller tank. Sizing from the colder start costs a slightly larger tank and protects the system all year.
FAQ
TVC-402/FAQDo I need a thermal expansion tank on my water heater?
If the cold supply passes through a check valve, pressure-reducing valve or backflow preventer — common on newer services and where a PRV is fitted — the plumbing code requires thermal expansion control. IPC Section 607.3 says the device goes on the heater's cold supply, downstream of those valves.
What size expansion tank for a 50 gallon water heater?
At 140 °F with 60 psi supply pressure and a 150 psi relief valve, the expanded water is 0.75 gal and the minimum tank is about 1.4 gal — an Amtrol ST-5 (2.0 gal tank, 0.9 gal acceptance) passes both checks. At 160 °F and 40 psi the expanded water (1.05 gal) exceeds the ST-5's acceptance, so it steps up to an ST-12.
What should the expansion tank precharge be?
Equal to the static water supply pressure — Amtrol's instructions are to set the air precharge to match the cold-water supply pressure. Measure it with a gauge on a hose bib with no water running, and set the tank before it is filled.
Why do different calculators give different sizes?
Mainly the cold-water start temperature. Amtrol's factors assume water starts at 40 °F, which is conservative; Watts' calculator assumes 90 °F, which gives less expansion and a smaller tank. The maximum-pressure basis differs too. This page uses Amtrol's published method throughout.
Can the tank be too big?
Not in the sense of causing harm — a larger tank simply holds pressure rise lower. Too small is the failure: pressure climbs toward the relief-valve setting on every heating cycle.
Sources and standards
- Therm-X-Trol Sizing (MC10007, 03/19)AMTROL Inc. (manufacturer)https://www.amtrol.com/wp-content/uploads/2019/06/9017-112-03_19-MC10007-Sizing-TXT.pdfThe method used here: Tv = design pressure factor × expanded water; DPF = (max pressure + 14.7) ÷ (max pressure − line pressure); expansion factors from a 40 °F start (Table 1); worked example 240 gal, 150 °F, 125 psi, 60 psi.
- Therm-X-Trol Sizing Card (MC8520, 06/24)AMTROL Inc. (manufacturer)https://www.amtrol.com/wp-content/uploads/2024/06/MC8520-06_24-TXT-Sizing-Card.pdfTank volume and maximum acceptance volume for ST-5 to ST-210V, factory precharge, and the quick sizing chart.
- Therm-X-Trol Brochure (MC4090, 09/25)AMTROL Inc. (manufacturer)https://www.amtrol.com/wp-content/uploads/2025/10/MC4090-09_25-ThermXtrol-Brochure.pdfQuick sizing chart basis: 40 °F incoming water, 150 psi T&P relief valve, precharge equal to static supply pressure.
- Thermal Expansion Tank Installation & Operation Manual (9015-942, 07/25)AMTROL Inc. (manufacturer)https://www.amtrol.com/wp-content/uploads/2025/08/9015-942-07_25-Thermal-Expansion-Tank-IO.pdfAdjust the air precharge to match the cold-water supply pressure.
- Thermal Expansion Control — memo quoting 2018 IPC Section 607.3Colorado Department of Local Affairs, Division of Housing (30 March 2020)https://www.icc-nta.org/wp-content/uploads/2020/04/DOLA_div_housing_memo-2020-Thermal-Expansion-Control.pdfState memo reproducing the code text. ICC's own code pages are behind a reader login; check the edition your jurisdiction has adopted.
- Potable Expansion Tank Sizing (PLT series)Watts Water Technologies (manufacturer)https://www.watts.com/resources/planning/etpUses a 90 °F cold-water start and a final pressure of initial + 50 % — a less conservative basis than Amtrol's.
Every linked source above was retrieved and checked on 2026-11-01.