# Expansion tank size calculator: the gallons a water heater needs from its volume, set temperature and line pressure

> Tank gal = heater gal x expansion factor x (max psi + 14.7) / (max psi - line psi). A 50 gal heater at 140 F on 60 psi needs 1.4 gal: an Amtrol ST-5 or Watts PLT-5.

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| Source | https://ownerspec.com/plumbing/calculators/expansion-tank-size-calculator/ |
| Publisher | OwnerSpec (https://ownerspec.com/) |
| Published | 2026-09-24 |
| Updated | 2026-09-24 |
| Facts verified | 2026-09-24 |
| Disclosure | Contains affiliate links. OwnerSpec may earn a commission at no cost to the reader. |

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## Short answer

A thermal expansion tank is sized in two steps, both from Amtrol's Therm-X-Trol sizing guide. First, expanded water = water heater volume x the expansion factor for the set temperature: 1.50% at 140 F, 1.79% at 150 F, 2.76% at 180 F, all measured from 40 F incoming water. Second, tank volume = expanded water x the design pressure factor, DPF = (maximum allowable pressure + 14.7) / (maximum allowable pressure - static line pressure), which is Boyle's law in gauge units. A 50-gallon heater set to 140 F on 60 psi supply under a 150 psi T&P valve expands 0.8 gallon and needs 0.8 x 1.8 = 1.4 gallons of tank: a 2.0-gallon Amtrol ST-5 or a 2.1-gallon Watts PLT-5. Amtrol's own worked example, a 240-gallon heater at 150 F, 125 psi maximum and 60 psi line, returns 4.3 x 2.1 = 9.0 gallons, and this calculator returns the same 9.0. The tank's acceptance volume must also exceed the expanded water.

## Fact sheet

- **Answer:** Amtrol's Therm-X-Trol sizing guide sizes a potable expansion tank as expanded water (heater gallons x the expansion factor for the set temperature, 0.0150 at 140 F) x a design pressure factor, DPF = (maximum allowable psi + 14.7) / (maximum allowable psi - line psi). A 50 gal heater at 140 F, 150 psi T&P and 60 psi line expands 0.8 gal and needs 0.8 x 1.8 = 1.4 gal of tank.
- **Formula:** `Tank volume (gal) = [heater volume (gal) x expansion factor for the set temperature, from 40 F incoming] x [(maximum allowable pressure + 14.7) / (maximum allowable pressure - static line pressure)]; the tank's acceptance volume must also exceed the expanded water`
- **Inputs:** Water heater volume: 50 gal; Set temperature: 140 F (expansion factor 0.0150); Incoming water temperature: 40 F, fixed by Amtrol's table; Maximum allowable pressure: 150 psi (the T&P relief setting); Static line pressure: 60 psi
- **Output:** 0.8 gal of expanded water, DPF 1.8, minimum tank volume 1.4 gal: an Amtrol ST-5 (2.0 gal, 0.9 gal acceptance) or a Watts PLT-5 (2.1 gal, 1.26 gal acceptance). On Amtrol's own example (240 gal, 150 F, 125 psi, 60 psi) the same arithmetic returns 4.3 x 2.1 = 9.0 gal, matching the guide.
- **Assumptions:** Expansion factors are Amtrol's Table 1, based on 40 F incoming water; warmer incoming water expands less, so 40 F is the conservative case; The tank precharge is set equal to the static line pressure before installation, as Amtrol instructs; a lower precharge fills part of the tank with water before heating starts; Expanded water and the DPF are each rounded to 0.1 before multiplying, the way Amtrol's worksheet does, which is why 0.75 gal shows as 0.8; Tank and acceptance volumes are the makers' published figures: Amtrol's June 2024 sizing card and Watts' thermal expansion product guide
- **Method:** The calculator multiplies heater volume by Amtrol's Table 1 expansion factor for the chosen set temperature, multiplies that by Amtrol's design pressure factor for the chosen maximum and line pressures, then walks a ladder of maker-listed tanks and returns the smallest whose total volume covers the result and whose acceptance volume exceeds the expanded water.
- **Primary source:** [Amtrol, Sizing the Therm-X-Trol, MC10007 (PDF)](https://www.amtrol.com/wp-content/uploads/2019/06/9017-112-03_19-MC10007-Sizing-TXT.pdf) (manufacturer or vendor document)
- **Facts verified:** 2026-09-24
- **Last updated:** 2026-09-24
- **Canonical URL (cite this):** https://ownerspec.com/plumbing/calculators/expansion-tank-size-calculator/

Do not pick a tank off a size chart until a gauge has given you your static pressure: the same 50-gallon heater needs 1.2 gallons of tank at 40 psi and 1.9 gallons at 80 psi, and the chart on the box knows only one of those houses.

> Interactive calculator, "Minimum expansion tank volume, and the next listed size": use it on the HTML page at the URL above; the formula, constants and sources are written out below.

## What is the formula?

Thermal expansion is the extra volume water takes up as a storage heater warms it, with nowhere to go once a check valve, pressure reducing valve or backflow preventer closes the system. Amtrol's Therm-X-Trol sizing guide sizes the tank in one line: Tv = design pressure factor x expanded water.

| Variable | Definition | Where it comes from |
| --- | --- | --- |
| Heater volume | Rated storage capacity in gallons | The heater's data plate |
| Expansion factor | Fraction the water grows from 40 F to the set temperature | Amtrol Table 1 (below) |
| Expanded water | Heater volume x expansion factor | Calculated |
| Maximum allowable pressure | The pressure the system may not reach, normally the T&P relief setting | The T&P valve tag; 150 psi on residential heaters, 125 psi in Amtrol's example |
| Static line pressure | House pressure with no water running | A gauge on a hose bibb |
| Design pressure factor (DPF) | (maximum allowable pressure + 14.7) / (maximum allowable pressure - line pressure) | Amtrol's equation for conditions not in its Table 2 |
| Tank volume | Expanded water x DPF | Calculated |

The DPF is Boyle's law in gauge units; written the other way, tank volume = expanded water / acceptance factor, where acceptance factor = 1 - (line + 14.7) / (maximum + 14.7). Amtrol's Table 1 expansion factors, from 40 F incoming water:

| Set temperature | Expansion factor | Percent |
| --- | --- | --- |
| 100 F | 0.0062 | 0.6% |
| 120 F | 0.0100 | 1.0% |
| 130 F | 0.0124 | 1.2% |
| 140 F | 0.0150 | 1.5% |
| 150 F | 0.0179 | 1.8% |
| 160 F | 0.0209 | 2.0% |
| 170 F | 0.0242 | 2.4% |
| 180 F | 0.0276 | 2.8% |

Amtrol prints no factor for other incoming temperatures, so the calculator holds that box at 40 F.

## What does a worked example look like?

Amtrol's own example, a 240-gallon heater at 150 F, 125 psi maximum and 60 psi line, beside the house case: a 50-gallon heater at 140 F on 60 psi under a 150 psi T&P valve.

| Step | Amtrol's example | House case |
| --- | --- | --- |
| Expansion factor (Table 1) | 0.0179 at 150 F | 0.0150 at 140 F |
| Expanded water | 240 x 0.0179 = 4.3 gal | 50 x 0.0150 = 0.75, rounded to 0.8 gal |
| DPF | (125 + 14.7) / (125 - 60) = 2.1, as Amtrol's Table 2 prints | (150 + 14.7) / (150 - 60) = 1.8 |
| Tank volume | 4.3 x 2.1 = 9.0 gal | 0.8 x 1.8 = 1.4 gal |
| Listed tank | ST-25V (10.3 gal), per Amtrol | Amtrol ST-5 (2.0 gal, accepts 0.9) or Watts PLT-5 (2.1 gal, accepts 1.26) |

Amtrol's 2024 sizing card puts that 50-gallon heater on an ST-5 at 40, 60 and 80 psi, which is what the formula returns. At 60 gallons the expanded water is 0.9 gal, the ST-5's acceptance volume exactly, and the calculator steps past it on a tie.

| Heater, set temperature | 40 psi line | 60 psi line | 80 psi line |
| --- | --- | --- | --- |
| 40 gal, 140 F | 0.6 x 1.5 = 0.9 gal | 0.6 x 1.8 = 1.1 gal | 0.6 x 2.4 = 1.4 gal |
| 50 gal, 140 F | 0.8 x 1.5 = 1.2 gal | 0.8 x 1.8 = 1.4 gal | 0.8 x 2.4 = 1.9 gal |
| 80 gal, 140 F | 1.2 x 1.5 = 1.8 gal | 1.2 x 1.8 = 2.2 gal | 1.2 x 2.4 = 2.9 gal |
| 50 gal, 160 F | 1.0 x 1.5 = 1.5 gal | 1.0 x 1.8 = 1.8 gal | 1.0 x 2.4 = 2.4 gal |

All rows at a 150 psi maximum. The tank grows by 60% between 40 and 80 psi, which is why the [high water pressure guide](/plumbing/guides/high-water-pressure-80-psi-when-you-need-a-prv/) comes first: the pressure reducing valve that fixes that number is also what makes the tank a code requirement.

## Which listed tank size is next?

Tank volume is the shell; acceptance volume is the water the diaphragm can take. Amtrol says a tank must meet both, so the calculator checks both against the sizes the two makers publish.

| Tank | Volume | Acceptance | Precharge | Source |
| --- | --- | --- | --- | --- |
| Amtrol ST-5 | 2.0 gal | 0.9 gal | 50 psig | Amtrol MC8520 |
| Watts PLT-5 / DET-5-M1 | 2.1 gal | 1.26 gal | 20 psi | Watts PG-ThermExpansion |
| Amtrol ST-12 | 4.4 gal | 3.2 gal | 50 psig | Amtrol MC8520 |
| Watts PLT-12 / DET-12-M1 | 4.5 gal | 2.8 gal | 20 psi | Watts PG-ThermExpansion |
| Watts DET-20-M1 | 8.5 gal | 3.2 gal | 20 psi | Watts PG-ThermExpansion |
| Amtrol ST-25V | 10.3 gal | 10.3 gal | 40 psig | Amtrol MC8520 |
| Amtrol ST-30V | 14.0 gal | 11.3 gal | 40 psig | Amtrol MC8520 |
| Watts DET-35-M1 | 14 gal | 5.6 gal | 20 psi | Watts PG-ThermExpansion |

The two 2-gallon tanks differ: an 80-gallon heater at 140 F (1.2 gal expanded) clears the Watts and not the Amtrol.

Precharge is the number the chart hides. Amtrol calls matching it to supply pressure before installation a critical step: a 70 psi supply pushing on a 40 psi charge fills part of the tank before heating begins. Watts recommends charging to supply pressure up to 80 psi, which the formula assumes.

## Where do the numbers come from?

Amtrol's MC10007 sizing guide supplies the equation, Tables 1 and 2 and the 240-gallon example. Its June 2024 MC8520 card supplies the ST-series volumes, acceptance volumes and precharges, and assumes 40 F incoming water, a 150 psi T&P valve and a precharge equal to supply pressure. Watts' product guide supplies the DET and PLT tables and bases its selection chart on a 150 psi relief setting, a maximum 40 F temperature rise and a 20 psi precharge.

That last line is the reason to run the formula: Watts' chart is built on a 40 F rise, Amtrol's factors on a 100 F rise at 140 F, and neither on your gauge.

Section 607.3 of the 2018 Plumbing Code, as North Carolina's Office of the State Fire Marshal prints it, requires a thermal expansion tank on the cold supply of a storage water heater fed through a check valve, pressure reducing valve or backflow preventer, sized per the tank manufacturer's instructions. Codes are adopted locally and vary. Whether your house is in that situation is the [thermal expansion tank guide's](/plumbing/guides/thermal-expansion-tank-when-required/) question.

**Only after the gauge has a number**

A hose-bibb gauge is the input this calculator cannot guess, and static pressure over 80 psi means a regulator first. Under 80 psi with a 40 to 50 gallon heater at 140 F, the Amtrol ST-5 is the tank Amtrol's own card points at. For a 60 or 80 gallon heater the Watts PLT-12 prints 2.8 gal of acceptance, an IAPMO listing and ANSI/NSF 61 in its catalog.

A tankless heater with no recirculation loop, and a house with no check valve, PRV or backflow preventer, are not this page's readers. A first expansion tank is plumbing work many jurisdictions permit and inspect.

## The picks

- **Amtrol ST-5 Therm-X-Trol Expansion Tank, 2.0 gal, 3/4 in NPTM**: A 40 to 50 gal heater at 140 F on supply up to 80 psi (Amtrol's sizing card lists 2.0 gal tank volume, 0.9 gal maximum acceptance volume, 150 psig working pressure, 200 F, 50 psig factory precharge; the submittal sheet lists the diaphragm as NSF/ANSI 61) Amazon: https://www.amazon.com/dp/B08BZTFDRD/?tag=ownerspec-20
- **Watts PLT-12 Potable Water Expansion Tank, 4.5 gal, 3/4 in male inlet**: A 60 to 80 gal heater, or any heater once the answer passes 2.0 gal (Watts' catalog lists 4.5 gal tank volume, 2.8 gal acceptance volume, 150 psi, 200 F, 20 psi precharge, Listed by IAPMO and certified to ANSI/NSF 61) Amazon: https://www.amazon.com/dp/B002JEQGYO/?tag=ownerspec-20

## Frequently asked questions

### What size expansion tank does a 50 gallon water heater need?

On Amtrol's formula, a 50 gal heater at 140 F on 60 psi under a 150 psi T&P expands 0.8 gal and needs 0.8 x 1.8 = 1.4 gal of tank: a 2.0 gal Amtrol ST-5 or a 2.1 gal Watts PLT-5. Amtrol's sizing card puts that heater on an ST-5 at 40, 60 and 80 psi. At 80 gal the answer moves to the 4.4 to 4.5 gal size.

### Is a 2 gallon or a 4.5 gallon expansion tank right for my heater?

Run the formula with your own line pressure. A 2 gal tank (Amtrol ST-5 accepts 0.9 gal, Watts PLT-5 accepts 1.26 gal) covers a 40 to 50 gal heater at 140 F on ordinary pressure. Amtrol's card moves 60 and 80 gal heaters at 140 F, and 40 gal heaters at 180 F, to the 4.4 gal ST-12 at every pressure it lists.

### Why does the calculator ask for line pressure if the tank has a fixed size?

Because the air cushion does the work. Amtrol's design pressure factor is (maximum pressure + 14.7) / (maximum pressure - line pressure): at a 150 psi T&P setting it is 1.5 at 40 psi line pressure and 2.4 at 80 psi, so the same 0.8 gal of expanded water needs 1.2 gal of tank at 40 psi and 1.9 gal at 80 psi.

### Does the expansion tank precharge have to match my house pressure?

Amtrol says equalizing the precharge to incoming supply pressure is a critical step: a 70 psi supply pushing on a 40 psi charge takes up tank space before the water has begun to heat. Its 2024 card lists a 50 psig factory precharge for the ST-5 and ST-12. Watts ships DET and PLT tanks at 20 psi and recommends charging to supply pressure, up to 80 psi.

### Is a thermal expansion tank required by code?

The model plumbing codes require one. Section 607.3 of the 2018 Plumbing Code, as North Carolina's Office of the State Fire Marshal prints it in a 2019 interpretation, says a storage water heater fed through a check valve, pressure reducing valve or backflow preventer needs a thermal expansion tank on its cold supply, downstream of those devices, sized per the tank manufacturer's instructions. Codes are adopted locally and vary.


## Sources

- [Amtrol, Sizing the Therm-X-Trol, MC10007 (PDF)](https://www.amtrol.com/wp-content/uploads/2019/06/9017-112-03_19-MC10007-Sizing-TXT.pdf) (manufacturer or vendor document)
- [Amtrol, Therm-X-Trol Quick Sizing Charts and Specifications, MC8520 06/24 (PDF)](https://www.amtrol.com/wp-content/uploads/2024/06/MC8520-06_24-TXT-Sizing-Card.pdf) (manufacturer or vendor document)
- [Amtrol, Therm-X-Trol ST Series Non-ASME submittal sheet, MC4400 01/24 (PDF)](https://www.amtrol.com/wp-content/uploads/2023/11/MC4400_01_24_Submittal_Data_Book-34.pdf) (manufacturer or vendor document)
- [Watts, Thermal Expansion Products guide, PG-ThermExpansion (PDF)](https://www.watts.com/dfsmedia/0533dbba17714b1ab581ab07a4cbb521/24277-source/pg-thermexpansion-pdf) (manufacturer or vendor document)
- [North Carolina Office of the State Fire Marshal, 2018 Plumbing Code Section 607.3 Thermal Expansion Control, interpretation of January 3, 2019 (PDF)](https://www.ncosfm.gov/plumbing/06073-thermal-expansion-control-0/open) (government or regulatory source)

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Citation: OwnerSpec, "Expansion tank size calculator: the gallons a water heater needs from its volume, set temperature and line pressure", https://ownerspec.com/plumbing/calculators/expansion-tank-size-calculator/, facts verified 24 September 2026.
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