Water heater expansion tank sizing chart
Lookup a residential screening tank size from heater capacity and incoming water pressure.
What this chart answers
This page reproduces the residential sizing table from an A. O. Smith expansion-tank installation document. Choose the row for the highest incoming water pressure and the column for the heater capacity. The cell is a nominal tank size for the source product family under the stated assumptions.
It is a lookup reference, not a universal product recommendation. A different tank family may have a different acceptance volume, factory precharge, maximum pressure, connection or temperature rating.
Residential expansion tank needed
source chart assumptions: 160°F max water temperature, 150 PSI max pressure| Incoming pressure | 30 gal | 40 gal | 52 gal | 66 gal | 82 gal | 100 gal | 120 gal |
|---|---|---|---|---|---|---|---|
| 40 psi | 2 gal | 2 gal | 2 gal | 5 gal | 5 gal | 5 gal | 5 gal |
| 50 psi | 2 gal | 2 gal | 2 gal | 5 gal | 5 gal | 5 gal | 5 gal |
| 60 psi | 2 gal | 2 gal | 5 gal | 5 gal | 5 gal | 5 gal | 10 gal |
| 70 psi | 2 gal | 2 gal | 5 gal | 5 gal | 5 gal | 10 gal | 10 gal |
| 80 psi | 2 gal | 5 gal | 5 gal | 5 gal | 5 gal | 10 gal | 10 gal |
Worked example:
A 50-gallon residential water heater with an incoming pressure of 60 psi falls between the 40 and 52 gallon columns. The source table does not provide a 50-gallon column, so use the next higher listed capacity, 52 gallons: the screening result is a 5 gallon nominal tank. Confirm the exact heater volume and tank model before ordering.
When the incoming pressure is above the chart range, the chart cannot be extrapolated safely. Install and adjust pressure regulation and tank precharge according to the current product instructions and local plumbing requirements.
How to use this chart
- Measure the highest cold-water supply pressure over the relevant operating period.
- Find the next higher water-heater capacity column when the exact capacity is absent.
- Check the selected tank's nominal volume and acceptance volume against the actual thermal expansion.
- Match the precharge to the cold supply pressure within the manufacturer's stated limits.
Important: Closed potable-water systems may need an expansion tank when a check valve, backflow preventer or pressure-reducing valve prevents thermal expansion from returning to the supply. The chart does not determine whether a local code requires one.
Use boundary: This chart does not calculate hydronic expansion, glycol expansion, commercial storage, multiple-tank arrangements, relief-valve compliance, or the current model's acceptance volume. Use the exact manufacturer document and a qualified plumbing review.
Common questions
Is a 5 gallon tank always correct for a 50 gallon heater?
No. This chart uses 52 gallons as the next higher listed capacity and belongs to one manufacturer reference. Verify actual heater volume, pressure, temperature and the selected tank's acceptance rating.
Can I use this chart for a hydronic boiler?
No. Potable water heater and closed hydronic systems use different fluid temperatures, pressure arrangements and tank selection data. Use the dedicated hydronic calculator and manufacturer data.
What does precharge mean?
Precharge is the air pressure set in the tank before water enters it. It must be checked against the cold supply pressure and the product instructions.
Cite or reuse this table
This dataset is free to cite with attribution. Use the stable URL below and preserve the source assumptions.
HVAC Calculator Pro. (2026-10-04). Water heater expansion tank sizing chart. Retrieved from https://hvaccalcpro.com/charts/water-heater-expansion-tank-sizinghttps://hvaccalcpro.com/charts/water-heater-expansion-tank-sizing