Code table
Water heater size by number of bathrooms
Where the Uniform Plumbing Code applies, Table 501.1(2) sets a minimum First Hour Rating by bathrooms and bedrooms: from 38 gallons for a one-bedroom, one-bath home to 74 gallons for larger homes. It is a floor, not a target. Your busiest hour can need more.
Updated
UPC Table 501.1(2): minimum First Hour Rating
Section 501.1 of the Uniform Plumbing Code says the minimum capacity for water heaters shall be in accordance with the first hour rating in Table 501.1(2). The values below are from the 2021 UPC as adopted in Washington State.
| Bathrooms | Bedrooms | Minimum FHR |
|---|---|---|
| 1 to 1.5 | 1 | 38 gal |
| 2 | 49 gal | |
| 3 | 49 gal | |
| 2 to 2.5 | 2 | 49 gal |
| 3 | 62 gal | |
| 4 | 62 gal | |
| 5 | 74 gal | |
| 3 to 3.5 | 3 | 62 gal |
| 4 | 74 gal | |
| 5 | 74 gal | |
| 6 | 74 gal |
The code’s footnotes: the first hour rating is found on the EnergyGuide label; nonstorage and solar water heaters must meet the same rating and deliver the maximum system demand flow; replacement water heaters are covered by Section 102.4. Confirm the edition and any local amendments with your building department.
Bathrooms are a stand-in for showers
The table uses bathrooms and bedrooms because they roughly predict how many people shower and how many showers can overlap. A busy family in a two-bath house can use more hot water in an hour than a couple in a three-bath house. That is why it is worth checking your own busiest hour against the code minimum:
| House | Example busiest hour | Code min. | Calc. at 50°F | Calc. at 40°F |
|---|---|---|---|---|
| 1 to 1.5 bath, 2 bed | 2 standard showers, 2 bathroom faucets, 1 kitchen faucet | 49 | 38 | 39 |
| 2 to 2.5 bath, 3 bed | 3 standard showers, 2 bathroom faucets, 1 kitchen faucet, a dishwasher load | 62 | 59 | 61 |
| 2 to 2.5 bath, 4 bed | 4 standard showers, 3 bathroom faucets, 1 kitchen faucet, a dishwasher load | 62 | 76 | 78 |
| 3 to 3.5 bath, 5 bed | 4 standard showers, 3 bathroom faucets, 1 kitchen faucet, a bath, a dishwasher load | 74 | 103 | 106 |
Choose a heater that meets the larger of the code minimum and your own calculated peak, and remember that FHR is rated at 58°F incoming water.
Tankless by bathrooms
A bathroom count maps more directly onto tankless sizing, because the question is how many showers can run at once. Two showers need 3.9 gpm through the heater at a 70°F rise; three need 5.9 gpm. Whole-house tankless sizing has the full table by climate.
Which codes use the table
Table 501.1(2) belongs to the Uniform Plumbing Code published by IAPMO. States and cities that adopt the UPC may amend it, and many jurisdictions use a different code, such as the International Residential Code, with its own requirements. Your building department can tell you which applies and whether a permit is needed to replace a water heater.
Bathroom sizing questions
What size water heater for a 2 bathroom house?
Under Uniform Plumbing Code Table 501.1(2), a house with 2 to 2.5 bathrooms needs a minimum First Hour Rating of 49 gallons with 2 bedrooms, 62 gallons with 3 or 4 bedrooms and 74 gallons with 5. Your own busiest hour may need more; check it with the calculator.
What size water heater for 3 bathrooms?
The UPC table sets a minimum First Hour Rating of 62 gallons for 3 to 3.5 bathrooms with 3 bedrooms and 74 gallons with 4 to 6 bedrooms. For tankless, three showers at once need about 5.9 gpm at a 70°F rise.
Does my state use this table?
The table is in the Uniform Plumbing Code, which is adopted, sometimes with amendments, in a number of Western and some other states and cities. Places that use a different plumbing code, such as the International Residential Code, have their own requirements. Your local building department can tell you which code applies.
Does the table apply to tankless water heaters?
Yes, in its own way. The code's footnote says nonstorage and solar water heaters must meet the table's First Hour Rating and also deliver hot water at the maximum system demand flow calculated under the code.
Sources
Water Heater Sizing Calculator for iPhone
Take the calculator to the job site
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