Reference tables
Water heater sizing chart
Quick-reference tables for tank First Hour Rating and tankless flow. Every cell is calculated by the same engine as the calculator from the assumptions stated under each table, so you can reproduce any number.
Updated
Chart 1: Minimum First Hour Rating by showers per hour
Standard showers (2.5 gpm, 8 min, 105°F) in the busiest hour, plus one kitchen-tap use and one dishwasher load. 120°F tank. Read across to your incoming water.
| Showers in busiest hour | 40°F in | 50°F in | 60°F in | 70°F in |
|---|---|---|---|---|
| 1 | 26 | 25 | 24 | 23 |
| 2 | 42 | 41 | 39 | 37 |
| 3 | 58 | 56 | 54 | 51 |
| 4 | 74 | 72 | 69 | 65 |
| 5 | 91 | 88 | 84 | 79 |
| 6 | 107 | 104 | 99 | 93 |
At 50°F incoming water, each extra standard shower adds 15.7 gallons and a 35-gallon bath adds 27.0.
Chart 2: Tankless flow by simultaneous showers
Standard showers at 105°F running at the same time, 120°F outlet. Only the hot share passes through the heater.
| Showers at once | 40°F in | 50°F in | 60°F in | 70°F in |
|---|---|---|---|---|
| 1 | 2.0 · 81k | 2.0 · 69k | 1.9 · 56k | 1.8 · 44k |
| 2 | 4.1 · 163k | 3.9 · 138k | 3.8 · 113k | 3.5 · 88k |
| 3 | 6.1 · 244k | 5.9 · 206k | 5.6 · 169k | 5.3 · 131k |
| 4 | 8.1 · 325k | 7.9 · 275k | 7.5 · 225k | 7.0 · 175k |
Chart 3: What a gas tankless unit can deliver at each rise
Flow = input × efficiency ÷ (500 × rise), the idealized full-fire output. 199,900 BTU/hr is the largest residential gas tankless input. Real units also have maximum flow limits; confirm on the manufacturer’s curve.
| Input · efficiency | 40°F | 50°F | 60°F | 67°F | 70°F | 80°F | 90°F |
|---|---|---|---|---|---|---|---|
| 120,000 · 82% | 4.9 | 3.9 | 3.3 | 2.9 | 2.8 | 2.5 | 2.2 |
| 160,000 · 82% | 6.6 | 5.2 | 4.4 | 3.9 | 3.7 | 3.3 | 2.9 |
| 199,900 · 82% | 8.2 | 6.6 | 5.5 | 4.9 | 4.7 | 4.1 | 3.6 |
| 120,000 · 95% | 5.7 | 4.6 | 3.8 | 3.4 | 3.3 | 2.9 | 2.5 |
| 160,000 · 95% | 7.6 | 6.1 | 5.1 | 4.5 | 4.3 | 3.8 | 3.4 |
| 199,900 · 95% | 9.5 | 7.6 | 6.3 | 5.7 | 5.4 | 4.7 | 4.2 |
To use Charts 2 and 3 together: find your required flow in Chart 2, then look for a row in Chart 3 that meets it at your rise. For electric units, see the electric tankless kW table.
Code minimums
In Uniform Plumbing Code jurisdictions, Table 501.1(2) also sets a minimum First Hour Rating by bathrooms and bedrooms. It is reproduced on the bathrooms page.
Assumptions
Fixture defaults are the app’s library: 2.5 gpm federal maximum showerheads for 8 minutes at 105°F; kitchen tap 2.2 gpm for 2 minutes at 110°F; dishwasher 5 gallons drawn straight from the heater. Hot fraction = (use − incoming) ÷ (heater − incoming). No design margin. Full details on the methodology page.
Sources
Water Heater Sizing Calculator for iPhone
Take the calculator to the job site
The app runs the same sizing engine as this site, offline. Basic tank and tankless sizing is free. Lifetime Pro, a one-time $4.99 purchase, adds tank vs. tankless comparison, custom fixtures, saved jobs with notes and PDF reports you can send to a customer.
