Enter simultaneous hot-water fixtures and average hot-water flow per fixture, not mixed-flow volume unless all that flow must be heated. Peak minutes is the duration of that draw; other demand adds all additional hot-water gallons in the same busiest hour. Enter required temperature rise from incoming water to delivered hot-water temperature.
All example values are scenarios, not household-size recommendations.
First-hour rating includes storage and recovery and is not identical to nominal tank gallons. The 500 coefficient is an approximate water-density/specific-heat conversion; delivered thermal power is not fuel input, which also depends on efficiency. Mixing, recovery timing, recirculation and actual draw patterns affect service.
This calculation does not approve installation, venting, electrical supply or a particular heater model.
Modeled peak-hour hot-water demand (gallons) from duration of simultaneous peak draw (minutes)
| Duration of simultaneous peak draw (minutes) | 10 |
|---|---|
| Other hot-water gallons during the same peak hour | 10 |
| Required temperature rise (°F) | 70 |
| Fixtures used at once | 2 |
| Average flow per fixture | 2.5 |
| Modeled peak-hour hot-water demand (gallons) | 60 |
| Simultaneous hot-water flow (GPM) | 5 |
| Approximate delivered heating power (Btu/h) | 175,000 |
Common error: Choosing a heater by household size alone. Two people who shower back to back can need more than four who stagger their use.
Water Heater Sizing: formula
IAPMO: Uniform Plumbing Code · PNNL Building America: First-hour demand
Water Heater Sizing: changing Duration of simultaneous peak draw (minutes)
| Duration of simultaneous peak draw (minutes) | Modeled peak-hour hot-water demand (gallons) | Simultaneous hot-water flow (GPM) | Approximate delivered heating power (Btu/h) |
|---|---|---|---|
| 8 | 50 | 5 | 175,000 |
| 9 | 55 | 5 | 175,000 |
| 10 (example) | 60 | 5 | 175,000 |
| 11 | 65 | 5 | 175,000 |
| 12 | 70 | 5 | 175,000 |
Water Heater Sizing: compare input ranges
| Input | Tested from | To | Modeled peak-hour hot-water demand (gallons): low input | Modeled peak-hour hot-water demand (gallons): high input | Swing |
|---|---|---|---|---|---|
| Fixtures used at once | 1 | 3 | 35 | 85 | 50 (83%) |
| Duration of simultaneous peak draw (minutes) | 9 | 11 | 55 | 65 | 10 (17%) |
| Average flow per fixture | 2.3 | 2.8 | 56 | 66 | 10 (17%) |
| Other hot-water gallons during the same peak hour | 9 | 11 | 59 | 61 | 2 (3.3%) |
| Required temperature rise (°F) | 63 | 77 | 60 | 60 | 0 |
Water Heater Sizing: individual input comparisons
| Input | Value | Modeled peak-hour hot-water demand (gallons) | Simultaneous hot-water flow (GPM) | Approximate delivered heating power (Btu/h) |
|---|---|---|---|---|
| Duration of simultaneous peak draw (minutes) | 8 | 50 | 5 | 175,000 |
| 9 | 55 | 5 | 175,000 | |
| 10 | 60 | 5 | 175,000 | |
| 11 | 65 | 5 | 175,000 | |
| 12 | 70 | 5 | 175,000 | |
| Other hot-water gallons during the same peak hour | 8 | 58 | 5 | 175,000 |
| 9 | 59 | 5 | 175,000 | |
| 10 | 60 | 5 | 175,000 | |
| 11 | 61 | 5 | 175,000 | |
| 12 | 62 | 5 | 175,000 | |
| Required temperature rise (°F) | 56 | 60 | 5 | 140,000 |
| 63 | 60 | 5 | 157,500 | |
| 70 | 60 | 5 | 175,000 | |
| 77 | 60 | 5 | 192,500 | |
| 84 | 60 | 5 | 210,000 | |
| Fixtures used at once | 0 | 10 | 0 | 0 |
| 1 | 35 | 2.5 | 87,500 | |
| 2 | 60 | 5 | 175,000 | |
| 3 | 85 | 7.5 | 262,500 | |
| 4 | 110 | 10 | 350,000 | |
| Average flow per fixture | 2 | 50 | 4 | 140,000 |
| 2.3 | 56 | 4.6 | 161,000 | |
| 2.5 | 60 | 5 | 175,000 | |
| 2.8 | 66 | 5.6 | 196,000 | |
| 3 | 70 | 6 | 210,000 |
Modeled peak-hour hot-water demand (gallons): Duration of simultaneous peak draw (minutes) against Other hot-water gallons during the same peak hour
| Duration of simultaneous peak draw (minutes) / Other hot-water gallons during the same peak hour | 8 | 10 | 12 |
|---|---|---|---|
| 8 | 48 | 50 | 52 |
| 9 | 53 | 55 | 57 |
| 10 | 58 | 60 | 62 |
| 11 | 63 | 65 | 67 |
| 12 | 68 | 70 | 72 |
Water Heater Sizing: values and definitions
| Quantity and definition | Value used |
|---|---|
| Duration of simultaneous peak draw (minutes) — Enter the duration of simultaneous peak draw (minutes) used in this estimate. | 10 |
| Other hot-water gallons during the same peak hour — Enter the other hot-water gallons during the same peak hour used in this estimate. | 10 |
| Required temperature rise (°F) — Enter the required temperature rise (°f) used in this estimate. | 70 |
| Fixtures used at once — Number of fixtures expected to run at the same time during peak demand. | 2 |
| Average flow per fixture — Typical flow rate per fixture, in gallons per minute. | 2.5 |
| Modeled peak-hour hot-water demand (gallons) | 60 |
| Simultaneous hot-water flow (GPM) | 5 |
| Approximate delivered heating power (Btu/h) | 175,000 |
Fixtures used at once: half and double
| Fixtures used at once | Modeled peak-hour hot-water demand (gallons) |
|---|---|
| 1 | 35 |
| 2 | 60 |
| 4 | 110 |
Sizing a water heater is a question of how much hot water you use at once, not how many people live in the house. A household of three is a common fit for a 40 gallon storage tank, and a tankless unit is sized by flow: two fixtures running at 2.5 gallons a minute each need a unit that heats 5 gallons a minute. Tanks give a reservoir and recover slowly, and tankless units give continuous hot water within their flow limit, so the peak demand decides which fits.
Note when you use the most hot water, such as morning showers plus a dishwasher, and size for that peak. The temperature rise in your area also sets the tankless capacity you need.
Check supply flow on the flow rate calculator, pipe losses on the pipe friction page and the volume of hot water lines on the pipe volume calculator. The watts to amps calculator sizes an electric heater's circuit, and the energy and power converters change units.
Can a 5-GPM label be used at every inlet temperature?
No. Available hot-water flow changes with required temperature rise. Check the actual performance curve for the intended conditions.