Water Heater Sizing Calculator
Size a storage water heater by peak-hour hot-water demand (First Hour Rating) or size a tankless unit by peak simultaneous flow, using DOE worksheet values.
Result
- Showers @ 10 gal20 gal
- Shaving @ 2 gal2 gal
- Shampoo @ 4 gal ยท hand dishwashing @ 4 gal8 gal
- Dishwasher @ 6 gal ยท food prep @ 5 gal5 gal
- Clothes washer @ 7 gal0 gal
- Suggested tank30โ40 gallon tank
Step-by-step
- Multiply each peak-hour activity by its DOE average hot-water use and sum them.
- Required First Hour Rating = 35 gallons.
- Choose a storage heater whose labelled First Hour Rating is within a few gallons of this number: 30โ40 gallon tank.
How to use this calculator
- Choose the sizing method: storage tank (First Hour Rating) or tankless (peak GPM).
- Count how many of each hot-water activity happen during your single busiest hour.
- For tankless, count only fixtures that may run at the same time.
- Read the required First Hour Rating in gallons, or the peak flow in GPM, plus the suggested unit size.
About this calculator
Picking the right water heater size is about meeting your peak hour of hot-water demand, not your daily total. The U.S. Department of Energy recommends sizing a storage tank by its First Hour Rating (FHR) โ the gallons of hot water it can deliver in a busy hour โ and matching that to your own peak-hour demand. This calculator follows the DOE worksheet: it multiplies the number of showers, shaves, shampoos, dishwashing sessions, dishwasher and clothes-washer loads, and food-prep tasks during your busiest hour by the average hot-water gallons each consumes, then sums them. For tankless (on-demand) heaters, which have no storage, what matters instead is the peak simultaneous flow rate in gallons per minute, so the calculator switches to summing fixture flow rates. It then suggests a tank size or required GPM.
How it works โ the formula
Tank: FHR (gal) = ฮฃ (count ร DOE gallons-per-use)
Tankless: Peak flow (GPM) = ฮฃ (simultaneous fixtures ร fixture GPM)Storage heaters are matched to peak-hour gallons (First Hour Rating); tankless heaters are matched to peak instantaneous flow. Both sum per-fixture figures from the DOE sizing worksheet.
Worked examples
- Inputs:
- showers=3, dishwasher=1, clothesWasher=1
- Output:
- 30 + 6 + 7 = 43 gal FHR โ 40โ50 gal tank
- Inputs:
- showers=2, shaves=2, foodPrep=1
- Output:
- 20 + 4 + 5 = 29 gal FHR โ 30โ40 gal tank
- Inputs:
- mode=tankless, showers=1, handDish=1
- Output:
- 2.5 + 1.5 = 4.0 GPM
Limitations
- Per-use gallons are DOE averages; high-flow fixtures use more.
- Tankless GPM capacity must be checked at your local temperature rise.
- Does not model recovery rate of a specific tank model โ match to the labelled FHR.
Sizing estimate. Confirm against the unit's EnergyGuide First Hour Rating or GPM-vs-temperature-rise chart.