Peak-hour hot-water demand calculator

Peak-hour demand is the input every other sizing decision hangs on. Add up the hot-water draws in your busiest hour and you have the target your tank’s First-Hour Rating has to clear.

Typical planning values. Your real hot-water use, incoming water temperature, recovery and efficiency vary by household, unit, fuel and region — confirm your unit’s rated First-Hour Rating, GPM and UEF on its EnergyGuide label and the manufacturer’s instructions. Round sizing up, and leave headroom for peak demand.

1 Enter your numbers

in the peak hour
About 20 gallons each.
in the peak hour
About 20 gallons each.
in the peak hour
in the peak hour
in the peak hour
in the peak hour
in the peak hour
in the peak hour
Your result
Peak-hour hot-water demand75 gallons
Showers (20 gal)60 gal
Baths (20 gal)0 gal
Other draws15 gal

Add up every hot-water use in your busiest hour — that peak, not the daily total, sets the First-Hour Rating you need. This routine peaks at about 75 gallons. Size a tank to an FHR at or above it.

The single most useful number in water-heater sizing is the peak hour — the busiest 60 minutes of hot-water use in a normal day. It is what the DOE first-hour-rating method is built around, and it is the figure that separates a tank that never runs cold from one that quits mid-shower.

Count only draws that overlap in that window. Two people showering back-to-back within the hour both count; a load of laundry you run at midnight does not belong in the morning peak. The output feeds straight into the what-size tool and the first-hour-rating tool.

Formula

Peak-hour demand is a straight weighted sum:

peak_hour_demand = Σ(number_of_uses × gallons_per_use)

with the LABELED DOE draws: shower 20 gal, bath 20, shave 2, hand/face 4, shampoo 4, dishwasher 6, food prep 5, clothes washer 7. Adjust any figure to your own fixtures — a low-flow shower and a short timer both cut the shower number.

Worked example

Busiest hour: 3 showers, 1 shave, 2 hand washes and one food-prep draw.

3×20 + 1×2 + 2×4 + 1×5 = 60 + 2 + 8 + 5 = 75 gallons

So the household needs a heater rated to deliver at least 75 gallons in the first hour. The showers dominate (60 of the 75), which is the practical lesson: shower behavior — count, length and flow rate — is the biggest lever on the size you buy.

Measure first, avoid a wrong size

What to measure first. Watch a real weekday morning and tally the overlapping draws; do not average across the day. The peak is spiky, and sizing to the average leaves the last person cold.

  • Showers are the swing factor. At ~20 gallons apiece they usually make up most of the peak; a 1.75-GPM head or a shorter shower can shave a whole tank size off the requirement.
  • Appliances rarely fall in the peak. Dishwashers and clothes washers can often be shifted off the morning rush — if they never overlap the peak, leave them out.
  • The peak drives FHR, not tank gallons. Once you have the peak-hour number, match it against a First-Hour Rating, which folds in how fast the unit reheats.

Reference table

LABELED DOE hot-water use per fixture, used to weight each draw.

Fixture / useGallons per use
Shower20 gal
Bath (tub)20 gal
Shaving2 gal
Hand / face wash4 gal
Hair shampoo (sink)4 gal
Hand dishwashing4 gal
Automatic dishwasher6 gal
Food prep5 gal
Clothes washer7 gal

Frequently asked questions

What is peak-hour hot-water demand?
It is the total gallons of hot water your household draws in its busiest single hour. It is the basis of the DOE first-hour-rating sizing method, because a heater has to keep up with the worst hour, not the daily average.
How many gallons is a shower?
The DOE planning figure is about 20 gallons per shower, but a low-flow head (1.5–1.75 GPM) and a short shower can bring that well under 15. Because showers dominate the peak, that adjustment matters most.
Should I include the dishwasher and washing machine?
Only if they actually run during your peak hour. Both can usually be scheduled off the morning rush; if they never overlap the peak, leaving them out gives a truer size.
What do I do with the peak-hour number?
Match it to a tank whose First-Hour Rating is at or above it, or to a tankless sized for the peak simultaneous flow. The what-size and first-hour-rating tools take it from here.

Related calculators

What Size Water Heater Do I NeedSize a water heater by the DOE first-hour-rating method — add your busiest hour of hot-water use to get the tank gallons and First-Hour Rating you need.First-Hour-Rating CalculatorFirst-Hour Rating is the hot water a tank really delivers in hour one: usable storage plus recovery. Compute the FHR from your tank size and burner input.Tankless GPM SizingSize a tankless water heater by flow, not gallons: add the GPM of the fixtures you run at once at your temperature rise, and get the required GPM rating.