Water Heater Size Calculator

1 Your busiest hour

Showers

Times in your busiest hour. AHRI counts about 20 gallons of hot water each.

Baths

Times in your busiest hour. AHRI counts about 20 gallons of hot water each.

Shaving

Times in your busiest hour. AHRI counts about 2 gallons of hot water each.

Hand and face washing

Times in your busiest hour. AHRI counts about 4 gallons of hot water each.

Hair shampoo

Times in your busiest hour. AHRI counts about 4 gallons of hot water each.

Hand dishwashing

Times in your busiest hour. AHRI counts about 4 gallons of hot water each.

Automatic dishwasher

Times in your busiest hour. AHRI counts about 14 gallons of hot water each.

Food preparation

Times in your busiest hour. AHRI counts about 5 gallons of hot water each.

Automatic clothes washer

Times in your busiest hour. AHRI counts about 32 gallons of hot water each.

Tankless: showers running at once

Tankless: clothes washer filling at once

Tankless: sinks running at once

0.5 gpm of hot water each, DOE's kitchen sink example.

Tankless: dishwasher filling at once

Incoming water temperature (tankless)From EPA's map of average shallow ground water temperatures. Mountain areas can be colder than their region, and winter water is often colder.

2 What size to buy

Tank: first-hour rating

70 gallons or more

your busiest hour uses about 70 gallons

Tankless: flow rate

4.5 gpm or more

at a 70°F temperature rise

EnergyGuide group

medium

for a tank rated at this first-hour rating

Showers	3 × 20 gallons = 60 gallons
Shaving	1 × 2 gallons = 2 gallons
Hair shampoo	1 × 4 gallons = 4 gallons
Hand dishwashing	1 × 4 gallons = 4 gallons
Peak-hour demand	70 gallons: buy a tank whose first-hour rating is at least this
1 shower	1.5 gpm
Clothes washer	2 gpm
1 sink	0.5 gpm
Dishwasher	0.5 gpm
Temperature rise	120°F hot water − 50°F incoming water = 70°F

The first-hour rating is on the yellow EnergyGuide label of a tank water heater, and a tankless model's label lists its maximum flow in gallons per minute. Check the tankless model's flow at your temperature rise in its specifications, since its flow drops as the rise gets bigger.

Set the thermostat to 120°F (49°C), as DOE and the Consumer Product Safety Commission recommend. CPSC warns that 140°F (60°C) water can burn in six seconds.