Home Energy Calculators

Tank water heater size.

The size that matters for a tank water heater is its first hour rating (FHR), which should match your household's busiest hour of hot water use. Enter the showers and other hot water in that hour and this works out the demand to match against the FHR on the EnergyGuide label. Runs in your browser; nothing is sent or stored.

Your numbers

Units

Switching resets the example values to the matching unit.

Showers in your busiest hour

Typically the morning rush.

Minutes per shower

Your own habit; 8 is an example.

Showerhead flow (gpm or L/min)

EPA: standard showerheads use 2.5 gpm; WaterSense labelled ones 2.0 gpm or less. Check your showerhead.

Other hot water in that hour (gallons or litres)

Dishwasher, laundry and sinks: your estimate of the hot water they draw from the heater. 10 is an example.

Shower temperature (°F or °C)

The mixed water temperature at the shower. Example value.

Cold water inlet temperature (°F or °C)

Varies by season and region. Example value.

Tank thermostat setting (°F or °C)

The temperature the heater stores. Example value.

First hour rating of a heater you are considering (optional)

In gallons or litres, from the EnergyGuide label or the maker's literature. 55 is an example.

Result

Fill the form and press Calculate.

Worked example

Two showers of 8 minutes at 2.5 gpm draw 40 gallons of mixed water at 105°F. With 50°F cold water and a 120°F tank, the mix is (105 - 50) / (120 - 50) = 79% hot, so the showers take 31.4 gallons from the heater. Add 10 gallons for the dishwasher and sinks and the peak-hour demand is 41.4 gallons. A heater with a first hour rating of 55 meets that with 13.6 gallons to spare; one rated 40 would be short by 1.4 gallons. These are example inputs: replace them with your own.

How it works

Peak-hour demand = showers x minutes x flow x hot share + other hot water. The hot share is (shower temperature - cold temperature) / (tank temperature - cold temperature), because shower water is a mix of hot water from the tank and cold from the line. The first hour rating is, per Vermont Energy Saver, the number of gallons of hot water the heater can supply per hour starting with a full tank of hot water; it depends on tank capacity and the size of the burner or element, and is listed on the EnergyGuide label. Vermont Energy Saver advises looking for a model whose first hour rating matches within 1 or 2 gallons of your peak hour demand.

Not covered. Tankless heaters are sized by flow instead (see the tankless calculator). Hot water from a dishwasher or washing machine varies by model, so check its manual. Treat the result as a planning estimate and confirm with a plumber or the manufacturer's sizing guide.

Sources and basis (pages opened 2026-10-08): Vermont Energy Saver, Storage Water Heaters; US EPA WaterSense, Showerheads.

This is a planning estimate and not professional plumbing, electrical or financial advice. Rates, fixtures and equipment vary, so confirm important decisions with your utility, the manufacturer or a licensed professional.

FAQ

What size water heater do I need?

Match the first hour rating (FHR) on the EnergyGuide label to your peak-hour hot water demand. This calculator estimates that demand from the showers and other hot water used in your busiest hour.

What is a first hour rating?

The number of gallons of hot water a storage heater can supply in an hour, starting with a full tank of hot water. It depends on tank size and the burner or element, and appears on the EnergyGuide label.

How close should the first hour rating be to my peak hour demand?

Vermont Energy Saver suggests a model whose first hour rating matches within 1 or 2 gallons of your peak hour demand. Going under risks running out of hot water in your busiest hour.

Does tank size in gallons equal first hour rating?

No. The first hour rating includes how fast the burner or element reheats water, so a smaller tank with a powerful burner can have a higher FHR than a larger, weaker one.

Can I use litres and degrees Celsius?

Yes. Switch the units setting. The calculator converts to US gallons and Fahrenheit internally and shows both gallons and litres.