Water Heater Size Calculator

What the water heater size calculator does

A storage tank has to cover the busiest hour of the day: the hot water already in the tank plus what the heater can reheat during that hour. That combined figure is the first hour rating, printed on the label of every tank heater sold in the US. A tankless heater stores nothing, so it has to heat the full flow of every fixture running at once. The water heater size calculator handles both: peak hour demand for tanks, peak flow and temperature rise for tankless units.

Water heater calculator formulas

For a tank, each use is counted with the typical volumes published by the US Department of Energy, such as 10 gallons (38 L) per shower and 20 gallons (76 L) per bath. The heater recovery rate follows from its input and the temperature rise:

The calculator solves the second line for the tank volume and rounds up to a standard size. Electric elements turn about 98% of their input into heat; a gas tank burner is taken at 76%. For tankless heaters the power follows from the flow and the specific heat of water, 4.186 kJ per kg and kelvin, with 98% efficiency for electric and 95% for condensing gas units.

Water heater size calculator example

A busiest hour with 3 showers, one shave, one dishwasher load, and some food preparation, 10 °C water heated to 50 °C, with a 4.5 kW electric tank:

A tankless heater for one 9 L/min shower at the same temperatures needs 25.6 kW electric, so a 27 kW unit, or about 90,200 BTU/h of gas input.

How to use the water heater sizing tool

Choose storage tank or tankless. Enter the cold water inlet temperature, about 10 °C in much of Europe and the northern US in winter, and the hot water temperature you want. For a tank, count the showers, baths, dishwasher and washer loads, and other uses in the busiest hour of a normal day and pick the heater input. For tankless, enter how many showers and faucets run at the same time and the shower flow rate. Press Calculate.

Reading your first hour rating

When you shop for a tank, compare the first hour rating on the label, not only the tank volume, because a heater with a strong burner or element can deliver more than a larger tank with a weak one. Choose a rating within a few liters or gallons of the peak hour demand; going much bigger only adds standby losses. For tankless units, the flow and the temperature rise decide the size: in winter, colder inlet water can cut the flow a unit delivers by a third, so size for the coldest month.

When to use a hot water demand calculator

• Replacing a tank water heater with the right size
• Switching from a tank to a tankless water heater
• Sizing hot water for a bathroom addition
• Checking whether an electric tankless unit fits the supply
• Comparing gas and electric tank recovery

Common water heater calculator mistakes

• Sizing by tank volume alone. Compare first hour ratings.
• Using a summer inlet temperature. Size for winter.
• Counting daily use instead of the busiest hour. The peak hour sets the size.
• Ignoring the electric supply. Tankless units need heavy circuits.

Tips for hot water heater size

• Low-flow shower heads shrink both tank and tankless sizes.
• Stagger showers and laundry to cut the peak hour.
• Keep stored water at 60 °C to control legionella and use a mixing valve.

Related water heater and energy tools

A water heater ties into the rest of the system. The expansion tank calculator sizes the tank for thermal expansion, the energy cost calculator prices the electricity it uses, and the heat transfer calculator works out the energy to heat any volume.

Tankless water heater sizing and electric supply

Electric tankless heaters draw large currents: 27 kW on a 400 V three-phase supply takes about 39 A per phase, and on a 240 V single-phase US supply a similar unit needs over 100 A. That is why most of Europe sizes electric instantaneous heaters at 18 to 27 kW on three phases, while gas or a storage tank suits larger homes. Every tank or closed hot water system also needs room for the water to expand as it heats, which the expansion tank calculator sizes.

Tank water heater size and running cost

Heating water takes the same energy whatever the heater type: 1 kWh warms about 21 liters by 40 K. A family using 150 liters of hot water a day needs about 7 kWh a day, which you can price with the energy cost calculator. An oversized tank adds standby loss on top, often 1 to 2 kWh a day for a 200 liter tank, so choose the smallest tank that meets the first hour rating. To work out the heating energy for any volume and temperature, use the heat transfer calculator.

Hot water use per activity

Tank water heater size by household

Typical busiest hour, water heated from 10 °C to 50 °C:

Tankless water heater sizing: electric power for 45 °C

Shower flow 9 L/min (2.4 gpm), faucet 5 L/min:

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