How to use the water heater size calculator
- Choose the heater type: Storage tank or Tankless.
- For a tank, list what uses hot water in your busiest hour: how many times, and how many hot gallons each time. The starting rows are examples, so change them to match your household.
- For a tankless heater, list the fixtures that run at the same time with their flow in gallons per minute, then enter the hot water temperature you want, the cold water coming in and the heater efficiency.
- Read the first hour rating to look for (tank) or the heating power needed (tankless). Print the result to take shopping.
Formula
Busiest-hour demand = sum of (times used x hot gallons each time)
First hour rating to look for = demand x (1 + margin %), rounded up to a whole gallon
Tankless heat needed = gallons per minute x temperature rise (°F) x 8.34 x 60 Btu per hour
kW = Btu per hour / 3,412.14, and heater input = delivered heat / efficiency
Water weighs about 8.34 pounds per gallon, and raising one pound by one degree Fahrenheit takes one Btu, so one gallon per minute heated by one degree takes 8.34 x 60 = 500.4 Btu per hour.
Worked examples
For a storage tank, 2 showers at 10 hot gallons, 1 dishwasher load at 6, 1 clothes washer load at 7 and 4 sink uses at 1.5 add up to 20 + 6 + 7 + 6 = 39 gallons in the busiest hour. A 10% margin gives 42.9, which rounds up to 43 gallons.
For a tankless heater, a shower at 2 gallons per minute, a kitchen sink at 1.5 and a dishwasher at 1.5 running together is 5 gallons per minute. From 55°F to 120°F is a rise of 65°F, so the heater must deliver 5 x 65 x 500.4 = 162,630 Btu per hour, which is 47.66 kW. At 80% efficiency the heater input is 203,288 Btu per hour, or 59.58 kW.
Tank or tankless
A storage tank heats a reservoir of water ahead of time, so it can supply a lot of hot water at once but runs out if demand is too high. The first hour rating published for a tank is how much hot water it can deliver in one hour, starting with a full tank. A common way to size a tank is to choose a model whose first hour rating is at least your busiest hour of use. Check the rating published by the manufacturer.
A tankless heater heats water as it flows, so it never runs out but is limited by how fast it can raise the temperature. The key numbers are the flow you need at the same time and the temperature rise. Colder incoming water in winter raises the needed heat, so use a realistic cold water temperature for your location. Manufacturers publish the flow each model can deliver at a given rise.
Using the estimate
The use figures in the tool are examples that you replace with real numbers for your household. Counting hot gallons rather than total gallons matters, because showers and sinks mix hot and cold water. This page is a planning aid and not an installer's sizing: fuel type, venting, gas line size, electrical capacity and local codes also decide what can be installed.
Nothing you enter leaves your browser. Related tools include the BTU calculator, the electricity cost calculator, the water bill calculator and the propane tank runtime calculator.
Assumptions and limits
- Usage, flow rates, temperatures and efficiency are numbers you enter. The starting values are examples and not recommendations.
- Gallons are US gallons and temperatures are in degrees Fahrenheit.
- The factor 8.34 pounds per gallon is used, so the heat needed is slightly more than the common 500 rule of thumb.
- Electrical service, gas supply, venting, code requirements and the heater's recovery rate are not modeled.
Frequently asked questions
What size water heater do I need?
For a storage tank, find your busiest hour of hot water use and choose a model whose first hour rating is at least that. For a tankless heater, add up the flow of fixtures used together and the temperature rise.
What is a first hour rating?
It is the amount of hot water a storage tank can deliver in one hour starting from a full tank of hot water. The manufacturer publishes it, and it is often more useful than the tank size alone.
Why does winter matter for a tankless heater?
Incoming water is colder in winter in many places, so the heater must raise the temperature further. The same flow then needs more power, and a unit that is fine in summer may not keep up.
Are the starting numbers recommendations?
No. They are examples so the tool shows a result. Replace the uses, flow rates and temperatures with your own, since households differ a lot.