Pick Tank or Tankless first. For a tank, enter how many showers, baths, shaves, dishwasher loads and so on happen in your household's busiest hour; the calculator gives the first-hour rating to look for. For tankless, enter how many outlets might run at once, the incoming water temperature and the heater setting; it returns the flow you need and the power that takes, electric or gas. The US units and Metric buttons switch gallons to litres and convert the temperatures.
Two heater types, two sums
A tank runs out. A tankless heater never runs out but can run short. So they are sized differently.
For a tank, what matters is the hot water you draw in your worst hour. The calculator multiplies each activity by a standard amount and adds them up. The total is your peak hour demand, and the heater's first-hour rating (FHR) should meet it.
| Activity | Hot water per use |
|---|---|
| Shower | 10 gallons (38 litres) |
| Bath | 20 gallons (76 litres) |
| Shave with the tap running | 2 gallons (8 litres) |
| Washing-up by hand | 4 gallons (15 litres) |
| Dishwasher load | 6 gallons (23 litres) |
| Food prep | 5 gallons (19 litres) |
| Hot washing-machine load | 7 gallons (26 litres) |
A tankless unit can only heat so many gallons a minute. The calculator adds up everything that might run together (a shower at 2.0 gpm, a bathroom faucet at 1.0, a kitchen faucet or dishwasher at 1.5, a washing machine at 2.0, a tub at 4.0) and the heat needed to lift that flow to your setting:
BTU per hour = gpm × 500 × temperature rise in °F
The 500 is the weight of a gallon of water, about 8.33 lb, times 60 minutes. Divide by 3,412 for electric kilowatts, or by the burner efficiency (90 % here) for gas input.
Worked example with the starting values
In Tank mode the calculator opens with three showers, one shave and one dishwasher load in the busiest hour:
3 × 10 + 1 × 2 + 1 × 6 = 38 gallons (144 litres)
So it asks for a first-hour rating of at least 38 gallons.
In Tankless mode it opens with two showers and the kitchen faucet running together, water arriving at 50 °F (10 °C) and the heater set to 120 °F (49 °C):
- Flow: 2 × 2.0 + 1.5 = 5.5 gpm (20.8 L/min)
- Temperature rise: 120 minus 50 = 70 °F (39 °C)
- Heat: 5.5 × 500 × 70 = 192,500 BTU per hour
- Electric: 192,500 ÷ 3,412 = about 56.4 kW
- Gas at 90 % efficiency: 192,500 ÷ 0.9 = about 214,000 BTU/h input
Reading the answer
For a tank, find the first-hour rating on the yellow EnergyGuide label or the spec sheet. Capacity alone tells you little: a gas tank reheats faster than an electric tank of the same size, so it usually has a higher first-hour rating.
For tankless, ignore the headline maximum and look for the flow the maker quotes at your temperature rise. The formula run backwards shows why: a 24 kW electric unit delivers about 2.3 gpm (8.9 L/min) at a 70 °F rise, which is one shower, not two. The 56.4 kW in the example would draw about 235 amps at 240 volts, more than the whole electrical service of many houses. Hence big tankless setups in cold regions tend to be gas, or two units side by side.
If the complaint is a long wait for hot water rather than running out, the heater is rarely to blame; the pipe volume calculator shows how much cold water sits in the line.
Limits, and when to call a professional
- The per-use figures assume ordinary habits. Long showers or a big soaking tub push the busiest hour past the table.
- Fixture flows are typical. A 1.5 gpm shower head needs less, an older head may need more; measure yours with the flow rate calculator.
- Incoming water is colder in winter, from roughly 40 °F (4 °C) in the northern US to 70 °F (21 °C) in the south. Size a tankless unit for the coldest month.
- Gas heaters lose some output at high altitude, and hard water scales up heating surfaces over the years (see hard water and limescale).
Installing a water heater involves gas pipework, venting, electrical supply or a temperature and pressure relief valve, often several of these. That is work for a licensed plumber, gas fitter or electrician, and permit rules differ by state and country. Ask the installer to confirm the size if your answer falls between two models.
Where the figures come from
The per-use gallons and the peak hour method come from the US Department of Energy's water heater sizing worksheet, which suggests a first-hour rating within a gallon or two of your peak hour demand. The 500 factor and the 3,412 BTU per kWh are physical constants. Shower heads sold new in the US are capped at 2.5 gpm by federal rules, and EPA WaterSense labelled heads use 2.0 gpm or less.
FAQ
What size water heater do I need for a family of 4?
Size it by the busiest hour, not the head count. Three showers, a shave and a dishwasher load in the same hour need a first-hour rating of about 38 gallons (144 litres); add a bath and it rises to 58 gallons. That often means a 40 or 50 gallon tank.
What size tankless water heater do I need?
Add up the flow of everything you run at once and the temperature rise in your coldest month. Two showers and a kitchen faucet at a 70 °F (39 °C) rise need 5.5 gpm, which is about 56.4 kW electric or 214,000 BTU/h of gas input. One 2.0 gpm shower at the same rise needs about 20.5 kW.
What is a standard water heater size?
In US homes, 40 and 50 gallon (151 and 189 litre) tanks are the most common, with smaller tanks in flats and larger ones for big households. A tank far bigger than your peak hour just keeps extra water hot all day, so compare first-hour ratings, not capacity.