Water Heater Size Calculator
Water Heater Size Calculator
Size a new water heater the way the US Department of Energy suggests: a tank by the hot water your home uses in its busiest hour, matched to the first hour rating on the label, or a tankless unit by the flow of the fixtures running at once and how much the water has to be heated.
In your busiest hour, how many of each?
How many of each running at the same time?
Where the hot water goes
Sizes against your demand
Gallons per use are the averages the US Department of Energy gives for sizing: shower 20, shave 2, hand dishwashing or food prep 3, dishwasher 7, top load washer 25, front load washer 15. A tank’s first hour rating, printed on its EnergyGuide label, should be at least the peak hour demand. The first hour ratings shown for common tanks are approximations, three quarters of the tank plus one hour of recovery at a 90 °F rise (gas at 80 percent recovery efficiency, electric with one 4,500 W element); the label on the model you buy is the real figure. Tankless sizing uses the fixture flows shown, the rise from the incoming water temperature to the setting (DOE suggests assuming 50 °F incoming and 120 °F hot unless you know otherwise), 500 BTU per hour per gpm per °F, 95 percent efficiency for gas and 99 percent for electric. The CSV, image and printed report contain the values you entered, nothing preloaded.
How to use this tool
Pick tank or tankless and the fuel. For a tank, count what happens in your busiest hour, usually the morning rush: showers, shaves, dishes and laundry. For tankless, count the fixtures that run at the same moment, choose your shower head flow, and set the incoming water temperature and your hot water setting. The size to look for appears at the top.
The first chart shows where the hot water goes, and the second compares common sizes with your demand. Switch between tank and tankless to compare the two for the same home.
How a tank is sized
A storage water heater is sized by its first hour rating, the hot water it can deliver in an hour starting from a full tank. The US Department of Energy suggests working out your peak hour demand, the most hot water used in any one hour, and choosing a tank whose first hour rating at least matches it. Its averages are 20 gallons for a shower, 2 for a shave, 3 for hand dishwashing or food prep, 7 for a dishwasher, 25 for a top load washer and 15 for a front load washer.
How a tankless heater is sized
A tankless heater has no store of hot water, so it must heat the full flow of everything running at once. Add up the flow of those fixtures in gallons per minute, then find the temperature rise from the incoming water to the setting. Heating water takes 8.33 BTU per gallon per degree, so the heat needed is gpm times 60 times 8.33 times the rise, about 500 times gpm times the rise, in BTU per hour.
Hot water per use
| Use | Gallons | Liters |
|---|---|---|
| Shower | 20 | 76 |
| Shave | 2 | 8 |
| Hand dishwashing or food prep | 3 | 11 |
| Dishwasher load | 7 | 26 |
| Top load washer | 25 | 95 |
| Front load washer | 15 | 57 |
Average hot water per use, from the US Department of Energy. Long showers, big tubs and older fixtures use more.
Tankless flow by size and incoming water temperature
| Tankless size | 40 °F in | 50 °F in | 60 °F in | 70 °F in |
|---|---|---|---|---|
| 120,000 BTU gas | 2.9 gpm | 3.3 gpm | 3.8 gpm | 4.6 gpm |
| 140,000 BTU gas | 3.3 gpm | 3.8 gpm | 4.4 gpm | 5.3 gpm |
| 160,000 BTU gas | 3.8 gpm | 4.3 gpm | 5.1 gpm | 6.1 gpm |
| 180,000 BTU gas | 4.3 gpm | 4.9 gpm | 5.7 gpm | 6.8 gpm |
| 199,000 BTU gas | 4.7 gpm | 5.4 gpm | 6.3 gpm | 7.6 gpm |
| 11 kW electric | 0.9 gpm | 1.1 gpm | 1.2 gpm | 1.5 gpm |
| 18 kW electric | 1.5 gpm | 1.7 gpm | 2.0 gpm | 2.4 gpm |
| 24 kW electric | 2.0 gpm | 2.3 gpm | 2.7 gpm | 3.2 gpm |
| 27 kW electric | 2.3 gpm | 2.6 gpm | 3.0 gpm | 3.6 gpm |
| 32 kW electric | 2.7 gpm | 3.1 gpm | 3.6 gpm | 4.3 gpm |
| 36 kW electric | 3.0 gpm | 3.5 gpm | 4.1 gpm | 4.9 gpm |
Most hot water flow each size gives with 120 °F output and the incoming water temperature shown, at 95 percent efficiency for gas and 99 percent for electric.
A worked example you can check on paper
In a home's busiest hour, three people shower, one shaves and someone washes a sink of dishes: 3 times 20, plus 2, plus 3, 65 gallons. A tank needs a first hour rating of at least 65 gallons. A typical 40 gallon gas tank with a 40,000 BTU burner manages about 73; a 50 gallon electric tank manages only about 58, so an electric home would need an 80 gallon tank.
On tankless, two of those showers at 2.0 gpm run together with a 0.75 gpm bathroom faucet: 4.75 gpm. From 50 °F to 120 °F is a 70 °F rise, so the heat is 4.75 times 500 times 70, about 166,000 BTU per hour, and a gas unit at 95 percent needs about 175,000 BTU of input: a 180,000 BTU size. Electric would take about 49 kW, more than one common unit.
Mistakes that catch people out
Sizing by the number of bedrooms. Two people who shower at the same time need more than four who spread out through the day.
Comparing tank sizes, not first hour ratings. Gas and electric tanks of the same size deliver very different amounts in the first hour.
Using summer water temperatures for tankless. Incoming water can be 10 to 20 °F colder in winter, and the unit's flow drops with it.
Forgetting the supply. Gas tankless units need a bigger gas line and new venting; electric units need heavy circuits and enough service capacity.
Popular calculations
Each opens the calculator with its own address, and every value can be changed once it is open.
Gas tanks
Electric tanks
Gas tankless
Electric tankless
Metric
How this was checked
The engine was tested in Node against the Department of Energy's worked examples: three showers, a shave and hand dishwashing add to 65 gallons of peak demand, and a 0.75 gpm faucet with a 2.5 gpm shower head gives the 3.25 gpm the DOE example uses. Tankless heat was checked by hand at 500 BTU per hour per gpm per °F, and the approximate first hour ratings of common tanks were compared with published ratings, which they match within a few gallons.
Every address in the lists above was then opened and checked for a distinct title, the right answer, charts that render, and clean CSV, image and print output containing only that calculation, at desktop, tablet and phone widths.
Common questions
What size water heater do I need for a family of 4?
Size it by the busiest hour rather than the number of people. If that hour has three showers, a shave and a sink of dishes, the demand is about 65 gallons, so look for a first hour rating of 65 or more: typically a 40 gallon gas tank or an 80 gallon electric tank.
What is a first hour rating?
The number of gallons of hot water a tank can deliver in an hour starting full, printed on the yellow EnergyGuide label. It depends on the tank size and how fast the burner or elements recover, which is why a 40 gallon gas tank can outperform a 50 gallon electric one.
What size tankless water heater do I need?
Add up the flow of the fixtures that run at once and the temperature rise from the incoming water to your hot water setting. Two 2.0 gpm showers and a 0.75 gpm faucet with water coming in at 50 °F need about 4.75 gpm at a 70 °F rise, around 175,000 BTU of gas input.
How cold is my incoming water?
The Department of Energy suggests assuming 50 °F unless you know otherwise. Groundwater is colder in the north, often around 40 to 45 °F in winter, and warmer in the South, 60 to 70 °F or more, so a tankless unit sized in the South can fall short up north.
Is an electric tankless water heater enough for a whole house?
In warm climates, often yes for one or two fixtures at a time. Heating 2.75 gpm through an 80 °F rise takes about 33 kW, roughly 136 amps at 240 V, so larger homes in cold climates usually need gas or a tank.
Is a bigger water heater better?
Not past what you need. A larger tank keeps more water hot all day and loses more heat, and a larger tankless unit costs more and may need a bigger gas line. Match the first hour rating or flow to your real peak.
Related Calculators and Reference
- Furnace Size Calculator: heating the house as well as the water.
- Flow Rate Converter: gpm, liters per minute and more.
- Energy Converter: BTU, kWh and therms.
- Watts to Amps Calculator: current for an electric heater.
- Pool Chlorine Calculator: dosing a pool or hot tub.
- Water Softener Size Calculator: hard water scale builds on heating elements, so check the softener size for the same supply.
- Septic Tank Size Calculator: size the septic tank and drain field for the same household.
- Energy Saver: Sizing a New Water Heater: the Department of Energy method used here.