LayoutWise

Water Heater Size Calculator

Tell the planner what happens in your busiest hour, or which fixtures run at the same time. You get the first-hour rating to look for in a tank, or the gpm, BTU and kW for a tankless heater.

Your hot water

1. Type
2. Your busiest hour

How many times each happens in the one hour of the day when you use the most hot water.

Showers20 gal each
Shaves2 gal each
Hand dishwashing or food prep3 gal each
Dishwasher loads7 gal each
Top-load washer loads25 gal each
Front-load washer loads15 gal each

Tank size

30 gal

gas, typical

Peak hour

45 gal

hot water you use

First-hour rating

45+ gal

look for at least

That tank

~55 gal

typical rating

Peak hour vs. first-hour rating

45 gal
4030-gal gas tank · ~55 gal first hourShowersShavesHand

Pick a tank whose first-hour rating reaches past the dashed line. Tank size alone does not tell you that — the rating is on the EnergyGuide label.

Typical first-hour ratings

30 gal gasabout 55 gal first hour
40 gal gasabout 70 gal first hour
50 gal gasabout 85 gal first hour
75 gal gasabout 125 gal first hour

Shopping list

  • ✓Water heater: gas tank, about 30 gallons with a first-hour rating of at least 45 gallons
  • ✓Expansion tank (required in many areas with closed plumbing systems)
  • ✓Drain pan and T&P relief valve discharge pipe
  • ✓ENERGY STAR model — check the "first hour rating" on the EnergyGuide label

Gallons per use follow the U.S. Department of Energy sizing worksheet. Ratings vary by model; compare the first-hour rating (tank) or the gpm at your temperature rise (tankless) on the spec sheet.

Tank water heaters: size by first-hour rating

The number that matters is the first-hour rating (FHR) — how many gallons of hot water the heater can deliver in an hour, starting with a full tank. It's on the yellow EnergyGuide label as "Capacity (first hour rating)". Pick a tank whose FHR is at least your peak-hour demand.

Find your peak-hour demand

Think of the one hour when your household uses the most hot water — often the morning rush. The Department of Energy's worksheet counts 20 gallons per shower, 2 per shave, 3 for hand dishwashing or food prep, 7 per dishwasher load, 25 per top-loading washer load and 15 for a front-loader. Three showers, a shave and some dishes come to 65 gallons — so you'd look for an FHR of 65 or more.

Why the same tank size rates differently

Gas burners heat water faster than electric elements, so a 40-gallon gas tank usually has a higher first-hour rating than a 50-gallon electric one. Heat pump water heaters are the most efficient but recover slowly; their FHR is in the maker's literature, since they don't always carry an EnergyGuide label. The typical ratings in the planner are midpoints — always check the model you buy.

Tankless: size by flow and temperature rise

Add up the flow of every fixture you might run at once, in gallons per minute. Then find the temperature rise: your target temperature minus the incoming water temperature. DOE suggests assuming 50 °F incoming water if you don't know it, which means a 70 °F rise for 120 °F hot water. A shower and a bathroom faucet at once need about 3 gpm at that rise.

Gas tankless units typically deliver a 70 °F rise at about 5 gpm; electric units at about 2 gpm. A whole-house electric unit can draw well over 100 amps, so check your electrical panel before choosing one.

Set it to 120 °F

120 °F is hot enough for most homes and lowers both scalding risk and energy use. Use 140 °F only for an appliance that needs it, such as a dishwasher without its own heater, and add an anti-scald valve.

Sources: U.S. Department of Energy — sizing a new water heater.