HowMuchIt / Water heater
What size water heater do I need?
Tank size alone does not tell you if you will run out of hot water. What matters is your busiest hour for a tank, and the flow you need at once for a tankless heater. Both start from what your home actually does.
Work out your water heater size
| Use | Gallons |
|---|---|
| Shower | 20 |
| Top-loader washer load | 25 |
| Dishwasher load | 7 |
| Hand dishwashing or food prep | 3 |
| Shaving | 2 |
Tankless: shower 2.5 gallons per minute (the federal cap), sink faucet about 1. Rise = hot you want minus cold coming in.
Tank: the first hour rating
Think of the busiest hour in your home, often the morning: two showers, a load of dishes, breakfast. Add the hot water each of those uses, and you have your peak-hour demand. A tank heater's first hour rating tells how many gallons of hot water it delivers in one hour, starting full. Pick a heater whose rating matches your peak hour or goes a little above it. The rating is printed on the yellow EnergyGuide label and in the maker's specifications.
Tankless: flow and temperature rise
A tankless heater warms water as it flows, so it is sized by how many fixtures run at the same moment, in gallons per minute. The second number is the temperature rise: the hot water you want minus the cold water coming into the house. Cold incoming water means a bigger rise, and the same heater then delivers less flow. That is why the same model can be plenty in a warm state and short in a cold one. Measure your cold tap in winter if you can.
Before you replace yours
- Note what your current heater does well or badly: running out in the morning is a peak-hour problem.
- Gas, venting and electrical work call for a licensed plumber or electrician.
- Check the space, the venting and the electrical supply before choosing tank or tankless.
Good to know
- A tank water heater's first hour rating (FHR) is how many gallons of hot water it can deliver in one hour, starting with a full tank of hot water.1,2,3
- Size a tank by your peak hour: add up the hot water used in your busiest hour of the day, then pick a model whose first hour rating at least matches that total.1,2,3
- A shower uses about 20 gallons of hot water.1,2
- An automatic dishwasher load uses about 7 gallons of hot water in the federal worksheet; a utility guide gives 5 to 10 gallons, depending on the machine.1,3
- A top-loading clothes washer uses about 25 gallons of hot water per load, or more.1,3
- Smaller uses are a few gallons each: shaving about 2 gallons, hand dishwashing or food prep about 3 gallons.1,3
- A tankless (demand) heater is sized by flow: add the gallons per minute of every hot water fixture you expect to run at the same time.1,4
- Fixture flow rates used for tankless sizing: a sink faucet about 1 gallon per minute; a shower about 2 to 2.5 gallons per minute.1,2,5
- Temperature rise is the hot water temperature you want minus the temperature of the cold water coming into the house.1,2,5
- A tankless unit's rating depends on both numbers together: the bigger the temperature rise, the fewer gallons per minute it can deliver.1,2,4
- Since January 1, 1994, US federal law has limited new showerheads to 2.5 gallons per minute.6,7,8
- Incoming cold water temperature depends on where you live and on the season; measure it in the coldest season if you can.4,5,9
How we checked this page
Checked on October 26, 2026. We kept a fact only when two independent publishers agree, and each one is numbered to its sources below. Where sources give different numbers, we keep only what they share. We never write a price. This page was written with an AI assistant and checked against the sources below.
Sources
- Sizing a New Water Heater, U.S. Department of Energy (Energy Saver)
- What Size Water Heater Do I Need?, Bob Vila
- Residential Water Heater Sizing Guide, CenterPoint Energy
- A. O. Smith Builder Guide (AOSRB80001), A. O. Smith
- Tankless Sizing (printable), Noritz
- Best Management Practice #7: Faucets and Showerheads, U.S. Department of Energy (FEMP)
- The Show's Over: Showerhead Efficiency Standard Is Restored, NRDC
- Trump's shower executive order, Fast Company
- North America Ground Water Temperature Map, Rinnai