Water heater size

Size a tank by its hot water in the first hour. Size a tankless heater by flow and temperature rise.

A tank's first-hour hot water should cover your busiest hour, 65 gallons in the example. Three showers at 20 gallons, a shave at 2, and hand dishwashing at 3 make 65 gallons. A dishwasher load adds 7 gallons if it runs in that hour.

A quiet hour with one shower and some food prep uses 23 gallons. A heavier hour with four showers and a top-loading washer load takes 105 gallons. In tests, 3 of 505 electric storage models give 105 gallons or more in the first hour. A front-loading washer load takes 15 gallons of hot water, against 25 for a top loader.

A tankless heater needs 3.25 gallons per minute for a shower and a faucet at once. In tests, 6 of 337 electric tankless models are certified at 3.25 gallons per minute or more. Add a faucet to two showers and the flow comes to 5.75 gallons per minute.

Tank or tankless, for your household · Replace your water heater

Your busiest hour

Example busiest hour: 65 gallons. Fixtures at once: 3.25 gal/min at a 70°F rise. Cold water: 50°F.

Cold water: Type the temperature of your cold water.

Tank water heaters

Add up the hot water your home uses in its busiest hour. Then pick a tank whose first-hour rating is at least that much.

Hot water each time

Gallons of hot water each use takes
Use	Gallons of hot water
Shower	20
Shave	2
Hand dishwashing or food prep	3
Automatic dishwasher load	7
Top-loading clothes washer load	25
Front-loading clothes washer load	15

Busiest hour, and the tanks that cover it

Busiest-hour examples and the basic models whose first-hour rating covers each
Busiest hour	Gallons	Gas storage models	Electric storage models	Heat pump models
Example busiest hour	65	629 of 763	95 of 505	365 of 481
One shower and one hand dishwashing	23	763 of 763	505 of 505	481 of 481
Three showers, one shave, and one hand dishwashing	65	629 of 763	95 of 505	365 of 481
Four showers and one top-loading clothes washer load	105	83 of 763	3 of 505	10 of 481

A basic model counts where every one of its listed models delivers at least that much hot water in the first hour.

Hot water in the first hour, by size

First-hour rating by tank size, lowest to highest
Size	Basic models	First-hour rating (gal)
30-gallon gas storage	105	46–170
40-gallon gas storage	287	51–91
50-gallon gas storage	325	65–187
65-gallon gas storage	1	157
80-gallon gas storage	16	130–206
30-gallon electric storage	95	29–60
40-gallon electric storage	181	38–66
50-gallon electric storage	144	40–78
80-gallon electric storage	7	83–86
40-gallon heat pump	87	43–73
50-gallon heat pump	118	41–77
65-gallon heat pump	121	46–87
80-gallon heat pump	115	55–105

Tankless water heaters

Add up the flow of the fixtures you run at once. The temperature rise is how much the heater warms the cold water: 50°F water heated to 120°F is a 70°F rise. A dishwasher with no heater of its own may want 140°F, a 90°F rise.

Each certified flow on this site is at a 67°F rise. Colder water coming in needs a bigger rise, and the same heater then delivers less flow.

Flow at once, and the heaters that cover it

Fixtures run at once and the basic models certified at that flow or more
Run at once	Gallons per minute	Gas tankless models	Electric tankless models
Example fixtures at once, at a 70°F rise	3.25	451 of 487	6 of 337
One shower	2.5	476 of 487	21 of 337
One shower and one faucet	3.25	459 of 487	6 of 337
Two showers	5.0	210 of 487	1 of 337
Two showers and one faucet	5.75	60 of 487	1 of 337

A shower head is counted at 2.5 gallons per minute and a faucet at 0.75. A basic model counts where every one of its listed models is certified at that flow or more, at a 67°F rise. The first row uses the rise named beside it.

Only models with a certified flow figure count in the tankless totals.

Certified flow, by group

Certified flow at a 67°F rise by flow group
Flow group	Basic models	Flow (gal/min)
Gas tankless, under 4.0 gallons per minute	88	1.6–3.9
Gas tankless, 4.0 to 4.9 gallons per minute	189	4.0–4.9
Gas tankless, 5.0 to 5.9 gallons per minute	193	5.0–5.9
Gas tankless, 6.0 gallons per minute and up	17	6.0–8.5
Electric tankless, under 1.0 gallon per minute	223	0.2–0.9
Electric tankless, 1.0 to 1.9 gallons per minute	82	1.0–1.9
Electric tankless, 2.0 gallons per minute and up	32	2.2–6.5

See water heaters by type

Updated Oct 26, 2026 · Methodology

Not covered

Daily hot water use. These examples cover the busiest hour only.
Water heaters that also heat the home.