Water Heater Size Calculator: First Hour Rating

The number that tells you whether a tank water heater is big enough is its first-hour rating (FHR): the gallons of hot water it can deliver in one hour starting from a full, hot tank. Tank size in gallons is only part of that; how fast the burner or element reheats the water is the rest. To size a water heater, estimate the hot water your household uses in its busiest hour, usually the morning showers, and pick a model whose FHR is at least that. For a tankless heater, the figure is the flow in gallons per minute it can deliver at your temperature rise. The calculator below does both, using the federal test conditions as defaults.

Water heater size calculator

Defaults: 58°F incoming water and a 125°F tank are the conditions of the DOE water heater test; the 105°F shower is our assumption for a comfortable shower. At those temperatures a shower is 70% hot water and 30% cold. Sinks are counted at the federal maximum faucet flow of 2.2 gpm, and dishwashers at the federal maximum of 5.0 gallons per cycle (most use less; enter your model’s figure). Enter the clothes washer’s hot water from its manual, or zero for cold-water washing.

First-hour rating needed for back-to-back showers

This is the “family of 4” answer, made explicit. If four people each take an 8-minute shower in the same morning hour with 2.0 gpm heads, the household needs about 45 gallons of hot water in that hour, so a tank heater with an FHR of 45 gallons or more. With 2.5 gpm heads it is 56 gallons. If the showers are spread across the morning and evening, the busiest hour may hold only two, and a smaller heater will do. The number of people matters less than how many of them use hot water in the same hour.

DOE usage bins: how water heaters are rated

Every residential water heater sold in the US is tested to the same federal method. Water enters at 58°F, the heater delivers it at 125°F, and in the first-hour test tank heaters are drained at 3 gpm (1.5 gpm for tanks under 20 gallons), with new draws started as the heater’s controls cycle, until the hour is up. The water delivered in that hour is the first-hour rating. Tankless heaters are rated instead by the maximum flow they can hold over a 67°F rise for 10 minutes.

The rating then puts the heater in a usage bin, and it is tested for efficiency on that bin’s daily draw pattern: 38 gallons a day for the low bin, 55 for medium, 84 for high. That is why the Uniform Energy Factor (UEF) on the label is quoted together with a bin, and why comparing the UEF of a medium-bin heater with a high-bin one is not like for like.

Tankless water heaters: flow and temperature rise

A tankless heater has no stored hot water to fall back on, so it must heat the full flow as it passes. The load is the flow times the temperature rise, which is why the same unit delivers far fewer gallons per minute in a northern winter than in a southern summer. With 58°F water and a 125°F setting the rise is 67°F; with 40°F winter water heated to 120°F it is 80°F. Two 2.0 gpm showers at 105°F, from 40°F water heated to 120°F, need about 3.3 gpm of hot water, close to 140,000 Btu/h of gas at 95% efficiency, or about 38 kW electric, roughly 160 A at 240 V. That electrical load is the practical limit for whole-house electric tankless heaters in cold climates.

Federal fixture flow limits used above

ENERGY STAR water heater criteria

ENERGY STAR’s own advice for a replacement is to start from the capacity of the heater you have, from its nameplate or yellow EnergyGuide label, and go larger if people, showers or water-using appliances have been added. That is a sensible floor; the first-hour rating comparison above tells you whether the old heater was actually big enough. ENERGY STAR also notes that heat pump water heaters need a space that stays between 40 and 90°F year-round with about 1,000 cu ft of air around the unit, and that in cold weather with cold incoming water they may switch to electric resistance heating for a while, which uses more energy.

Worked example: family of four, one morning rush

Three showers between 6:30 and 7:30, each 8 minutes with 1.8 gpm heads: 3 x 1.8 x 8 = 43.2 gallons of mixed water, 70% hot, so 30.3 gallons at 125°F. Add 4 minutes of hot water at the kitchen sink (4 x 2.2 x 0.70 = 6.2 gal) and a dishwasher run started before leaving (we assume 4 gal; check your model’s figure, which cannot exceed the 5.0 gallon federal limit): about 41 gallons. A tank heater with an FHR of 41 gallons or more covers the rush, which is in the DOE low usage bin. If a fourth person showers in the same hour, it rises to about 51 gallons, the edge of the medium bin.

Other things that change the answer

• Incoming water temperature. Groundwater and municipal supply temperatures vary with region and season. Colder water means each shower takes more hot water and each tankless gallon needs more heat. Measure it at a cold tap in winter if you can.
• Tank temperature setting. A hotter tank stretches the stored water further because less of it is needed per shower, but it increases standby loss and scald risk; mixing valves let a tank run hotter while delivering safer water at the taps.
• Big tubs and body sprays. A large soaking tub can take more hot water in a few minutes than several showers; enter its fill volume in the bath field.
• Recirculation. A recirculating loop brings hot water to the tap faster but adds heat loss from the pipes; it does not change the first-hour rating you need.

Related calculators

• Bathroom exhaust fan CFM calculator: clearing shower steam
• Insulation R-values by climate zone
• All construction calculators

Frequently asked questions

What size water heater do I need for a family of 4?

It depends on how many showers fall in the same hour. Four 8-minute showers at 2.0 gpm in one hour need about 45 gallons of hot water, so a tank heater with a first-hour rating of 45 gallons or more; with showers spread out, less.

What is a first-hour rating?

The gallons of hot water a tank water heater can deliver in one hour, starting fully heated, under the DOE test (58°F in, 125°F out, drawn at 3 gpm). It combines tank size and recovery speed.

Is a 40 or 50 gallon water heater better?

Compare first-hour ratings rather than tank size: a 40 gallon gas heater with fast recovery can out-deliver a 50 gallon electric one. Choose the one whose FHR covers your busiest hour.

How many gpm tankless water heater do I need?

Add the flows that run at the same time, as hot water: two 2.0 gpm showers mixed to 105°F take about 2.8 gpm of 125°F water from 58°F supply. Check that the heater delivers that at your winter temperature rise.

What do the DOE usage bins mean?

Very small, low, medium and high bins group water heaters by first-hour rating (under 18, 18 to 50, 51 to 74, 75 gallons and up). Each bin is tested on its own daily draw pattern for the efficiency rating.

Sources

1. 10 CFR Part 430, Subpart B, Appendix E, Uniform Test Method for Measuring the Energy Consumption of Water Heaters: sections 2.3 to 2.5 (test temperatures), 5.3 (first-hour and maximum GPM tests), 5.4.1 Tables I and II (draw pattern bins), Tables III.1 to III.4 (daily volumes). US federal regulation, read at law.cornell.edu (cached 25 October 2026).
2. 10 CFR 430.32(f), (o) and (p): dishwasher water, faucet and showerhead maximums.
3. ENERGY STAR Water Heater Key Product Criteria and ENERGY STAR heat pump water heaters (read 25 October 2026).
4. Shower temperature (105°F), shower length (8 minutes) and the household examples are our assumptions; the arithmetic is shown with each table.

Last reviewed: 25 October 2026. Numbers in the tables are our arithmetic from the sources listed; code sections are summarised, not quoted. How we check our work: methodology and corrections. Spotted an error? Please let us know.