What does "first-hour rating" mean?

First-hour rating (FHR) is how many gallons of hot water a tank heater can deliver in one hour starting with a full tank. It combines stored volume plus how fast the burner/element reheats. Match your peak-hour demand to the FHR, not just the tank volume.

What GPM do I need for a tankless heater?

Add up the flow rates of all fixtures that may run simultaneously during peak use. A typical shower is 1.5–2.5 GPM and a dishwasher is 1–1.5 GPM. If two showers run at once you need at least 3–5 GPM. The calculator totals your selected concurrent loads.

Is a bigger tank always better?

No. Oversized tanks heat and store more water than you use, wasting energy every hour through standby heat loss. Size to your actual peak demand, not maximum possible use.

How accurate is this estimate?

The result is a reliable starting range based on Department of Energy sizing guidelines and appliance flow-rate averages. For final purchase decisions, cross-check with the manufacturer's FHR table and your local plumber.

Does water temperature affect the sizing?

Yes. The calculator assumes a standard 120°F delivery temperature. If you set your heater higher (e.g. 140°F for a dishwasher without a booster) the effective capacity increases slightly because you mix more cold water in — but scalding risk goes up too.

Water Heater Size Calculator

Name: Water Heater Size Calculator
Availability: InStock
Author: Nham Vu

Enter your household size and hot-water habits to get a recommended tank size (gallons) or tankless flow rate (GPM).

Your Household Details

Heater type

Number of people 3

110

Showers per day 3

012

Average shower duration 8 min

3 min20 min

Baths per week 2

014

Dishwasher

Hot-water laundry loads per week 3

014

Fill in your details and click Calculate Size.

Recommendation

Tank

gallons (FHR)

Acceptable range: 40 – 50 gal

Peak-hour demand breakdown

Total peak-hour demand — gal

Standard sizing reference

Household	Tank (gal)	Tankless (GPM)

Summary

Enter your household size and hot-water habits to get a recommended tank size (gallons) or tankless flow rate (GPM).

How it works

Select your heater type: tank (storage) or tankless (on-demand).
Enter the number of people in your household.
Adjust the sliders for showers per day, dishwasher use, and laundry loads per week.
The calculator totals the peak hot-water demand and applies standard sizing formulas.
Review the recommended size range and the breakdown of where demand comes from.

Use cases

Replacing an aging tank and choosing the right gallon capacity.
Deciding whether to switch from a tank to a tankless system.
Sizing a new water heater for a newly built home.
Estimating GPM requirements before buying a tankless unit.
Comparing energy cost trade-offs between tank sizes.
Advising renters or landlords on appropriate heater capacity.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu