How this calculator works
Enter your household size, number of bathrooms, and how many showers and appliance loads typically happen during your busiest hot-water hour, plus your water heater's fuel type, and this calculator estimates the storage tank size needed using the Department of Energy's first-hour rating (FHR) method. It also estimates the continuous flow rate, in gallons per minute, that a tankless unit would need to cover several fixtures running at the exact same moment.
Storage tank water heaters are properly sized by first-hour rating, not raw tank capacity, because a fast-recovering gas unit can supply more hot water in an hour from a smaller tank than a slow-recovering electric unit with a bigger tank. Tankless (on-demand) water heaters don't store hot water at all, so they're sized differently — by the combined flow rate of every fixture that might run simultaneously, since a tankless unit has a hard limit on how many gallons per minute it can heat at once, no matter how long the demand lasts.
The formula
Peak-hour demand (gal) = showers × 17.5 + laundry loads × 20 + dishwasher loads × 6 (typical gallons per use)First-hour rating ≈ 0.7 × tank capacity + recovery rate (gal/hr); solved for tank size: tank = (required FHR − recovery) ÷ 0.7Tankless GPM required = simultaneous showers × 2.5 + simultaneous faucets × 1.5 + simultaneous appliances × 2.0The 0.7 usable-fraction figure, per-fixture gallons-per-use estimates, and typical recovery rates by fuel type (roughly 40 gal/hr for gas, 21 for electric resistance, 10 for heat pump electric) are commonly used planning approximations — check your specific water heater's EnergyGuide label for its actual first-hour rating and recovery rate.
Worked example: 2 showers + 1 laundry load, gas water heater
- Peak-hour demand: 2 × 17.5 (showers) + 1 × 20 (laundry) + 0 (dishwasher) = 55 gallons.
- Gas recovery rate: about 40 gal/hr. Required tank: (55 − 40) ÷ 0.7 ≈ 21.4, rounded up to 22 gallons.
- Check: 22 × 0.7 + 40 ≈ 55.4 gallons of first-hour rating, which covers the 55 gallon peak demand.
- For a tankless unit instead, with 2 showers and 1 faucet running at the same moment: 2 × 2.5 + 1 × 1.5 = 6.5 GPM required.
Frequently asked questions
What is a first-hour rating, and where do I find it?
First-hour rating (FHR) is the number of gallons of hot water a storage water heater can deliver in one hour, starting from a full tank, printed on its yellow EnergyGuide label alongside the tank's rated capacity. It's the standard, DOE-recognized way to compare how different models actually perform, since it accounts for both stored volume and how fast the unit reheats.
Why would an electric water heater need a bigger tank than a gas one for the same household?
Gas burners transfer heat much faster than electric resistance elements, so a gas unit can recover (reheat) a much larger volume of water per hour. To hit the same first-hour rating, a slower-recovering electric or heat pump unit needs more stored capacity to make up for its slower reheating, which is why this calculator recommends a larger tank for electric and heat pump models given the same usage pattern.
How is tankless sizing different from tank sizing?
A storage tank water heater draws down its reserve during a burst of demand and reheats afterward, so it's sized around total gallons used in an hour. A tankless unit has no reserve — it must heat water in real time to match whatever combined flow rate is happening right now, so it's sized to the gallons-per-minute of every fixture that could plausibly run at the exact same moment, not the total used over an hour.
What if I regularly run more fixtures at once than this calculator assumes?
Increase the 'running at the same moment' fields to match your household's actual worst case — for example, two full bathrooms both showering while a load of laundry starts. Undersizing a tankless unit for your true simultaneous demand is the most common tankless installation complaint, since the water noticeably cools when demand exceeds the unit's rated GPM.
Are these gallons-per-use and flow-rate figures medical-grade precise?
No — they are typical, commonly cited planning figures (a showerhead's federal maximum flow rate, an average laundry or dishwasher cycle's water use) meant for sizing guidance, not an exact measurement of your specific fixtures. Low-flow fixtures, high-efficiency appliances, and your household's actual habits can shift real usage meaningfully in either direction.