Water Heater Size Calculator
Tank size and first-hour rating target by household size.
You need
50 gallons
Compare models by first-hour rating, not just tank volume.
- First-hour rating target
- 36 gallons
Estimates only. Verify quantities before ordering.
How this is calculated
Tank size: 40 gal for 1 to 2 people, 50 gal for 3 to 4, 65 gal or more for 5 plus, with about 10 gallons added for electric models. First-hour rating target: roughly 12 gallons per person in the busiest hour, plus about 20 gallons if laundry or dishwasher runs during it.
Enter your household size and fuel type, and this calculator recommends a storage tank size plus the first-hour rating (FHR) to look for on the yellow EnergyGuide label. FHR, how many gallons of hot water the unit delivers in a busy hour, is the number that actually matters; tank volume is just the everyday shorthand for it.
Electric models are sized about 10 gallons larger than gas for the same household because electric elements reheat water more slowly than a burner. If your busy hour stacks multiple showers with laundry and the dishwasher, buy toward the high end or shift chores off the peak; an undersized tank shows up as the third shower running cold.
Water heater sizes by household
Rows assume no laundry or dishwasher in the peak hour; add about 20 gallons of FHR if there is.
| Household | Tank size | FHR target |
|---|---|---|
| 1-2 people, gas | 40 gallons | 24 gallons |
| 3 people, gas | 50 gallons | 36 gallons |
| 4 people, gas | 50 gallons | 48 gallons |
| 4 people, electric | 60 gallons | 48 gallons |
| 6 people, electric | 75 gallons | 72 gallons |
Tank or tankless
A storage tank heater keeps 40 to 80 gallons hot continuously and delivers a large volume quickly, then needs time to recover. It is cheap to buy, simple to replace, and loses some energy through the tank walls around the clock. Expect somewhere between 8 and 15 years of service depending on water quality and whether the sacrificial anode was ever replaced.
Tankless units heat water on demand and never run out, at two to three times the purchase and installation cost. They last longer, often 20 years, and eliminate standby loss. The constraint is flow rate rather than volume: a unit is rated in gallons per minute at a given temperature rise, and in cold climates that rise is large enough to cut the effective capacity substantially. A unit that comfortably runs two showers in Florida may struggle with one shower and a sink in Minnesota.
Heat pump water heaters are the efficiency outlier, using roughly a third of the electricity of a resistance model by moving heat from the surrounding air. They need a reasonably large, reasonably warm space to draw from and they cool and dehumidify that space, which is a bonus in a basement and a problem in a small closet.
First hour rating and recovery
First hour rating is the number to compare, and it appears on the yellow EnergyGuide label. It combines the stored volume with how fast the burner or element can reheat, giving the gallons the unit can deliver in a busy hour starting from fully hot. Two 50 gallon heaters can have noticeably different first hour ratings depending on their input.
Gas heaters recover much faster than electric ones, which is why a 40 gallon gas unit often matches a 50 gallon electric on first hour rating. That difference is the reason electric models are generally sized about 10 gallons larger for the same household. If you are switching fuel types, compare the ratings rather than the tank sizes.
Size to your peak hour, not your daily total. Count what genuinely overlaps: back-to-back showers on a weekday morning, or a shower plus a dishwasher plus laundry. A household that uses a lot of hot water spread through the day needs less capacity than one that concentrates it into 45 minutes.
Venting, space, and the details that change the price
Atmospheric gas heaters vent through a conventional flue and rely on natural draft. Power-vented and condensing units use a fan and can vent horizontally through a wall in PVC, which opens up locations without a chimney but requires electricity and a condensate drain. Tankless units almost always need their own dedicated, correctly sized vent and often a larger gas line than the tank they replace.
Electric conversions have their own requirements. A resistance tank typically needs a 30 amp 240 volt circuit; whole-house electric tankless can require 100 amps or more of dedicated capacity, which frequently means a service upgrade and turns a straightforward swap into a significant electrical project.
Budget for the code items too, since they are commonly missed in an online price comparison: an expansion tank where there is a backflow preventer on the supply, a properly routed temperature and pressure relief discharge line, seismic strapping in some regions, and a drain pan with a drain where a leak would damage finished space.
Common questions
What size water heater for a family of 4?
A 50 gallon gas or 55 to 60 gallon electric tank is the standard recommendation for 3 to 4 people. More useful: look for a first-hour rating around 65 to 70 gallons if your mornings stack two showers with a laundry load.
What is a first-hour rating?
FHR is how much hot water the heater can deliver in one busy hour, combining the stored volume with how fast the unit reheats. It is printed on the EnergyGuide label. Two 50 gallon tanks can differ by 20 gallons of FHR, which is why it beats tank size for comparisons.
Should I get a tankless water heater instead?
Tankless units are sized completely differently: by flow rate (gallons per minute) and the temperature rise from your groundwater, not by tank gallons. They never run out of hot water but limit how many fixtures run at once. This calculator covers storage tanks; for tankless, add up the GPM of fixtures that run simultaneously.
How long should a water heater last?
Storage tanks typically last 8 to 12 years; tankless units 15 to 20 with maintenance. If your tank is past 10 years and you are seeing rusty water or moisture at the base, size the replacement before it fails, since emergency replacements rarely leave time to compare models.