Water heater sizing calculator
Nobody runs out of hot water over a day. They run out in the forty minutes when three people shower and the dishwasher starts. Size for that hour, not for the household, and the two technologies answer it in completely different units — gallons for a tank, gallons per minute for a tankless.
Count what actually happens in the busiest hour of the day, not what happens all day.
- Shower (10 min)20 gal · 2 gpm3
- Bath fill20 gal · 4 gpm0
- Shaving at the lav2 gal · 1 gpm1
- Hands and face4 gal · 1 gpm2
- Dishes by hand4 gal · 1.5 gpm1
- Dishwasher cycle6 gal · 1 gpm1
- Laundry, warm7 gal · 2 gpm0
Setpoint minus the coldest incoming water.
Tank size, first-hour rating 80 gal
Tankless flow at a 70 °F rise
Peak hour demand is 80 gallons. A tankless unit must be rated for 3.5 gpm at a 70 °F rise — the headline GPM on the box is quoted at a much smaller rise.
| Tank | First hour rating | Covers peak |
|---|---|---|
| 30 gal | 70 gal | no |
| 40 gal | 80 gal | yes |
| 50 gal | 90 gal | yes |
| 66 gal | 106 gal | yes |
| 80 gal | 120 gal | yes |
First hour rating, not capacity
A tank's useful number is its first hour rating: the gallons it can deliver in one hour starting full. That is capacity plus recovery, and recovery depends on the burner or the elements. A 40 gallon gas heater with a strong burner can outperform a 50 gallon electric one on a busy morning, which is why the yellow label is worth reading and the nameplate gallons alone are not.
Recovery is the quiet half
Why tankless GPM is quoted optimistically
A tankless unit's flow rate is only meaningful with a temperature rise attached. The same unit that delivers 9 gpm at a 35 °F rise in Florida delivers around 4 gpm at a 75 °F rise in Minnesota. Incoming water temperature is a climate fact, not a preference.
Take the rise from your own coldest incoming temperature to your setpoint, then check the manufacturer's flow chart at that rise. Two simultaneous showers at 2 gpm each is a real requirement of 4 gpm at your rise, not at theirs.
Code notes
IPC 501 covers water heater installation generally; IPC 504 the temperature and pressure relief valve and its discharge. Setpoints above 120 °F raise the scald risk — several jurisdictions require a thermostatic mixing valve when storage is kept hotter for legionella control. Confirm with your authority having jurisdiction (AHJ).