Water Heater Size Calculator
The right water heater matches your household's Peak Hour Demand - the gallons of hot water used during your busiest hour. This water heater size calculator uses the DOE Energy Saver sizing method to recommend a tank size in gallons and a tankless unit GPM rating based on your number of occupants, bathrooms, appliances, and usage pattern.
How Water Heater Sizing Works
Tank and tankless water heaters size differently:
- Tank heaters are sized by Peak Hour Demand (PHD) - the gallons of hot water you use in your busiest hour. The tank plus recovery must cover PHD.
- Tankless heaters are sized by GPM (gallons per minute) - the flow rate needed when multiple fixtures run at the same time.
Peak Hour Demand (Tank Sizing)
Typical peak hour water usage per person:
- Shower or bath: 10 gallons
- Shave: 2 gallons
- Hand dishwashing / food prep: 4 gallons
- Total per person: about 12 gallons in a normal household
Add appliance peak hour loads: clothes washer 7 gal, dishwasher 6 gal. So a family of 4 with a washer and dishwasher running = 4 x 12 + 13 = 61 peak hour gallons, which points to a 50 gallon water heater with recovery.
Tank Size Recommendations
| Household | Peak Hour | Recommended Tank |
|---|---|---|
| 1-2 people | under 30 gal | 30-40 gallon |
| 2-3 people | 30-50 gal | 40-50 gallon |
| 3-4 people | 50-65 gal | 50 gallon water heater |
| 4-5 people | 65-80 gal | 50 gallon gas or 65-75 gal electric |
| 5+ people | 80+ gal | 75-80 gallon |
Electric water heaters recover more slowly than gas, so 50 gallon water heater electric models often need to be sized up to 65 or 75 gallons for the same household. A 50 gallon water heater gas or 50 gallon water heater propane recovers twice as fast as the same tank on electric resistance.
Tankless Water Heater Size Calculator (GPM)
Tankless units heat water on demand, so size by peak simultaneous flow rate:
| Fixture | Flow Rate |
|---|---|
| Low-flow shower | 2.0 GPM |
| Standard shower | 2.5 GPM |
| Kitchen sink | 1.5 GPM |
| Bathroom sink | 1.0 GPM |
| Dishwasher | 1.5 GPM |
| Clothes washer | 2.0 GPM |
Add up the fixtures you might run at the same time. Two showers + kitchen sink = 2 x 2.0 + 1.5 = 5.5 GPM, which fits a mid-size gas tankless unit. For whole-house tankless in a cold climate, aim for 8-10 GPM; a 10 GPM tankless water heater covers most 3-4 bathroom homes.
Tankless Water Heater Size Chart
| GPM Rating | Best For |
|---|---|
| 4-6 GPM | 1-2 bathroom home, warm climates |
| 7-9 GPM | 2-3 bathroom home, moderate climates |
| 9-11 GPM | 3+ bathroom home, cold inlet water |
| 11+ GPM | Large homes, simultaneous heavy use |
Water Heater Size for Family of 4
A typical family of 4 uses 50-70 gallons in the peak hour. A 50 gallon water heater (gas or propane) is the standard recommendation. Electric families of 4 often need a 65-75 gallon tank to avoid running cold. Brands like Bradford White 50 gallon water heater and Home Depot 50 gallon water heater gas (Rheem, AO Smith, GE) are popular picks.
Should I Choose Tank or Tankless
- Tank: lower upfront cost ($500-1,500 installed), simpler install, handles simultaneous use fine if sized right.
- Tankless: higher upfront cost ($1,500-3,500 installed), endless hot water, 20-30% lower operating cost, longer lifespan (20+ years vs 10-12 for tanks), but may struggle with multiple simultaneous fixtures in cold climates unless oversized.