Water Heater Size Calculator
Undersized Means Cold Showers, Oversized Wastes Energy
A storage water heater that's too small runs out of hot water during peak household demand — typically overlapping morning showers — while one that's oversized keeps far more water hot than necessary around the clock, adding to standby energy loss. Sizing guidance built around household headcount gives a practical starting point before factoring in usage patterns like back-to-back showers or a large soaking tub.
The Formula
2 people = 40 gal
3 people = 50 gal
4 people = 60 gal
5 people = 80 gal
Beyond 5 = 80 gal + 10 gal per additional person
This reflects standard household-size sizing guidance commonly used as a baseline by water heater manufacturers and plumbing references for typical usage patterns.
Where This Calculation Matters
- New home construction — specifying tank capacity before rough-in plumbing is finalized.
- Water heater replacement — confirming the household hasn't outgrown the capacity of the unit being replaced.
- Home additions — adding bedrooms or bathrooms often means adding household members and pushing demand into a higher capacity tier.
- Tankless conversion decisions — the same household demand figures inform the flow rate a tankless unit would need to match.
Recommended Tank Capacity by Household Size
| Household size | Recommended tank capacity |
|---|---|
| 1 person | 30 gal |
| 2 people | 40 gal |
| 3 people | 50 gal |
| 4 people | 60 gal |
| 5 people | 80 gal |
| Each additional | +10 gal |
This is a general baseline — households with high-demand fixtures like large soaking tubs or multiple simultaneous showers should size up a tier.
How to Use This Calculator
- Enter the household size (number of people).
- Select Calculate to get the recommended tank capacity in gallons.
Related Calculations
To size the overall water storage or supply for the same household, use the Water Tank Size Calculator. For expected static pressure feeding that water heater, see the Water Pressure Calculator.