Water Heater Size Calculator
Water heater sizing should be based on First Hour Rating (FHR) — not just tank size. FHR is how many gallons of hot water the unit can deliver in the first busy hour, combining stored volume with recovery rate. A 50-gallon tank with a low-BTU burner may have a lower FHR than a 40-gallon tank with a high-BTU burner. Enter household size, peak-hour showers, baths, and dishwasher use to get your calculated FHR, recommended tank size for gas or electric, estimated annual energy cost, and potential annual savings from upgrading to a heat pump water heater. Gas water heaters cost less to operate than electric resistance units; heat pump water heaters cost 60–70% less to operate than standard electric. The 30% federal tax credit for heat pump water heaters (available through 2032) significantly improves the payback period.
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How to Use This Calculator
- 1Enter your measurements in the fields above. Use the units shown next to each field (ft, in, etc.).
- 2Click Calculate to get your material estimate instantly.
- 3Review the highlighted result — it shows the key quantity you need to order.
- 4Use Copy Results to save your estimate, or bookmark the URL — your inputs are encoded in the link.
Pro tip: Always add 10% to your material order to account for waste, cuts, and defects.
Frequently Asked Questions
What size water heater do I need for a family of 4?
A family of 4 typically needs a First Hour Rating of 50–70 gallons. A 50-gallon gas or 55-gallon electric tank water heater covers most 4-person households. If you have two back-to-back showers in the morning, size up to a 60-gallon unit.
What is First Hour Rating (FHR)?
First Hour Rating is how many gallons of hot water a heater can deliver in a busy first hour, starting with a full tank. It accounts for heating capacity plus stored volume. The FHR appears on the EnergyGuide label. Match your peak demand to the FHR, not just the tank size.
Gas vs. electric water heater — which is better?
Gas water heaters cost $300–$600 less per year to operate than standard electric resistance units. However, gas requires a gas line, flue venting, and has higher install cost if neither exists. Heat pump water heaters (electric) close the gap — they operate at 3× the efficiency of resistance heating and qualify for a 30% federal tax credit through 2032. If you have gas, stay with gas. If converting or building new, a heat pump water heater is often the better choice.
How long does a water heater last?
Traditional tank water heaters last 8–12 years. Annual flushing to remove sediment and anode rod inspection every 3–5 years extends life to the upper end of that range. Tankless (on-demand) water heaters last 20+ years with regular descaling. Heat pump water heaters last 10–15 years. Most manufacturers warrant water heaters for 6–12 years depending on model.