Water Heater Size Calculator
Choosing the right water heater size prevents cold showers during peak demand and avoids wasting energy heating excess water. This calculator estimates the required tank size or tankless flow rate based on your household’s hot water usage patterns.
Tank Water Heater Sizing
Sizing is based on first-hour rating (FHR) methodology. Gas water heaters recover faster than electric and may use a smaller tank. Heat pump water heaters are the most efficient but have slower recovery rates.
Water Heater Sizing Guide
First-Hour Rating (FHR)
The most important sizing metric for tank water heaters is the first-hour rating — the number of gallons of hot water the unit can deliver in the first hour starting with a full tank. This accounts for both stored hot water and recovery speed.
Typical Hot Water Usage by Fixture
| Fixture | Gallons per Use | Flow Rate |
|---|---|---|
| Shower | 15–20 gal | 2.0–2.5 GPM |
| Bath | 30–50 gal | — |
| Dishwasher | 6–10 gal | 1.0–1.5 GPM |
| Clothes Washer (warm) | 15–25 gal | 2.0 GPM |
| Hand Washing | 2–4 gal | 0.5–1.0 GPM |
| Kitchen Sink | 3–5 gal | 1.0–1.5 GPM |
Tank vs. Tankless
Tank water heaters store a reservoir of pre-heated water and are sized by gallon capacity and first-hour rating. Tankless (on-demand) units heat water as it flows through and are sized by flow rate (GPM) and temperature rise. Tankless systems never run out of hot water but may struggle to supply multiple simultaneous fixtures if undersized.
Frequently Asked Questions
What size water heater do I need for a family of four?
A family of four with average usage typically needs a 50-gallon electric tank or a 40-gallon gas tank (gas recovers faster). For tankless, plan for at least 7–8 GPM capacity for comfortable simultaneous usage.
Is a bigger tank always better?
No. An oversized tank wastes energy maintaining a large volume of hot water that may never be used. The goal is to match the first-hour rating to your peak hour demand, not to buy the largest available tank.