Water heaters are a big, expensive purchase most people size by guesswork, or by whatever the old one happened to be. Get a real recommendation based on your actual household instead.
How This Calculator Works
- 12 gallons per person, plus a 15-gallon household baseline. This follows DOE Energy Saver guidance and is the most commonly cited formula for estimating peak-hour hot water demand, the busiest single hour of your day, not total daily use.
- 3+ bathroom adjustment: +10%. More bathrooms means a higher chance of overlapping simultaneous use during that peak hour.
- Rounded up to the nearest standard tank size (30, 40, 50, 65, 80, or 100 gallons), since undersizing causes real daily problems (running out of hot water mid-shower) while modest oversizing just costs a bit more upfront with no downside.
Never round down from your calculated number. Undersizing a water heater causes daily frustration, oversizing by one tier costs more upfront but causes no ongoing problems.
Last Updated: August 2026.