Water Heater Size Calculator
Free water heater sizing calculator. Find the right tank size in gallons or tankless GPM rating for your household based on people, fixtures, and peak hour use.
Quick answer
Tank size: 30-40 gal for 1-2 people, 40-50 gal for 3-4, 50-80 gal for 5+. Tankless GPM = sum of simultaneous fixture demand (a shower is ~2.5 GPM, a sink is ~1 GPM). Rule of thumb: 1.5 hours of cold-water rise capacity matters more than peak GPM for typical homes.
Water Heater Size Calculator
How it works
Tank heaters are sized by first-hour rating (FHR) — gallons delivered in the first hour from a full tank. Gas tanks recover faster than electric, so they need a smaller gallon size for the same FHR target. Tankless heaters are sized by GPM (gallons per minute) based on the number of fixtures that might run simultaneously and the temperature rise needed in your area.
When to use it
Before buying a replacement water heater, when comparing tank vs tankless, or when your family is always running out of hot water and you suspect your current unit is undersized.
Common mistakes
Buying the same size that's already there without checking whether it was right to begin with. A 40-gallon tank from a 1-bathroom era may be wrong for a 4-person, 3-bathroom home.
How the water heater sizing calculator works
For tank water heaters, sizing is based on first-hour rating (FHR) — how many gallons of hot water the heater delivers in the first hour of demand. A typical morning rush (showers, dishwasher) hits peak demand quickly. The calculator estimates FHR needed from household size, number of bathrooms, and peak-hour fixture use. For tankless, sizing is based on flow (gallons per minute, GPM) — it must support all simultaneous fixtures plus a temperature rise enough to deliver hot water at the inlet temperature of your area.
When to use it
Replacing a failed water heater and choosing a size for the new one. Comparing tank vs. tankless options. Evaluating whether your existing unit is undersized (running out of hot water during showers is a clue). Sizing for an addition (new bathroom, finished basement) that will increase hot-water demand.
Common mistakes
- Sizing tankless on GPM alone. A 7 GPM tankless looks generous on paper, but if your inlet water is 45°F (cold climate) and you want 110°F output, the unit needs to raise temperature 65°F — and its rated GPM at that rise is much lower than the headline number. Always check the temperature-rise chart.
- Oversizing 'just in case.' Bigger tanks waste more standby energy. Bigger tankless units cost more upfront. Size to actual peak hour, not worst-case scenario.
- Forgetting recovery rate. Tank water heaters refill at a fixed rate (electric: ~20 gal/hour, gas: ~40 gal/hour). For high-demand homes with infrequent peaks, a smaller tank with high recovery often beats a bigger tank with slow recovery.
Frequently asked questions
What size water heater do I need for a family of 4?
A 50-gallon tank is the typical recommendation for a 4-person household with normal usage. Tankless equivalent: 7-9 GPM at your local temperature rise. Heavy users (long showers, simultaneous bathroom use) may need to step up a size.
Is tankless better than a tank water heater?
Tankless saves 20-30% on energy cost but costs 2-3× more upfront. They last longer (20+ years vs. 10-12) but need higher gas line capacity or electrical service. Tankless wins for long-term cost; tank wins for upfront cost. Both deliver hot water reliably when properly sized.
How long should a water heater last?
Tank water heaters: 8-12 years typical. Tankless: 20+ years. Annual flushing extends life significantly — sediment in the bottom of a tank causes premature failure. Hard water areas see shorter lifespans without softening.