Water Heater Size Calculator
Work out a recommended tank size in gallons, or a tankless flow rate in GPM, based on your household size and which fixtures tend to run at the same time.
How this calculator works
Tank sizing starts from a baseline by household size, then adds capacity if two or more fixtures commonly run at once:
// Tank mode base gallons by household: 1-2 → 30, 3-4 → 40, 5-6 → 50, 7+ → 80 tank size = base gallons + (10 gal if 2+ fixtures checked) // Tankless mode flow rate (GPM) = sum of GPM for every checked fixture shower ≈ 2.5 GPM, dishwasher ≈ 1.5 GPM, washing machine ≈ 2.0 GPM
Worked example
A household of 3–4 people, tank heater, shower and dishwasher checked:
- Base size for 3–4 people: 40 gallons.
- 2 fixtures checked (shower + dishwasher) → +10 gallons.
- Recommended tank size: 40 + 10 = 50 gallons.
The same household in tankless mode, with the same two fixtures checked:
- Shower: 2.5 GPM + Dishwasher: 1.5 GPM.
- Recommended flow rate: 4.0 GPM.
Tank size by household — quick reference
| Household size | Baseline tank | With frequent simultaneous use |
|---|---|---|
| 1–2 people | 30 gal | 40 gal |
| 3–4 people | 40 gal | 50 gal |
| 5–6 people | 50 gal | 60 gal |
| 7+ people | 80 gal | 90 gal |
Tank vs tankless
A tank heater stores hot water ahead of time, so sizing is about having enough volume for peak usage before it runs out and needs to reheat. A tankless unit heats water as it flows through, so sizing is about matching or exceeding the combined flow rate of everything that might run at once — undersizing a tankless unit means the water pressure or temperature drops when a second fixture opens, rather than running out entirely like a tank does.
Frequently asked questions
What tank size do I need for a family of 4?
A household of 3 to 4 people typically needs around a 40 gallon tank as a baseline, rising to about 50 gallons if the household regularly runs a shower, dishwasher and washing machine at overlapping times, since simultaneous draws use up capacity faster than it can reheat.
How is tankless GPM different from tank gallons?
A tank stores a fixed volume of hot water, measured in gallons, that gets used and then reheated. A tankless unit heats water on demand and is rated by gallons per minute (GPM) it can heat at once, so sizing it means adding up the flow rate of everything that might run simultaneously, not estimating a storage volume.
Why does recovery rate matter, not just tank size?
Recovery rate is how quickly a tank reheats after being drawn down, and it varies by fuel type — gas units generally recover faster than electric ones of the same size. Two tanks with the same gallon capacity can run out of usable hot water at very different rates depending on recovery speed.
Electric vs gas water heaters — what's the general tradeoff?
Gas water heaters typically recover faster and cost less per gallon heated in most markets, but need a gas line and proper venting. Electric units install more flexibly anywhere with the right circuit, but often recover more slowly and can cost more to run depending on local energy prices.