How to Size a Water Heater (the First-Hour-Rating Method)
The deterministic method: size a tank by the busiest hour of hot-water use, a tankless by flow at your temperature rise.
1 Enter your numbers
Size by the busy hour, not the headcount — a tank by First-Hour Rating (≥ 75 gal here → a 50-gallon tank), a tankless by GPM at your temperature rise. Round up and leave headroom.
The single most common sizing mistake is buying by headcount – “four people, get a 50” – when the honest, deterministic method sizes by the busiest hour of hot-water use. Two four-person homes can have very different peaks: back-to-back morning showers plus a load of laundry is a completely different demand from a staggered schedule.
The method has two branches. For a storage tank, add up every hot-water draw in your peak hour to get the peak-hour demand, then choose a tank whose First-Hour Rating (FHR) meets or beats it – FHR is what the tank actually delivers in that first busy hour (usable stored water plus what the burner or element reheats). For a tankless, size by flow: add the GPM of the fixtures you will run at once, at your temperature rise ΔT = output − inlet. This tool computes the peak and hands you both a tank recommendation and a tankless flow starting point.
Formula
Tank branch – add up the busiest hour, then match the First-Hour Rating:
peak_hour_demand = Σ(number_of_uses × gallons_per_use)
choose a tank with First-Hour Rating ≥ peak_hour_demand
Tankless branch – add simultaneous flow at your temperature rise:
required_GPM = Σ simultaneous fixture GPM, at ΔT = output_temp − inlet_temp
DOE planning draws (gallons per use): shower 20, bath 20, shave 2, hand/face 4, shampoo 4, dishwasher 6, food prep 5, clothes washer 7. The related peak-hour demand and tankless GPM tools drill into each branch.
Worked example
Take a family of four on a busy morning: 3 showers (3 × 20 = 60), 1 shave (2), 2 hand washes (2 × 4 = 8) and food prep (5). Peak-hour demand = 60 + 2 + 8 + 5 = 75 gallons. So you want a tank rated to an FHR of about 75 – in practice a 50-gallon gas tank with a strong burner reaches an FHR near 75, which is why “a 50” often is the answer for a family of four, just for the right reason.
On the tankless side, at 120 °F output and 50 °F inlet you are sizing for a 70 °F rise, and running two showers at once (about 2.0 GPM each) puts you near 4–5.5 GPM – a whole-house gas tankless class. Note the trap: a colder northern inlet raises the required rise and cuts a tankless unit’s usable GPM, so size for your winter, not your summer.
Measure the busy hour, not the headcount
Getting a right-sized unit comes down to a few honest inputs:
- Find your real peak hour. It is usually the morning rush or the evening bath-plus-dishes window. Count the draws that actually overlap, not the daily total.
- Use your winter inlet temperature for a tankless. Southern inlet water might be 65–70 °F; northern can be 40–50 °F, and the colder number is what strains a tankless.
- Round up and leave headroom. An undersized unit runs out of hot water mid-shower; a slightly oversized one just costs a little more to buy. When in doubt, round up.
Common mistake: sizing a tankless by gallons, or a tank by GPM – they use different currencies (flow vs first-hour delivery). Confirm the unit’s rated FHR and GPM on its EnergyGuide label before you buy, and read the full sizing guide for the reasoning.
Reference table
| Household | Storage tank (gal) | First-Hour Rating (gal) | Tankless (GPM) |
|---|---|---|---|
| 1–2 people | 30–40 | 40–55 | 3–5 |
| 2–3 people | 40–50 | 50–65 | 5–7 |
| 3–4 people | 50–60 | 60–75 | 6–8 |
| 5+ people | 60–80 | 75–90 | 8–10 |
Labeled starting bands – always beat them with your own peak-hour demand. See the full size-by-household table.