What size water heater do I need?
The honest answer is not “a 50 for a family of four.” It is: size by the busiest hour of hot-water use, match a tank’s First-Hour Rating to that peak, or a tankless’s GPM to your simultaneous flow — then round up.
Almost every “what size” chart you will find sizes by the number of people in the house. That is a shortcut, and it is the reason so many households end up short of hot water at 7 a.m. or oversized (and paying standby losses) the rest of the day. Two four-person households can have wildly different peaks: one showers back-to-back before work, the other spreads use across the day. The quantity that actually sizes a tank is the peak-hour demand — the gallons of hot water drawn in your single busiest hour — and the specification that has to meet it is the First-Hour Rating (FHR), printed on every unit’s EnergyGuide label.
The method in three steps
1. Add up your peak hour. Walk through your busiest 60 minutes and sum the hot-water draws using the DOE planning typicals: a shower is about 20 gallons, a bath about 20, a shave about 2, a hand or face wash about 4, running the dishwasher about 6, a clothes-washer load about 7, food prep about 5. The formula is simply peak_hour_demand = Σ(uses × gallons per use).
2. Match the First-Hour Rating. For a storage tank, choose a model whose FHR is at least your peak-hour demand. FHR is not the tank’s gallon rating — it is the usable stored hot water (about 70% of the tank) plus what the burner or element reheats during that hour: FHR ≈ 0.70 × gallons + recovery. That is why a strong gas 40-gallon and a weak gas 40-gallon can have very different FHRs.
3. Or size a tankless by flow. A tankless has no storage, so it is sized by the simultaneous GPM you draw at once (two showers plus a kitchen faucet, say) at your temperature rise (ΔT = desired output − incoming water). Cold northern inlet water means a bigger rise and less usable flow from the same unit.
A worked example: a busy family of four
Say the peak hour is three showers, one shave, two hand washes and food prep. That is 3×20 + 1×2 + 2×4 + 1×5 = 60 + 2 + 8 + 5 = 75 gallons of hot water in one hour. A 50-gallon gas tank with a healthy burner reaches an FHR near 75, so it fits — you do not need a 75-gallon tank. On the tankless side, if two showers (2.0 GPM each) and a kitchen faucet (1.5) can run together, that is 5.5 GPM; at a 120 °F output and a 50 °F inlet the rise is 70 °F, so you need a whole-house gas tankless rated for at least 5.5 GPM at a 70 °F rise.
Run your own numbers in the what-size-water-heater calculator and the peak-hour-demand calculator; the household starting bands live in the size-by-household reference.
Household starting bands (then beat them with your peak)
| 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 planning typicals — a starting point only. Always confirm your unit’s rated FHR and GPM on its EnergyGuide label.
The edge cases the brand widgets ignore
- Fuel changes the answer. Gas recovers fast, so a smaller gas tank can hit the FHR an electric tank needs a bigger body to reach. An electric-resistance household often sizes up a step for the same peak.
- Cold inlet water. A northern winter inlet near 40 °F raises ΔT and cuts a tankless’s usable GPM — a unit that hits 5.5 GPM in Florida may only manage 4 in Minnesota. Size for winter, not the spec-sheet best case.
- Big tubs and rain heads. A soaking tub can be 40–60 gallons and a rain shower head can double the flow — both blow past the DOE typicals, so measure them rather than assuming.
- Simultaneity, not totals. A tankless cares about what runs at the same time; a tank cares about the hour’s total. Two very different questions.
Sizing for the awkward households
The peak-hour method shines exactly where the people-count charts fail. A two-person household with a huge soaking tub can out-demand a four-person family of quick showerers: one 50-gallon tub fill is two-and-a-half showers of hot water in a few minutes, so that couple may need a 50 where the chart says 40. A household with staggered schedules — shift workers who never overlap — can go a size down, because their draws never stack into a single peak hour. A home with a large primary suite and a rain head plus body sprays can run 4–5 GPM at one shower, which quietly doubles the assumed draw. The rule is always the same: build your own busiest hour from your real fixtures, then match the First-Hour Rating or the simultaneous GPM to it.
Guests and growth deserve a line too. If you host family for holidays or a teenager is a few years from marathon showers, buy a little headroom now — the cost difference between a 40 and a 50 is small next to the cost of running short for years or replacing early. But headroom is not a blank check: two sizes up “to be safe” just pays for standby losses on water you never draw. One step of headroom, sized from a real peak, is the sweet spot. If you are genuinely between a bigger tank and a tankless, let the deciding factor be whether your limit is total volume in an hour (favor a tank’s buffer) or endless flow within a GPM cap (favor a tankless) — the tank vs tankless guide walks that fork.
What to measure first, and the common mistakes
Before you shop, time your real morning and count the overlapping fixtures; if you can, measure a shower’s flow with a bucket and a stopwatch, and note your incoming water temperature. The recurring mistakes are sizing by people instead of the peak hour, reading the gallon number instead of the FHR, ignoring the temperature rise on a tankless, and buying the biggest tank “to be safe” — which just burns standby energy. Round up to the next real model, leave a little headroom for guests and a future bathroom, and let the First-Hour Rating, not the sticker gallons, be your guide.