What it is

Sizing a water heater means matching what the household actually draws during its busiest hour to what the heater can deliver in that hour. The rookie move is to size by tank gallons alone ("they had a 40, give them a 40"). The right way is to compare the home's peak-hour demand to the heater's first-hour rating for a tank, or to GPM at the design temperature rise for a tankless. A 40-gallon tank with a big burner can out-deliver a 50-gallon tank with a small one, because recovery matters as much as storage.

How it works

First-hour rating (FHR) is the gallons of hot water a full storage tank can deliver in one hour starting fully hot. It combines two things: the tank volume (the hot water already sitting there) plus the recovery (how fast the burner/elements reheat incoming cold while you're drawing). A tank with a strong burner keeps refilling the top with hot water as you draw, so its FHR is well above its raw gallon count. FHR is the number that actually matches real-world morning use.

Recovery rate is gallons per hour the heater can raise by a given temperature rise. It scales with input (BTU/h for gas, watts for electric) and inversely with the rise you demand: heating 40°F well water up to 120°F is an 80°F rise; the same heater does fewer GPH at a bigger rise.

Peak-hour demand is how much hot water the household uses in its heaviest hour — typically the morning rush of showers, plus maybe a load of laundry or the dishwasher. You size so the heater's FHR meets or beats that peak hour.

In the field

To size a storage tank:

  1. Estimate peak-hour demand. Add up the hot water used in the busiest 60 minutes. Rough per-use numbers: a shower ~10–20 gallons (10 min at ~2 GPM, less with low-flow), a tub fill ~20+ gallons, a dishwasher ~6 gallons, a clothes washer ~7–15 gallons (warm/hot cycle), shaving/handwash a few gallons. A typical family of four's morning peak often lands around 60–80 gallons.
  1. Match FHR to that demand. Pick a heater whose first-hour rating meets or slightly exceeds the peak hour — not whose tank size matches it. Look at the spec/EnergyGuide for FHR.
  1. Account for incoming water temperature. In cold climates with cold inlet water (40°F), recovery slows and you may need more storage or more input. Warm inlet regions can run smaller.
  1. Don't blindly oversize. A tank far bigger than the peak hour just stores more water you pay to keep hot 24/7 (standby loss). Size to the real peak, with a sensible margin.

For a tankless, there's no FHR — size by GPM at the design rise. Figure the maximum simultaneous flow (e.g., two showers + a sink = ~4–5 GPM) and the worst-case rise (coldest inlet to setpoint), then pick a unit rated for that GPM at that rise. A unit that does 7 GPM at a 45°F rise might only do 3.5 GPM at a 70°F winter rise — size for winter, not the brochure.

Normal values & targets

  • FHR ≈ tank gallons available hot + one hour of recovery. Always use the published FHR, not raw tank size.
  • Recovery (gas): a 40,000 BTU/h gas tank recovers roughly 40+ GPH at a 90°F rise. Recovery (GPH) ≈ input × efficiency ÷ (8.33 × rise).
  • Recovery (electric): a single 4500W element recovers roughly 20–21 GPH at a 90°F rise — much slower; size electric tanks with more storage.
  • Per-use hot water (rough): shower 10–20 gal, tub 20+ gal, dishwasher ~6 gal, clothes washer 7–15 gal (warm/hot).
  • Tankless: rate by GPM at design rise. ~199K BTU/h unit ≈ 5–7 GPM at 45°F rise, ~3–4 GPM at 70°F rise.
  • Typical family-of-four morning peak: ~60–80 gallons in the busy hour.

Common faults & what they mean

  • "We run out of hot water every morning." FHR (or recovery) is below the household's peak hour. Either the tank's too small for the burner it has, or it's an electric unit with slow recovery getting outrun. Upsize FHR or move to a bigger burner / gas / tankless.
  • Electric tank can't keep up with back-to-back showers. Slow recovery is the usual culprit — electric recovers ~half what comparable gas does. Bigger storage or a higher-input/hybrid unit fixes it.
  • Tankless goes lukewarm in winter only. Sized on summer GPM, not winter rise. The colder inlet drops its deliverable GPM below demand. Right-size for the coldest design rise.
  • Huge tank, sky-high standby costs. Oversized for the actual demand — paying to keep gallons hot that never get used. Match the peak, don't gold-plate.

Tech tips & gotchas

  • FHR beats tank size every time. When a homeowner says "I want a bigger one because we run out," check the FHR and recovery — a 40-gallon high-recovery gas unit can beat a 50-gallon low-recovery one. Sometimes the fix is input, not gallons.
  • Gas recovers about twice as fast as electric for equivalent storage. If they keep running out on electric, the answer is often more storage or a hybrid/gas swap, not just a same-type bigger tank.
  • Size tankless for the worst-case winter rise, and verify the gas line and meter can feed the input. A perfectly sized tankless on an undersized gas line still won't perform.
  • Don't oversize "to be safe." Standby loss is real money and a bigger tank costs more up front. Match the peak hour with a reasonable margin and stop there.
  • Low-flow fixtures and a recirculation system both change the math — low-flow lowers demand, recirc can raise standby and effective demand. Factor the actual fixtures.

Safety / code notes

  • Energy efficiency and standby loss are governed by the energy code / appliance standards; the EnergyGuide label carries the FHR you should size from.
  • Larger or hotter tanks raise scald risk — where a tank runs above ~120°F at the tap (for demand or sanitization), temper the delivered water with a thermostatic mixing valve per the plumbing code scald-protection provisions.
  • Whatever size you install still needs a correctly rated T&P valve and discharge line (IPC water-heater/relief provisions) and, on a closed system, thermal-expansion control (expansion tank).