Sizing a water heater wrong is the single most common new-install mistake. Undersized: cold showers at peak demand. Oversized: paying for capacity you never use (and slightly higher standby losses). The right size depends on what you're buying — tanks size by capacity and First Hour Rating (FHR), tankless sizes by flow rate (GPM) at a given temperature rise.
Tank water heater sizing — quick reference
| Household size | Bathrooms | Tank size (gas) | Tank size (electric) |
|---|---|---|---|
| 1-2 people | 1 | 30-40 gallon | 30-40 gallon |
| 2-3 people | 1-2 | 40 gallon | 50 gallon |
| 3-4 people | 2-3 | 40-50 gallon | 50-66 gallon |
| 4-5 people | 3 | 50 gallon | 66-80 gallon |
| 5-6 people | 3-4 | 50-75 gallon | 80 gallon |
| 6+ people | 4+ | 75-100 gallon or tankless | 80+ gallon or tankless |
Electric tanks need 10-15 gallons more capacity than gas to hit the same effective output — because electric units recover (reheat) much slower than gas. A 50-gallon gas tank delivers similar peak performance to a 66-gallon electric.
First Hour Rating (FHR) — the real number
FHR is the gallons of hot water a tank can deliver in the first hour of peak demand, starting from a full hot tank. It's printed on the EnergyGuide label and is more useful than raw capacity for sizing.
Calculate your peak hour demand:
- Shower: 17 gallons (8-min average, 2.1 GPM mixed)
- Bath: 20 gallons
- Dishwasher: 6 gallons
- Washing machine: 7 gallons (warm) / 14 gallons (hot)
- Kitchen sink (dishes): 4 gallons
- Bathroom sink (handwashing, shaving): 2 gallons
Add up the worst-case morning: 2 showers (34) + dishwasher (6) + bathroom sinks (4) = 44 gallons FHR. Pick a tank with FHR ≥ this number.
Tankless sizing — GPM at temperature rise
Tankless units are sized by GPM (gallons per minute) at a given temperature rise. Critical: the same unit produces different GPM in cold climates vs warm climates.
| Region | Incoming water temp | Target output | Required rise |
|---|---|---|---|
| Southern US (FL, TX, AZ, CA) | 65-75°F | 120°F | 45-55°F |
| Mid-South / Mid-Atlantic | 55-65°F | 120°F | 55-65°F |
| Northeast / Midwest | 45-55°F | 120°F | 65-75°F |
| Northern (MN, ME, ND, MT) | 35-45°F | 120°F | 75-85°F |
Then size by simultaneous-fixture peak GPM:
- Shower: 2.0-2.5 GPM (low-flow)
- Kitchen sink: 1.5 GPM
- Bathroom sink: 1.0 GPM
- Dishwasher: 1.5 GPM
- Washing machine: 1.5 GPM (hot fill)
Two showers + a sink at the same time = 5.5 GPM peak. In a 65°F-rise region, you need a tankless rated 5.5 GPM at 65°F rise — typically the 180-199 kBTU class (Rinnai RU180iN, Navien NPE-210A2, Rheem RTGH-95).
Quick tankless sizing by bathroom count
| Bathrooms | Warm climate (50°F rise) | Cold climate (75°F rise) |
|---|---|---|
| 1 | 5.0-6.5 GPM (150 kBTU) | 4.0-5.0 GPM (180 kBTU) |
| 2 | 6.5-8.0 GPM (180 kBTU) | 5.0-7.5 GPM (199 kBTU) |
| 3 | 8.0-10.0 GPM (199 kBTU) | 7.5-9.0 GPM (199 kBTU + recirc) |
| 4+ | 10+ GPM (199 kBTU max + recirc, or hybrid like Rinnai Demand Duo) | 2 units in parallel or Demand Duo |
Heat pump / hybrid sizing
Heat pumps use larger tanks than gas because their recovery rate is slow (heat pump mode produces 8-12 GPH vs 40-50 GPH for gas). Standard sizing:
- 2-3 people: 50 gallon
- 3-4 people: 65-66 gallon (most popular size)
- 4-5 people: 80 gallon
- 5+ people: 80 gallon or pair with a tankless backup
Should you oversize?
Mild oversizing (one tier up) is generally safe and gives margin for guests or future bathroom additions. Severe oversizing (50% over) wastes money on the unit and slightly on standby losses. The cost difference between a 50 and 75-gallon gas tank is $300-500; if you're on the bubble, size up.
Undersizing risks
- Cold showers at peak demand
- Tank recovers cold faster, burner runs more, lifespan shortens 15-25%
- On tankless: unit modulates at full burn constantly, more wear, more scale formation
- On heat pump: defaults to resistance mode (which eliminates the efficiency advantage)
By-brand sizing guides
- Rheem lineup — capacities across Performance / Performance Plus / Platinum tiers
- AO Smith lineup
- Rinnai sizing — GPM at temperature rise tables for Sensei and RL series
- Navien sizing — NPE-A2 sizing for combi vs tankless-only
Related guides
- 40 vs 50 gallon — the most common upgrade decision
- Tank vs tankless
- Replacement planning
- Installation
Bottom line
For a typical 3-person, 2-bathroom US home: 40-50 gallon gas tank (FHR 60+), 50-66 gallon electric, 180k BTU tankless (~8 GPM in warm climates, ~6.5 GPM in cold), or 65-gallon heat pump. When in doubt, size one tier up — the cost premium is small relative to running short on hot water for the next 12 years.