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Size a water heater on its First Hour Rating, not on gallons. For most households that means a 50-gallon gas tank or a 66 to 80-gallon electric one for a family of four; 40 gallons gas or 50 electric for two or three people; and 30 to 40 gallons for one or two. If you are going tankless, forget volume entirely and size on gallons per minute at your local temperature rise. The number that decides all of this is your peak hour — the busiest sixty minutes of your day — and there is a worksheet below to calculate it in about two minutes.
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Why gallons is the wrong number
A 50-gallon tank does not deliver 50 gallons of hot water. It delivers perhaps 35 to 40 gallons of genuinely usable hot water before incoming cold water dilutes what is left below a comfortable temperature — and then it delivers however much more the burner or elements can reheat while you are still drawing.
That second part is recovery, and it is where gas and electric part company completely.
| Heater | Input | Approx. recovery at 90°F rise | What that feels like |
|---|---|---|---|
| Gas tank | ~40,000 BTU/hr | ~40 gallons per hour | Replaces a shower's worth while you shower |
| Electric tank | 4,500 W (~15,350 BTU/hr) | ~20 gallons per hour | Roughly half as fast |
| Heat pump (hybrid) | Varies by mode | Slower still in efficient mode | Needs more storage to compensate |
| Tankless gas | 150,000–199,000 BTU/hr | Continuous — no storage at all | Never runs out; limited by flow rate |
This is why a 40-gallon gas heater often outperforms a 50-gallon electric one in the same house, and why electric households need noticeably larger tanks for the same number of people. First Hour Rating captures both halves of the equation — usable storage plus one hour of recovery — in a single figure, and it is printed on the yellow EnergyGuide label on every water heater sold in the US. Compare that, not the gallon count.
The peak hour worksheet
Pick the busiest hour in your week — for most families, a weekday morning between about 6:30 and 8:00 — and add up what actually happens in it.
| Activity | Hot water used | Times in your peak hour |
|---|---|---|
| Shower | 20 gallons | × |
| Bath | 20 gallons | × |
| Washing hair at a sink | 4 gallons | × |
| Shaving at a sink | 2–4 gallons | × |
| Hand dishwashing | 4 gallons | × |
| Automatic dishwasher | 6 gallons | × |
| Clothes washer (warm) | 7 gallons | × |
| Food preparation | 5 gallons | × |
A worked example: three showers, a dishwasher run and a load of laundry in the same hour comes to 60 plus 6 plus 7, or 73 gallons. That household wants a First Hour Rating of at least 73 — which in practice is a 50-gallon gas tank, a 75 to 80-gallon electric one, or any competent tankless unit.
Two adults with staggered showers and no morning laundry might total 24 gallons, and a 40-gallon gas heater covers that with room to spare. The spread between those two households is enormous, which is exactly why "one size per bedroom count" advice fails so often.
Quick sizing by household
| People | Target FHR | Gas tank | Electric tank | Heat pump |
|---|---|---|---|---|
| 1–2 | ~30–40 gal | 30–40 gallon | 40–50 gallon | 50 gallon |
| 2–3 | ~40–50 gal | 40 gallon | 50 gallon | 50–65 gallon |
| 3–4 | ~50–70 gal | 40–50 gallon | 66–80 gallon | 65–80 gallon |
| 5+ | ~70–90 gal | 50–75 gallon | 80+ gallon | 80 gallon |
Use this as a sanity check on the worksheet rather than as a substitute for it. Households with a large soaking tub, a multi-head shower, or teenagers who shower simultaneously routinely need a size more than the table suggests. Our 50-gallon electric picks and gas water heater guide cover specific models.
Sizing a tankless unit is a different calculation
A tankless heater has no storage, so household size is irrelevant. What matters is how many gallons per minute you need at once, and how cold the water arriving at your house is.
First add up simultaneous demand:
- Shower: 1.8 to 2.5 GPM (WaterSense-labeled heads are 2.0 or less)
- Kitchen faucet: 1.5 to 2.2 GPM
- Bathroom faucet: 0.5 to 1.5 GPM
- Dishwasher: 1 to 1.5 GPM
- Clothes washer: around 2 GPM
Two showers at once is roughly 4 to 5 GPM. A shower plus a kitchen sink is about 4.
Then check the unit's output at your temperature rise, which is the step almost everyone skips. The formula is GPM = BTU per hour ÷ (500 × temperature rise in °F). Groundwater is around 70°F across the southern states and can be 40°F in the northern tier in winter; a comfortable shower is 105 to 110°F. So the same 180,000 BTU gas unit that delivers over 7 GPM in Houston delivers about 5 GPM in Minneapolis in February — still plenty, but a 100,000 BTU unit in the same house drops to under 3.
Electric tankless is where this bites hardest, because the largest units an ordinary residential panel can feed produce under 100,000 BTU per hour. The full arithmetic, including the amperage each size demands, is in best tankless water heater.
The physical trap: modern tanks are bigger
This one catches people replacing like-for-like. Federal efficiency standards that took effect in April 2015 required substantially more insulation on residential storage water heaters, and manufacturers added it to the outside. A modern 50-gallon tank is typically two to three inches wider and several inches taller than the one it replaces.
Before you order anything, measure three things: the floor footprint, the ceiling height above the tank — you need room to pull the anode rod, which on a tall tank is around 44 inches of clearance — and the width of every doorway and stair turn between the front door and the installation point. If a 50-gallon tank will not fit, the answers are a "lowboy" or "short" model with the same capacity in a wider, shorter body, or a tankless unit that hangs on a wall.
Also check whether the space has adequate combustion air for a gas unit, and whether a heat pump water heater is viable — hybrids need roughly 700 to 1,000 cubic feet of surrounding air space, so a sealed closet will not do without louvered doors or ducting.
Four sizing mistakes worth avoiding
- Sizing on bedrooms. A three-bedroom house with two adults and a three-bedroom house with four teenagers have completely different peak hours.
- Buying much bigger "to be safe." An oversized tank keeps more water hot around the clock for no benefit, and standby loss is a real running cost. One size of headroom is sensible; three is waste.
- Ignoring what a soaking tub does. A 60-gallon freestanding tub is three showers in one draw and will empty most tanks by itself. Households with one should size on the tub, or accept waiting.
- Forgetting that capacity fades. Sediment accumulates on the bottom of a tank and displaces water while insulating the burner from it, so an unflushed heater delivers less hot water each year. That is a maintenance problem, not a sizing one — see how to flush a water heater and why hot water runs out so fast.
Before you buy
Check the ENERGY STAR water heater listings for efficiency comparisons across the size you have settled on, and be aware that replacing a water heater usually requires a permit and an inspection — requirements are set and enforced locally, so check with your city or county building department. Our page on whether you need a permit to replace a water heater sets out what is typically involved, and how much it costs to run a water heater puts numbers on the fuel choice. Our appliance running cost calculator will do the arithmetic with your own utility rate.
Related reading: how long a water heater lasts, what temperature to set it to, and how often to flush it.