Quick answer: Size a storage tank by peak-hour demand and First Hour Rating (FHR), not just gallons - roughly a 40-gallon tank for 1-2 people, 40-50 for 3-4, and 50-80 for 5+. Size a tankless unit by flow rate (GPM) and temperature rise: add up the GPM of fixtures used at once and make sure the unit delivers that at your coldest incoming water temperature. Size it for your home with the water heater sizing calculator.
Size by demand, not just gallons
The most common sizing mistake is choosing a water heater by tank gallons alone. What actually matters is whether the heater can meet your busiest hour of hot-water use - several showers, the dishwasher, and laundry stacking up. That is why storage heaters are rated by First Hour Rating and tankless heaters by flow rate and temperature rise. Get the peak-demand match right and you never run out; get it wrong and you get cold showers even with a big tank.
Storage tanks: First Hour Rating
First Hour Rating (FHR) is the number of gallons of hot water a full tank can deliver in one hour, combining the stored volume with how fast the burner or element reheats. It is a much better sizing number than tank size, because a 40-gallon tank with a strong burner can have an FHR well above 40 gallons.
To size a tank:
- Estimate your peak-hour demand - the total hot water used during your busiest hour (count showers, shaving, dishwasher, laundry that overlap).
- Choose a heater whose FHR meets or exceeds that number.
Rough peak-hour figures: a shower uses about 10-20 gallons of hot water, shaving 2 gallons, a dishwasher 6 gallons, a clothes washer 7 gallons per load. A family with three back-to-back morning showers plus a dishwasher can peak around 40-50 gallons in that hour.
| Household | Typical tank size | Target FHR |
|---|---|---|
| 1 - 2 people | 30 - 40 gal | ~30 - 45 gal |
| 3 - 4 people | 40 - 50 gal | ~45 - 60 gal |
| 5+ people | 50 - 80 gal | ~60 - 80 gal |
Electric tanks reheat slower than gas, so an electric heater often needs a larger tank (or higher FHR) to hit the same peak-hour delivery.
Tankless: flow rate and temperature rise
Tankless (on-demand) heaters do not store water - they heat it as it flows, so they are sized by two numbers:
- Flow rate (GPM): the total gallons per minute of the fixtures you might run at once. A shower is about 2-2.5 GPM, a kitchen or bathroom faucet 1-1.5 GPM. Add up your realistic simultaneous demand.
- Temperature rise: the difference between your incoming cold water temperature and the desired hot temperature (often 120°F).
The unit must deliver your peak GPM at that temperature rise. This is the crucial catch with tankless: its GPM output falls as the required rise increases. The same heater delivers plenty of hot water in a warm climate and far less in a cold one.
Why temperature rise makes or breaks tankless sizing
Suppose you want 120°F hot water and two showers at once (about 5 GPM total). In a warm climate where groundwater is 70°F, the rise is only 50°F and a mid-size tankless handles it. In a cold climate where groundwater is 45°F, the rise is 75°F, and the same unit might only deliver 3-4 GPM - not enough for two showers. That is why you always size tankless for your coldest incoming water, not the yearly average. Undersizing a tankless in a cold climate is the number one cause of "lukewarm" complaints.
| Simultaneous use | Approx. GPM needed |
|---|---|
| 1 shower | 2 - 2.5 |
| 1 shower + 1 faucet | 3 - 3.5 |
| 2 showers | 4 - 5 |
| 2 showers + kitchen | 5.5 - 6.5 |
Tank vs tankless
Both work well when sized right; the choice depends on your habits, climate, and budget:
- Storage tank: lower upfront cost, handles big simultaneous draws up to its capacity, simple. Downsides: standby heat loss, and it can run out during a long peak until it reheats.
- Tankless: endless hot water for continuous use, saves standby energy, lasts longer, and is compact. Downsides: higher upfront cost, limited by peak GPM, and needs adequate gas line or electrical capacity, plus proper venting for gas.
A useful way to decide: if your problem is running out during long or continuous use, tankless shines; if your problem is many fixtures at the exact same moment, a well-sized tank (or a larger tankless / two units) is often simpler.
Fuel type and recovery rate
Recovery rate is how many gallons per hour a heater can reheat, and it strongly affects the effective capacity. Gas heaters recover faster than electric, so for the same household a gas tank can be smaller than an electric one. Heat-pump (hybrid) electric heaters are very efficient but recover slowly, so they are usually sized generously. Match the fuel and recovery to your peak pattern, not just the number of people.
Worked example
A family of four, cold-ish climate (incoming water ~50°F), wanting 120°F, busy mornings:
- Tank route: peak hour is two showers (~30 gal) plus a dishwasher (~6 gal) = ~36 gal, so pick a 50-gallon gas tank with an FHR around 60-70 gal for comfortable headroom.
- Tankless route: peak is possibly two showers (~5 GPM) at a 70°F rise, which needs a high-capacity unit rated for roughly 8-9 GPM at low rise so it still delivers ~5 GPM at 70°F - or two smaller units.
Signs your water heater is the wrong size
You can often tell a sizing problem from the symptoms:
- Running out of hot water during back-to-back showers or a long shower means the tank is too small or its recovery is too slow - or a tankless unit is undersized for your peak GPM.
- Lukewarm water when two fixtures run at once is the classic tankless-undersized symptom (not enough GPM at your temperature rise).
- Water that never quite gets hot in winter but is fine in summer points to a tankless unit that cannot meet the higher cold-season temperature rise.
- High standby energy use from a tank far bigger than you need - oversizing a tank wastes energy keeping water hot that you rarely use.
Right-sizing fixes these without simply buying the biggest unit, which for a tank just raises standby losses.
Recovery rate and "how long does hot water last"
A tank does not just empty and stop - it is reheating the whole time you draw, which is why First Hour Rating beats raw gallons. Recovery rate (gallons per hour the heater can reheat) sets how fast it catches up. A gas tank with a strong burner might recover 40+ gallons per hour, while a standard electric tank recovers far less, and a heat-pump water heater slower still. So two 50-gallon tanks can behave very differently: the fast-recovery gas unit shrugs off three morning showers, while the slow-recovery electric one runs cold before the third. When you compare heaters, look at the recovery rate and First Hour Rating together, not just the tank size on the label - that pairing is what determines whether you run out.
Common water-heater sizing mistakes
- Sizing tanks by gallons alone instead of First Hour Rating and peak-hour demand.
- Sizing tankless on ideal GPM without checking the temperature rise for your cold-season incoming water.
- Ignoring simultaneous use - two showers plus a kitchen faucet is a very different load than one shower.
- Forgetting recovery rate, so an electric tank runs cold sooner than expected.
- Undersizing gas line or electrical service for a tankless unit.
Standards and references
| Reference | What it covers |
|---|---|
| First Hour Rating (FHR) | DOE storage-tank peak-hour delivery metric |
| UEF (Uniform Energy Factor) | Water heater efficiency rating |
| IPC / UPC | Plumbing code requirements for water heaters |
| Manufacturer GPM curves | Tankless output vs temperature rise |
The bottom line
Size a water heater by peak demand: for a tank, match the First Hour Rating to your busiest hour (about 40 gal for 1-2 people, 50 for 3-4, 50-80 for 5+); for tankless, match the peak GPM at your coldest incoming water temperature, because output falls as the temperature rise grows. Consider fuel type and recovery rate, and decide between tank (handles simultaneous draws cheaply) and tankless (endless hot water, saves standby energy). Size yours with the water heater sizing calculator (or the solar water heater sizing calculator for solar systems), and confirm the selection with a qualified plumber.
Frequently asked questions
What size water heater do I need for my family?
For a storage tank, size by peak-hour hot water use, not just gallons. Rough guides: 1-2 people need about a 30-40 gallon tank, 3-4 people about a 40-50 gallon tank, and 5 or more about a 50-80 gallon tank, matched to a First Hour Rating that meets your busiest hour. For tankless, size by flow rate in gallons per minute and the temperature rise you need. Actual demand depends on fixtures used at once and incoming water temperature.
What is First Hour Rating on a water heater?
First Hour Rating (FHR) is the number of gallons of hot water a storage tank can deliver in the busiest hour, starting with a full tank of hot water. It combines the tank volume with how fast the heater reheats, so it is a better sizing number than tank gallons alone. You estimate your peak-hour demand, then choose a heater whose FHR meets or exceeds it - a 40 gallon tank can have an FHR well above 40 gallons.
How do I size a tankless water heater?
Tankless heaters are sized by flow rate (gallons per minute) and temperature rise, not by tank size. Add up the GPM of the fixtures you might run at once (a shower is about 2-2.5 GPM, a faucet about 1-1.5 GPM), then find the temperature rise from your incoming cold water temperature to the target hot temperature (often to 120 F). The unit must deliver your peak GPM at that temperature rise - cold climates need bigger units because the rise is larger.
What is temperature rise and why does it matter for tankless?
Temperature rise is the difference between your incoming cold water temperature and the desired hot water temperature. A tankless heater's GPM output falls as the required rise increases, so the same unit delivers more hot water in a warm climate than a cold one. If your groundwater is 50 F and you want 120 F, that is a 70 F rise, which limits GPM more than a 40 F rise would. Always size for your coldest incoming water.
Is tankless or tank water heater better?
Tankless heats water on demand, saves standby energy, never runs out for continuous use, and lasts longer, but costs more upfront and is limited by peak GPM. A storage tank costs less, handles simultaneous heavy draws up to its capacity, but has standby losses and can run out during a big demand. The best choice depends on your hot-water pattern, climate, fuel, budget, and space.
How many gallons of hot water does a shower use?
A typical shower flows about 2 to 2.5 gallons per minute, so a 10 minute shower uses roughly 20 to 25 gallons of mixed water, of which a large share is hot. Low-flow showerheads cut this to about 1.5 to 2 GPM. Because several showers back to back are the usual peak, sizing a tank to your peak-hour showers and other draws - the First Hour Rating - is the key to never running out.
What size water heater for one or two people?
One or two people usually do well with a 30 to 40 gallon storage tank, or a tankless unit sized for about 3 to 4 GPM at your temperature rise. If you have a large soaking tub, a high-flow shower, or run laundry and a shower at the same time, step up a size. Peak simultaneous demand, not just the number of people, determines the right size.
Is a water heater sizing calculator accurate?
A calculator that sizes a tank by peak-hour demand and First Hour Rating, or a tankless unit by peak GPM and temperature rise for your climate, gives a reliable size for most homes. A complete selection also considers fuel type and recovery rate, simultaneous fixture use, incoming water temperature, efficiency, and venting or electrical capacity, and larger or commercial systems should be confirmed by a qualified plumber or engineer.