Size a tank heater by its First-Hour Rating, matched to your busiest hour of use. A rough guide is about 12 gallons per person, so a family of four needs near 50 gallons. Size a tankless unit by flow rate in gallons per minute plus the temperature rise it must produce.
How do I size a tank water heater?
Size a tank by its First-Hour Rating (FHR), not by gallons alone. The FHR is the number of gallons a full, heated tank can supply in one hour of heavy use. It combines tank volume with how fast the burner or element reheats incoming water. Two tanks of the same size can have different FHR numbers, so always check the yellow EnergyGuide label.
Start by finding your peak-hour demand. Pick the busiest hour in your home, often a weekday morning, and add up the hot water each fixture uses in that hour. Then buy a tank with an FHR within a gallon or two of that total.
A simple rule of thumb works for a first guess: about 12 gallons per person. Here is how that scales:
| Household size | Suggested tank (gallons) | Typical FHR target |
|---|---|---|
| 1 to 2 people | 30 to 40 | 30 to 50 |
| 3 to 4 people | 50 | 50 to 70 |
| 5 to 7 people | 60 to 80 | 70 to 90 |
The U.S. Department of Energy frames the goal plainly: a water heater "with a first hour rating or maximum gpm that matches within 1 or 2 gallons of your peak hour demand" is the right pick. Match the rating to the hour, and you avoid both cold showers and wasted standby heat.
One Phoenix note. Hard water builds scale on tank bottoms and heating surfaces, which slowly cuts how fast a tank reheats. That can drag down real-world FHR over the years. Flushing the tank each year helps the unit hold its rated output.
Recovery rate matters too. A gas tank reheats faster than an electric one of the same size, so a 40-gallon gas heater can post a higher FHR than a 40-gallon electric model. If your home leans on electric, you often need a larger tank to reach the same first-hour number. Check the EnergyGuide label rather than trusting the gallon count on the box.
How do I size a tankless water heater?
Size a tankless unit by two numbers: the flow rate you need in gallons per minute (GPM), and the temperature rise it must deliver. A tankless heater never runs out of hot water, but it can only heat so much at once. If you open more fixtures than it can handle, the water turns lukewarm.
Flow rate is the sum of the fixtures you expect to run at the same time. Common values:
| Fixture | Typical flow (GPM) |
|---|---|
| Shower | 2.5 |
| Bathroom faucet | 0.75 |
| Kitchen faucet | 1.5 |
| Dishwasher | 1.5 |
| Clothes washer | 2.0 |
Add up the fixtures that could run together during your busy hour. A shower plus a kitchen faucet is about 3.25 GPM, which is roughly the minimum a whole-home tankless unit should handle. Two showers at once push you past 5 GPM.
Temperature rise is the gap between your incoming water and your target. Set the target at 120 degrees, the temperature the Department of Energy recommends. If your cold water enters at 50 degrees, you need a 70-degree rise. The DOE uses that same 70-degree rise as its benchmark, noting a gas unit can heat about 5 GPM at that rise while an electric unit manages only about 2 GPM. Gas tankless units carry more power, so they serve larger homes.
Match both numbers together. A unit rated for 5 GPM at a 70-degree rise will deliver less flow if it must raise the temperature further, and more flow if the rise is smaller. Always read the manufacturer's chart, which plots GPM against temperature rise.
The two numbers pull against each other, and that catches many buyers off guard. A spec sheet headline like "up to 9 GPM" assumes a tiny temperature rise that no real home sees. Read the fine print. Find the row on the chart that matches your local rise, then read across to the honest flow. That is the number you size against, not the marketing figure on the front of the box.
Point-of-use sizing is different. A small electric tankless unit at one far sink only has to feed one fixture, so a 2 GPM rating is plenty. Whole-home sizing is where the rise and flow math gets tight, and where gas usually wins for a busy household.
Does Phoenix water make tankless sizing easier?
Yes, and it is a real advantage. Tankless capacity drops as the required temperature rise climbs. In cold-climate states, winter ground water can enter near 40 degrees, forcing an 80-degree rise that chokes a tankless unit's flow. Phoenix incoming water stays warm by comparison, often well above the national average even in winter.
Warmer incoming water means a smaller temperature rise to reach 120 degrees. A smaller rise lets a tankless unit deliver more of its rated flow. The same gas unit that struggles in Minnesota can run two showers here without strain. That is why tankless heaters perform better in the desert than in northern markets.
The flip side is hard water. Phoenix water is mineral-rich, and scale collects in a tankless unit's narrow heat exchanger faster than in cold, soft-water regions. Scale shrinks the effective flow and can trip error codes. Plan to descale the unit at least once a year so it keeps the capacity you paid for.
If you want the full trade-off between holding a tank of hot water and heating on demand, see our tank vs tankless page. This page is only about getting the size right.
How do I add up my fixtures and pick a size?
Walk through your busiest hour, fixture by fixture. The method is the same idea for both heater types, but the math differs.
For a tankless unit, list every fixture that might run at the same time and add the GPM. Picture a weekday morning: one person showers (2.5 GPM) while another runs the kitchen sink (1.5 GPM). That is 4 GPM at once. Now check the temperature rise. If Phoenix water enters at 65 degrees and you want 120, that is a 55-degree rise. Find a unit that delivers 4 GPM at a 55-degree rise, and you are covered.
For a tank, you size by the whole hour, not the same instant. Add up all the hot water drawn across that peak hour: showers, a load of laundry, the dishwasher, hand washing. The DOE notes a shower uses gallons fast and a tub fill uses even more, so a home with back-to-back morning showers needs a higher FHR. Match the total to a tank's FHR within a gallon or two.
A quick worked example. A family of four with two morning showers, a shaving sink, and a breakfast cleanup might draw 40 to 45 gallons in the peak hour. A 50-gallon tank with an FHR near 60 covers that with margin. The same family going tankless would want a gas unit rated for at least 5 GPM at their local temperature rise.
A few habits change the answer. A large soaking tub, a multi-head shower, or a big jetted tub raises peak demand sharply and may push you to a bigger tank or a higher-output tankless model. Cutting demand also helps. ENERGY STAR points buyers toward efficient units and low-flow fixtures, which lower both your peak draw and your energy bill. A 2.5 GPM shower head instead of an older 4 GPM head can drop your sizing by a full fixture.
When the numbers fall between two sizes, lean to the larger tank FHR or the higher tankless GPM. A small cushion handles guests and cold snaps. Going one notch too big on a tank costs a little standby energy. Going too small means cold showers every busy morning, which no setting can fix. Water heating is the second-largest energy use in a typical home, so the size you pick shows up on your bill for the next decade.
Think about the future, not just today. A growing family, a finished basement bathroom, or a new soaking tub all raise demand. It is easier to size with that in mind now than to swap a unit early. For replacement timing and how long a unit should last in our hard water, see our water heater lifespan page.