If you’re shopping for a new tank and wondering what size water heater do I need, here’s the short answer: most homes with 1–2 people do fine with 30–40 gallons, 3–4 people usually need 50–60 gallons, and larger households need 60–80 gallons or a properly sized tankless unit. But gallons are only half the story. The number that tells you whether you’ll run out of hot water during the morning rush is the first hour rating.
This guide answers what size water heater do I need using the U.S. Department of Energy’s sizing method, shows you how to do the math in about five minutes, and explains what each size costs. It’s written for homeowners in Los Angeles, Orange County and the surrounding communities, where closet installs, earthquake strapping and gas venting all affect which size fits.
What This Guide Covers
- Quick answer: water heater size by household
- Why first hour rating matters more than gallons
- How to calculate your peak hour demand
- How gas and electric models change the answer
- Sizing a tankless water heater by flow rate
- Signs your current heater is the wrong size
- Space, strapping and code checks in LA and Orange County
- What each size costs to install
- Frequently asked questions
What size water heater do I need for my household?
Household size is the fastest starting point. Angi’s sizing chart adds roughly 10 gallons for every extra person in the home:
| People in the home | Typical tank size |
|---|---|
| 1–2 | 30–40 gallons |
| 2–3 | 40–50 gallons |
| 3–4 | 50–60 gallons |
| 4–5 | 60–70 gallons |
| 5–6 | 70–80 gallons |
| 6–7 | 80–90 gallons (or tankless) |
Treat this table as a first answer to what size water heater do I need, not a final one. A family of four that showers at night over three hours needs far less hot water per hour than a family of four with three people showering back to back at 6:30 a.m. The next section measures that difference.
Why first hour rating matters more than gallons
The Department of Energy’s advice is to pick a storage water heater whose first hour rating (FHR) meets or exceeds your household’s peak hour demand. The FHR is how many gallons of hot water the heater can deliver in one hour, starting with a full tank. It depends on tank capacity, the heat source, and the size of the burner or heating element.
You’ll find it on the yellow EnergyGuide label as “Capacity (first hour rating),” or in the manufacturer’s spec sheet. Two 50-gallon tanks can have different first hour ratings, so the real question behind what size water heater do I need is often “what first hour rating do I need?”
How to calculate your peak hour demand (step by step)
The most reliable way to answer what size water heater do I need is the DOE worksheet. Pick your home’s busiest hour, count the hot water uses in it, and add them up using these averages:
- Shower: 20 gallons each
- Shaving: 2 gallons
- Hand dishwashing or food prep: 3 gallons
- Automatic dishwasher: 7 gallons
- Clothes washer, top-loader: 25 gallons
- Clothes washer, front-loader (H-axis): 15 gallons
Step 1: Pick your busiest hour
For most families it’s a weekday morning; for others it’s the evening, when baths, dishes and laundry overlap.
Step 2: Add up every hot water use in that hour
The DOE’s example: three showers (60 gallons) plus one shave (2 gallons) plus hand-washing dishes (3 gallons) equals a peak hour demand of 66 gallons.
Step 3: Match it to a first hour rating
In that example, you’d shop for a heater with a first hour rating of at least 66 gallons. Add a top-loading washer in the same hour and demand jumps to 91 gallons, a very different answer to what size water heater do I need. Moving laundry to another time of day can keep you from paying for a bigger tank.
A worked example for a family of four
Two adults and two teens all shower between 6:00 and 7:00 a.m., and the dishwasher runs: 4 × 20 + 7 = 87 gallons. The household chart suggests 50–60 gallons, but this family needs a first hour rating near 90 gallons, which points to a high-recovery tank, a larger tank or a tankless unit.
How gas and electric models change the answer
Fuel type also changes what size water heater do I need, because the first hour rating depends on the heat source and burner or element size. A gas tank and an electric tank with the same gallons can deliver different amounts of hot water in an hour, so compare the FHR printed on each label rather than tank size alone.
- Gas tank: a common fit when the home already has a gas line and a properly vented location.
- Standard electric tank: no flue to vent, but check the first hour rating closely if your family showers back to back.
- Heat pump (hybrid) electric: efficient, but it needs space and airflow around the unit, so plan the location first.
If you’re switching fuels or moving the heater, the size decision and the installation plan go together. A licensed plumber handling your water heater replacement can confirm gas capacity, venting, electrical supply and clearances before you commit to a model.
Sizing a tankless water heater by flow rate
With tankless, what size water heater do I need is answered in gallons per minute (GPM), not gallons stored. The DOE method:
- List the fixtures you’ll run at the same time and add their flow rates. A 2.5 GPM shower plus a 0.75 GPM faucet needs at least 3.25 GPM.
- Work out the temperature rise. Subtract the incoming water temperature from your target. If you don’t know it, the DOE suggests assuming 50°F, so heating to 120°F means a 70°F rise.
- Check the unit’s rating at that rise. Per the DOE, many tankless models are rated for a 70°F rise at about 5 GPM for gas and about 2 GPM for electric.
Angi’s tankless chart gives a rough household guide: about 6–8 GPM for 2 people, 8–10 GPM for 4 and 10–12 GPM for 6, with warmer-climate homes at the lower end. For brands and features, see our guide to picking the right tankless model, and if you’re still deciding between styles, read our breakdown of tankless vs. tank for LA homes.
Signs your current heater is the wrong size
If you’re asking what size water heater do I need because your current one isn’t keeping up, check these signs first.
Too small
- The last person to shower regularly gets lukewarm or cold water.
- You wait between showers, laundry and dishes for the tank to catch up.
- Your household has grown, or you’ve added a bathroom or a large tub since the heater went in.
Too large
- You never come close to running out, even on the busiest days.
- You pay to keep extra water hot around the clock (standby heat loss).
Running out of hot water isn’t always a sizing problem. A failing element, a broken dip tube or heavy sediment can shrink usable capacity, so if the heater used to keep up and suddenly doesn’t, have it checked before buying a bigger one.
Space, strapping and code checks in LA and Orange County
Once you know what size water heater do I need on paper, make sure that size fits and can be installed correctly where your current heater sits.
- Earthquake strapping: California Health and Safety Code Section 19211 requires new, replacement and existing residential water heaters to be braced, anchored or strapped against earthquake motion, at a minimum per the California Plumbing Code. Home sellers must also certify compliance to buyers.
- Physical space: moving up from 40 to 50 or 75 gallons adds height and width. Measure the closet, including the door opening, and leave room for service.
- Venting and gas supply: a larger or higher-input gas burner may need a different vent or more gas capacity.
- Thermal expansion: if your home has a pressure regulator or check valve, ask whether you also need an expansion tank.
- Permits: ask whether your city requires a permit for the replacement; a licensed plumber can pull one when needed.
What each size costs to install
Your answer to what size water heater do I need also drives the price. According to Angi, Los Angeles homeowners pay about $1,740 on average for water heater installation, with most projects between $1,140 and $2,410. Angi’s national figures by size show how cost rises with capacity:
| Tank size | Typical household | Installed cost range (Angi) |
|---|---|---|
| 30 gallons | 1–2 people | $925–$2,750 |
| 40 gallons | 1–4 people | $1,050–$3,000 |
| 50 gallons | 3–5 people | $1,050–$3,275 |
| 75 gallons | 4–6 people | $1,450–$5,250 |
| 80 gallons | 6+ people or high-demand appliances | $1,700–$9,825 |
| Tankless | Sized by GPM | $1,850–$5,100 |
The 5 Star Plumbing’s typical price for water heater installation is $2,000–$12,000+; the exact price is confirmed after the technician sees the job.
The bottom line
So, what size water heater do I need? Start with household size, then confirm with peak hour demand: choose a tank whose first hour rating meets or beats the hot water your family uses in its busiest hour. For tankless, size by the combined GPM of fixtures used at once and the temperature rise you need. Then make sure the unit fits, can be strapped properly and works with your gas or electric supply.
If you’d rather have a pro size it for your home, The 5 Star Plumbing has offered water heater installation and repair with experience since 2006. We’re a family-owned, licensed plumber (CA license #1041772) serving Los Angeles, Orange County and the surrounding communities, 24/7. Call (888) 909-0120 or request service online, and ask about our financing options.
Frequently asked questions: what size water heater do I need
What size water heater do I need for a family of 4?
Most families of four need a 50–60 gallon tank, according to Angi’s chart. If three or four people shower in the same hour, calculate your peak hour demand and choose a first hour rating at or above it, which can mean a high-recovery tank or a tankless unit.
Is a 40-gallon water heater enough for 2 people?
Usually, yes. For couples asking what size water heater do I need, a 30–40 gallon tank covers most 1–2 person homes, as long as its first hour rating covers your busiest hour.
What size water heater do I need for 3 bathrooms?
The number of bathrooms matters less than how many are used at the same time. Add up your busiest hour’s showers and appliance use, then match it to the first hour rating or, for tankless, the combined GPM.
Is a bigger water heater always better?
No. An oversized tank keeps extra water hot all day and takes up space. The best answer to what size water heater do I need is the smallest heater that covers your real peak hour demand.
Where do I find a water heater’s first hour rating?
On the EnergyGuide label, listed as “Capacity (first hour rating),” and in the manufacturer’s specifications. When you work out what size water heater do I need, compare that figure, not just tank gallons.
How many GPM tankless water heater do I need?
Add the flow rates of the fixtures you’ll run at once. The DOE’s one-shower-plus-one-faucet example comes to about 3.25 GPM, while Angi suggests roughly 8–10 GPM for a family of four. Check the GPM at your required temperature rise.
Do I need strapping when I upsize my water heater?
Yes. California law requires residential water heaters to be braced, anchored or strapped for earthquakes, whatever their size.
Sources: U.S. Department of Energy — Sizing a New Water Heater · Angi — What Size Water Heater Do I Need? · Angi — Water Heater Installation Cost, Los Angeles CA · California Health and Safety Code § 19211 (Justia)
