Water Heater Size Calculator: What Size Tank Do You Need?
Most water heaters are bought by tank size, but tank size alone is the wrong number to shop by. What actually decides whether the last person in the morning queue gets a hot shower is how much hot water the heater can deliver in its busiest hour. A small tank with a powerful burner or element can out-deliver a bigger, weaker one, and a large household can drain a tank that looks generous on paper. Enter the number of people in your home and how heavily you use hot water, and this calculator gives you a first-hour rating target, a tank size that meets it, and the tankless flow rate to compare.
Water Heater Size Calculator
Choose how many people live in your home and how heavily you use hot water. Results update as you change the inputs, or press Calculate. No sign-up and no email needed, just your numbers.
Enter your household size and usage, then press Calculate to see your results.
First-hour rating (FHR) explained
The first-hour rating (FHR) is the number of gallons of hot water a heater can supply in one busy hour, starting with a full, hot tank. It combines two things: the hot water already stored in the tank, plus the extra water the heater can heat up while that hour is running. You will find it on the energy label of most tank models, and it is a far better guide to real performance than the tank’s gallon figure alone, because two heaters with the same tank can have very different ratings.
The hour that matters is your peak hour — usually early morning, when showers run back to back, someone fills the kitchen sink and the washing machine starts a warm cycle. If the FHR covers that hour, the heater keeps up for the rest of the day without trying. If it falls short, the water does not stop, but it arrives lukewarm, and it arrives at the worst possible moment. Sizing by the peak hour, rather than by average daily use, is what stops that from happening.
Typical tank sizes by household
As a starting point, most homes land in these ranges. The calculator above sharpens the estimate by adjusting for how heavily your household uses hot water, which is why two homes with the same headcount can honestly need different tanks.
| People in household | Typical tank size |
|---|---|
| 1–2 | 30–40 gal |
| 3–4 | 40–50 gal |
| 5–6 | 50–65 gal |
| 7+ | 65–100 gal or tankless |
Read the table as a range, not a promise. A couple who take short showers and run the dishwasher overnight may be comfortable at the bottom of their band, while a family with teenagers, a soaking tub and a serious laundry habit belongs at the top of theirs — or one size beyond it.
Tank or tankless?
A tank heater stores hot water and keeps it ready around the clock. A tankless, or on-demand, heater stores nothing: it heats water as it flows, so it is sized in gallons per minute (GPM) rather than gallons in a tank. Tankless units never run out of hot water mid-shower, they take up little wall space, and they avoid the energy a tank loses just sitting there. The trade-offs are a higher upfront price, possible upgrades to the gas line or electrical supply, and a hard limit on flow.
That flow limit is what to test. One shower uses roughly 2 GPM, so two showers and a running tap at once can ask for 5 GPM or more, and a big soaking tub fills fastest of all. Multi-shower mornings and large tubs favour a large tank, two units, or a high-flow tankless model. Smaller homes with steady, spread-out use are where tankless tends to shine, because the demand never arrives all at once.
Do not oversize
It is tempting to buy one size up just to be safe, but an oversized tank costs you money every single day. A bigger tank holds more hot water that nobody uses, and all of it slowly loses heat through the tank walls — this is called standby loss — so the heater keeps reheating water simply to stand still. You also pay more for the unit itself, and you may need more floor space and a larger flue or electrical circuit. The right size is the one that covers your peak hour with a little to spare, and no more. If your household is about to grow, size for that future; otherwise, trust the peak-hour figure.
What to calculate next
Once the new heater is in, these tools help you check what it — and the rest of the house — costs to run:
- Electricity Cost Calculator — what the heater, or any appliance, costs to run per hour, month and year.
- Fridge Running Cost Calculator — the daily, monthly and yearly cost of the appliance that never switches off.
- Calculators hub — every free Wolveyes calculator in one place.
Frequently asked questions
What size water heater does a family of 4 need?
Most families of four need a 50-gallon tank with a first-hour rating of about 72 gallons at average usage. Heavy use — long showers, a soaking tub, back-to-back laundry — can push you to a 65-gallon tank, while light use may be fine with 40 gallons.
What is first-hour rating?
It is the amount of hot water a heater can deliver in its busiest hour, starting with a full tank: the stored hot water plus what it can heat during that hour. It appears on the energy label and predicts real performance better than tank size alone.
How is tankless sizing different?
Tankless heaters are sized in gallons per minute of simultaneous flow, not in stored gallons. Add up the flow of everything that might run at once — about 2 GPM per shower — and choose a unit that can deliver that at your incoming water temperature.
Is a bigger water heater always better?
No. A tank that is bigger than your peak hour needs just stores water nobody uses, and loses heat around the clock through standby losses. Size to your busiest hour with a little headroom, and the heater will keep up without wasting energy.
Estimates only — a plumber can confirm against your fixtures and incoming water temperature.