Water Heater Size Calculator
Size a tank, tankless, or heat pump water heater from your real peak-hour use — first-hour rating, flow rate and temperature rise — with U.S. pricing, annual energy cost, a model comparison, and a downloadable PDF.
Tanks are sized by peak-hour demand; tankless units by flow rate at your temperature rise — cold incoming water shrinks a tankless unit's output fast. Estimates use 2024-2025 U.S. prices. For planning purposes only — venting, gas line, and electrical work need a licensed pro.
| Model | Delivers | You Need | Fits? | Est. Unit Cost |
|---|
How to use the water heater size calculator
Four quick steps turn your household’s hot-water habits into the right size — a tank in gallons or a tankless unit in gallons per minute — with peak demand and temperature rise worked out, a comparison, and a downloadable PDF.
Pick the type
Choose a tank or tankless heater — the calculator sizes each the right way.
Enter your usage
Household size, or the showers, baths, and appliances used in your busiest hour.
Set the temperatures
For tankless, add your incoming water temp so the temperature rise is right.
Read the size & save a PDF
Get the recommended gallons or GPM, a demand breakdown, and a downloadable summary.
How the water heater size is calculated
Tank and tankless heaters are sized in completely different ways. A tank is matched to your peak-hour demand in gallons; a tankless is matched to your flow rate and temperature rise. Here is the exact math for both.
Tank — busy morning hour
Demand: 3 showers (30) + shave (2) + kitchen (4)
Peak hour: = 36 gallons
Pick: FHR ≥ 36 gal
A 40–50 gal tank with an FHR of 36+ covers it.
Tankless — same peak
Flow: 2 showers (4.0) + kitchen (1.5)
Total: = 5.5 GPM
Rise: 120 − 50 = 70°F
Pick a unit rated 5.5+ GPM at a 70°F rise.
Water heater sizing charts
Handy lookups for the questions people ask most — tank size by household, hot-water use per activity, fixture flow rates, and tankless output by temperature rise. Figures are typical; your usage and climate will vary.
| Household | Tank size | Typical FHR needed |
|---|---|---|
| 1–2 people | 30–40 gal | ~30–40 gal |
| 2–3 people | 40–50 gal | ~40–50 gal |
| 3–4 people | 50–60 gal | ~50–60 gal |
| 5–6 people | 60–80 gal | ~60–80 gal |
A guide only — match the first-hour rating to your real peak-hour demand, especially if several showers run close together in the morning.
| Activity | Gallons (hot) |
|---|---|
| Shower | ~10 |
| Bath | ~20 |
| Clothes washer | ~7 |
| Dishwasher | ~6 |
| Kitchen / hair wash | ~4 |
| Shaving / hand wash | ~2 |
Add up whatever overlaps in your busiest hour to get peak-hour demand for a tank heater. Low-flow fixtures use less than these figures.
| Fixture | Flow (GPM) |
|---|---|
| Shower | ~2.0 |
| Clothes washer | ~2.0 |
| Kitchen faucet | ~1.5 |
| Bathroom faucet | ~1.5 |
| Dishwasher | ~1.5 |
Add the GPM of the fixtures you’d run at the same time — that total is the flow a tankless unit must deliver at your temperature rise.
| Incoming water | Rise to 120°F | Typical region |
|---|---|---|
| 70°F | 50°F rise | Warm south |
| 60°F | 60°F rise | Mild / coastal |
| 50°F | 70°F rise | Moderate / midwest |
| 40°F | 80°F rise | Cold north / winter |
The bigger the rise, the less flow a tankless unit delivers — a heater good for 8 GPM at a 40°F rise may give only ~5 GPM at 70°F. Size for your coldest incoming water.
gallons + FHR
GPM + rise
target − incoming
The right size depends on the type — a tank stores a batch of hot water, a tankless heats on demand as it flows.
- Tank — match the first-hour rating to the gallons you use in your busiest hour.
- Tankless — match the flow rate to the fixtures you run at once, in GPM.
- Temperature rise — colder incoming water means more rise and a bigger unit.
Everything the calculator works out
One set of habits gives you a complete sizing answer — for a tank or a tankless heater — with the demand, the rise, and a comparison.
Key figures behind water heater sizing
Built for any hot-water decision
From a first replacement to a tankless upgrade, the same calculator sizes the heater to how your household actually uses hot water.
Their old tank died and they want the right size replacement without running out during morning showers.
- Count your busy-hour showers
- Match the first-hour rating
- Size up if mornings overlap
Switching to tankless for endless hot water and needs the right GPM for their climate and fixtures.
- Add up simultaneous fixtures
- Use your coldest incoming temp
- Size for the higher rise
Specifying a heater for a client or new build and wants demand, FHR, and GPM figured quickly.
- Work from peak-hour demand
- Compare tank vs tankless
- Save the PDF for the quote
7 tips for sizing a water heater right
Small habits keep your sizing honest and your showers hot, whether you choose a tank or tankless.
Water heater size calculator FAQ
The sizing, first-hour rating, tankless, and temperature-rise questions people ask most.
It depends on the type. For a tank heater, match the first-hour rating to your peak-hour hot-water demand, which for most families lands around a 40 to 60 gallon tank. For a tankless heater, size by the flow rate in gallons per minute you need at once and your temperature rise.
As a rough guide, a household of 3 to 4 people usually needs a 50 to 60 gallon tank, or a tankless unit that delivers around 5 to 7 gallons per minute.
Add up the gallons of hot water your household uses in its busiest hour. A shower is about 10 gallons, a bath about 20, shaving about 2, and running the dishwasher or kitchen sink a few gallons each.
For example, three back-to-back showers plus shaving and the kitchen sink is roughly 36 gallons in the peak hour. Your tank’s first-hour rating should meet or beat that number.
First-hour rating, or FHR, is how many gallons of hot water a tank heater can deliver in a single busy hour, starting with a full tank. It combines the tank volume with how fast the heater reheats.
It’s the number to match against your peak-hour demand, and it’s printed on the heater’s EnergyGuide label. Two tanks of the same size can have different first-hour ratings depending on their burner or element power.
Size a tankless heater by two things: the total flow rate in gallons per minute of the fixtures you want to run at once, and the temperature rise, which is your target temperature minus the incoming water temperature.
Add the GPM of the fixtures, work out the rise, then choose a unit rated for that flow at that rise. A tankless heater delivers less flow the colder your incoming water, so cold climates need a more powerful unit.
Temperature rise is how much the heater must warm the water, calculated as your target temperature minus the incoming groundwater temperature. Most homes target about 120 degrees Fahrenheit at the tap.
If your incoming water is 50 degrees, the rise is 70 degrees. Incoming water is colder in northern climates and in winter, so the required rise, and the size of tankless unit you need, goes up accordingly.
Each has trade-offs. Tankless heaters give endless hot water and save space and standby energy, but cost more upfront and have a flow-rate limit. Tank heaters cost less to buy and can deliver a big burst of hot water, but run out and use standby energy keeping the tank warm.
Tankless suits homes wanting continuous hot water and long-term efficiency; a tank suits lower upfront cost and high simultaneous demand. The right choice depends on your usage, budget, and climate.
A family of four typically needs a 50 to 60 gallon tank heater, or a tankless unit that delivers roughly 6 to 8 gallons per minute, depending on how many showers and appliances run at the same time.
If several people shower close together in the morning, size toward the higher end or check that the tank’s first-hour rating covers that peak. Bigger homes with two bathrooms in use at once may need more.
Yes. A tank that’s too small, or has too low a first-hour rating, runs out during back-to-back showers and takes time to recover. A tankless unit that’s undersized can’t keep up with the flow, so the water turns lukewarm when too many fixtures run at once.
Sizing to your real peak demand avoids both. It’s usually worth a little extra capacity so a busy morning doesn’t leave the last person with a cold shower.
General Estimating Notice: This calculator recommends a water heater size from your household usage, peak-hour demand, fixture flow, and temperature rise, using typical hot-water figures and standard sizing rules. Actual needs vary with your fixtures, habits, climate, and incoming water temperature, and manufacturer ratings differ between models. Water heater selection, gas, venting, and installation should follow the manufacturer’s specifications and local code, and gas or electrical work should be done by a qualified professional. For planning purposes only.