Water Heater Size Calculator (Tank & Tankless)
What size water heater you need: tank first-hour rating or tankless GPM and BTU.
Changing these fills in a typical busiest hour below. Edit the counts to match your real morning.
Busiest hour of hot water10 gal per shower
20 gal per tub fill
2 gal per shave
4 gal hand & face wash, hand dishwashing, food prep
6 gal per load
7 gal per warm load
Estimates for planning only. A licensed plumber or gas fitter should confirm sizing, venting, gas line and electrical capacity, and local code.
About the Water Heater Size Calculator (Tank & Tankless)
What size water heater do you need for a family of 4? Start with your busiest hour, not the tank size. In Tank mode, enter your household and bathrooms, adjust the showers, sink uses, dishwasher and washer loads for that hour, and pick gas, electric or heat pump. You get the first-hour rating (FHR) to look for on the EnergyGuide label plus a suggested tank size. In Tankless mode, list the fixtures running at once and pick your US region or Canadian province for the winter inlet temperature. The calculator shows the GPM, the temperature rise, and the BTU/h or kW a unit needs. Switch between °F and °C any time.
What size water heater for a family of 4?
A family of 4 typically needs a water heater with a first-hour rating (FHR) of about 64 gallons. That usually means a 40-gallon gas tank, a 50 to 65-gallon electric tank, or a 50 to 65-gallon heat pump water heater. For tankless, plan on about 6.5 GPM of fixture flow for two showers and a bathroom faucet, which in a cold-water region calls for a large (roughly 190,000 to 220,000 BTU/h) gas unit.
The 64 gallons comes from a typical busiest hour: four 10-gallon showers, one 2-gallon shave, four 4-gallon sink uses and one 6-gallon dishwasher load. Your own morning may be busier or quieter, so edit the counts in the calculator.
Water heater size by family size
This table uses the calculator's default peak hour for each household (up to 3 showers per bathroom per hour), modern water-saving fixtures, and its rule-of-thumb tank gallons per FHR. The tankless columns assume 40°F winter inlet water (Ontario, the US Northeast and Midwest), a 120°F outlet, showers and taps mixed to 105°F, and a 95% efficient gas unit.
| People (bathrooms) | Peak-hour FHR | Gas tank | Electric tank | Heat pump | Tankless fixtures at once | Fixture flow | Heater flow / gas input |
|---|---|---|---|---|---|---|---|
| 2 (1) | 30 gal | 30 gal | 30 gal | 50 gal | 1 shower + 1 bath faucet | 4.0 GPM | 3.3 GPM / 137,000 BTU/h |
| 3 (2) | 50 gal | 30–40 gal | 40–50 gal | 50 gal | 2 showers + 1 bath faucet | 6.5 GPM | 5.3 GPM / 222,000 BTU/h |
| 4 (2) | 64 gal | 40 gal | 50–65 gal | 50–65 gal | 2 showers + 1 bath faucet | 6.5 GPM | 5.3 GPM / 222,000 BTU/h |
| 5 (2) | 78 gal | 50 gal | 65–80 gal | 65 gal | 2 showers + bath + kitchen faucets | 8.7 GPM | 7.1 GPM / 298,000 BTU/h |
| 6 (3) | 92 gal | 50–75 gal | 80 gal | 65–80 gal | 3 showers + bath + kitchen faucets | 11.2 GPM | 9.1 GPM / 383,000 BTU/h |
A range such as 50–65 gal means a high-FHR model of the smaller size can work, while the larger size is the safe pick. Anything above 199,000 BTU/h is beyond most single residential gas tankless units.
First-hour rating, explained
The FHR is how many gallons of hot water a tank heater can deliver in one hour, starting full and hot. It combines the stored water with how fast the burner or element reheats. That is why the U.S. Department of Energy and Natural Resources Canada both say to judge a tank by its FHR, not its size: a 40-gallon gas tank with a big burner can out-deliver a 50-gallon electric tank.
- Gas tanks typically deliver about 1.6 to 1.9 gallons of FHR per gallon of tank.
- Electric resistance tanks recover slowly, at about 1.15 to 1.3.
- Heat pump tanks vary widely by mode; in hybrid mode many land around 1.2 to 1.5.
These ratios are rules of thumb built from typical models. Always read the actual FHR on the EnergyGuide or EnerGuide label and make sure it meets your peak-hour number.
Tankless sizing: GPM and temperature rise
Tankless heaters are rated by flow at a given temperature rise, so you need two numbers. First, add up the GPM of everything that runs at once (showers are 2.5 GPM at the federal maximum, 2.0 GPM for WaterSense). Second, subtract your coldest inlet water temperature from the outlet temperature. Gas input is GPM × 500 × rise ÷ efficiency, and electric kW is GPM × rise ÷ 6.83. The colder your ground water, the less flow the same heater can deliver:
| Region (approx. winter inlet) | Inlet | Rise to 120°F | 199,000 BTU/h gas (95%) | 27 kW electric | 36 kW electric |
|---|---|---|---|---|---|
| US Gulf Coast & Florida | 65°F (18.3°C) | 55°F (30.6°C) | 6.9 GPM | 3.4 GPM | 4.5 GPM |
| US Southwest desert | 60°F (15.6°C) | 60°F (33.3°C) | 6.3 GPM | 3.1 GPM | 4.1 GPM |
| California coast, US Southeast | 55°F (12.8°C) | 65°F (36.1°C) | 5.8 GPM | 2.8 GPM | 3.8 GPM |
| US Mid-Atlantic | 50°F (10.0°C) | 70°F (38.9°C) | 5.4 GPM | 2.6 GPM | 3.5 GPM |
| Pacific Northwest, BC coast | 45°F (7.2°C) | 75°F (41.7°C) | 5.0 GPM | 2.5 GPM | 3.3 GPM |
| US Northeast & Midwest, Ontario, BC Interior, Atlantic Canada | 40°F (4.4°C) | 80°F (44.4°C) | 4.7 GPM | 2.3 GPM | 3.1 GPM |
| Alberta, Quebec | 37°F (2.8°C) | 83°F (46.1°C) | 4.6 GPM | 2.2 GPM | 3.0 GPM |
| US northern tier, Saskatchewan, Manitoba | 35°F (1.7°C) | 85°F (47.2°C) | 4.4 GPM | 2.2 GPM | 2.9 GPM |
| Yukon, NWT, Nunavut | 34°F (1.1°C) | 86°F (47.8°C) | 4.4 GPM | 2.1 GPM | 2.9 GPM |
Inlet temperatures are approximate coldest-month values. They vary with well depth, municipal supply and pipe routing, so measure yours at a cold tap in winter if you can.
Worked example: family of 4 in Ontario
- Tank: 4 showers × 10 + 1 shave × 2 + 4 sink uses × 4 + 1 dishwasher × 6 = 64 gallons FHR (about 242 L). Shop for a gas tank with an FHR of 64 or more (typically 40 gallons) or an electric tank of 50 to 65 gallons with the same FHR.
- Tankless flow: two 2.5 GPM showers plus a 1.5 GPM faucet = 6.5 GPM at the taps. With ~40°F water heated to 120°F and showers mixed to 105°F, each gallon at the tap needs (105 − 40) ÷ (120 − 40) = 81% hot water, so the heater carries 5.3 GPM at an 80°F (44.4°C) rise.
- Gas: 5.3 × 500 × 80 ÷ 0.95 ≈ 222,000 BTU/h, more than a 199,000 BTU/h unit. Switching to 2.0 GPM WaterSense showerheads drops it to about 188,000 BTU/h, which fits.
- Electric: 5.3 × 80 ÷ 6.83 ≈ 62 kW (about 258 A at 240 V), far beyond a typical 200 A home service. In a cold climate, a family of 4 is better served by gas tankless, a tank, or a heat pump tank.
Common sizing mistakes
- Buying by tank gallons. Two 50-gallon tanks can differ by 20 or more gallons of FHR.
- Using a summer inlet temperature. A tankless unit that is fine in August can run lukewarm in February.
- Ignoring gas and electrical supply. Large gas tankless units often need a 3/4 in. gas line and new venting; electric tankless can need 150 A or more of dedicated breakers.
- Oversizing a tank. Natural Resources Canada warns that a bigger tank than you need costs more to buy and loses more heat standing by.
- Forgetting appliances. An older top-load washer can use over 30 gallons of hot water per load, which can drain a tank on its own.
Get it confirmed
These are planning estimates. A licensed plumber or gas fitter should confirm the final size, venting, gas line, electrical capacity and code requirements (including anti-scald protection) before you buy. Tankless units also need regular descaling in hard-water areas.
Sources
- U.S. Department of Energy, Energy Saver: Sizing a New Water Heater (peak-hour demand and flow-rate method, gallons per use).
- Air-Conditioning, Heating and Refrigeration Institute (AHRI): Worksheet for Estimating Peak Hour Demand/First Hour Rating (2011, no-conservation values).
- Natural Resources Canada: Water Heater Guide (first-hour rating, tankless sizing, winter inlet water as cold as 5°C).
- U.S. EPA WaterSense: showerhead (2.5 GPM standard, 2.0 GPM WaterSense) and bathroom faucet (2.2 GPM standard, 1.5 GPM WaterSense) flow rates.
- U.S. DOE Building America: Strategy Guideline, Proper Water Heater Selection (average annual groundwater temperature map).
How to use the Water Heater Size Calculator (Tank & Tankless)
- 01Choose Tank (first-hour rating) or Tankless (GPM), and set °F or °C.
- 02Enter how many people live in the home and how many bathrooms have a shower or tub. This fills in a typical busiest hour and a typical set of fixtures running at once.
- 03Tank: adjust the showers, baths, sink uses, dishwasher and washer loads in your peak hour, choose modern or older fixtures, and pick the fuel. Tankless: set the fixtures running together, their GPM, your region or a custom inlet temperature, and the outlet temperature.
- 04Read the required first-hour rating and tank size, or the required GPM, temperature rise, gas BTU/h and electric kW. Shop by those numbers, then have a licensed plumber confirm.
Frequently asked questions
What size water heater do I need for a family of 5?
A typical busy hour for 5 people (five 10-gallon showers, a shave, five sink uses and a dishwasher load) needs a first-hour rating of about 78 gallons. That is usually a 50-gallon gas tank, a 65 to 80-gallon electric tank, or a 65-gallon heat pump model. Check that the FHR on the label is at least 78.
What size water heater do I need for a family of 2?
Two people usually need a first-hour rating of only about 30 gallons (two showers, a shave and two sink uses). Almost any 30 or 40-gallon tank covers that, and smaller heat pump models start at 50 gallons. For tankless, one shower plus a bathroom faucet is about 4 GPM of fixture flow.
What size tankless water heater do I need for 2 bathrooms?
Two 2.5 GPM showers plus a 1.5 GPM bathroom faucet add up to 6.5 GPM. With 40°F winter water heated to 120°F, and showers mixed to about 105°F, the heater must deliver about 5.3 GPM at an 80°F rise, which is roughly 222,000 BTU/h for a 95% gas unit. That is above the usual 199,000 BTU/h limit, so use 2.0 GPM WaterSense showerheads (about 188,000 BTU/h) or plan for two units.
What size tankless water heater do I need in Canada?
Size for winter. Natural Resources Canada notes inlet water can drop to about 5°C (41°F), and it is colder still on the Prairies. At a 4°C inlet and 49°C outlet (about a 44°C rise), a 199,000 BTU/h condensing gas unit delivers about 4.7 GPM (18 L/min), enough for about two showers. A 27 kW electric unit delivers only about 2.3 GPM at that rise.
Is a 40-gallon water heater enough for a family of 4?
Often yes if it is gas. A family of 4 typically needs a first-hour rating of about 64 gallons, and many 40-gallon gas tanks are rated 65 to 75 gallons. A 40-gallon electric tank is usually rated only about 45 to 55 gallons, so electric households generally need 50 gallons or more, ideally with an FHR of 64 or higher.
Where do I find the first-hour rating?
It is printed on the yellow EnergyGuide label (US) or the EnerGuide label and spec sheet (Canada), and in the manufacturer's product literature. It is listed in gallons, sometimes as first-hour delivery. Tankless units list GPM instead; multiply GPM by 60 to compare with an FHR.