Size a tank by your busiest hour of hot water use, matched to its first-hour rating (FHR), not its gallon label. Add about 38 L per shower, 75 L per bath, 23 L per dishwasher run, 27 L per hot or warm wash and 15 L for sinks. Most households of three or four need a 40–50 gallon gas or 50–65 gallon electric tank.
- A tank's real capacity is its first-hour rating: stored hot water plus what it can reheat in that hour.
- A gas tank with a bigger burner can out-deliver a larger electric tank, which is why gas and electric size charts differ.
- Oversizing costs more to buy and loses more standby heat, so size for the realistic busiest hour, not a rare event.
- Storing at 60 °C and mixing down to 49 °C makes each stored litre stretch about 1.26 times further with 7 °C inlet water.
- Tankless sizing is a different question: litres per minute of fixtures running together, at your winter temperature rise.
01 /Why is gallon size the wrong way to size a water heater?
Gallon size is the wrong starting point because the tank's label volume tells you only what it stores, not what it can deliver when you need it. Natural Resources Canada's Water Heater Guide says the capacity of a gas-fired water heater should be judged by its first-hour rating, not its tank size, and notes that some gas water heaters with smaller tanks actually have a higher FHR than models with larger tanks because of their larger burners.
The first-hour rating is the amount of hot water a heater can supply in one hour starting from a full, hot tank. It combines two things: the hot water already stored, and the recovery, meaning how much new water the burner or elements can heat during that hour. A gas burner of about 40,000 BTU/h at typical efficiency can reheat roughly 170 litres an hour through a 48 °C rise; a single 4,500-watt electric element manages about 80. That is why a 40-gallon gas tank and a 40-gallon electric tank do not do the same job.
Stratification is also why you run out suddenly rather than gradually. Hot water sits in a layer on top of the cold. As you draw water from the top, the cold layer rises. The water at the outlet stays hot until the boundary reaches it, then the temperature drops quickly. Press the 'Heat it' button in the Tank Explorer to watch the layers form.
02 /Step 1: How do you estimate your peak-hour demand?
Estimate peak-hour demand by counting what really happens in your busiest hour, usually a weekday morning, and adding up the hot water each use takes. The Water Heater Size Calculator uses these editorial planning assumptions:
| Use in the peak hour | Litres of hot water each | Notes |
|---|---|---|
| Shower | 38 | About 8 minutes; low-flow heads use less, rain heads more |
| Bath | 75 | Standard tub; large soaker tubs can take far more |
| Dishwasher run | 23 | Modern dishwashers heat some water internally |
| Clothes washer on hot or warm | 27 | Cold washes use none |
| Sinks and handwashing | 15 (flat) | Added once per peak hour |
For comparison, Natural Resources Canada's Water Heater Guide estimates about 30 litres of hot water for an 8-minute shower with an energy-saving showerhead, and suggests a tank should cover at least three showers plus handwashing with hot water to spare. The calculator's 38 litres builds in some margin for ordinary showerheads.
| Household morning | Calculation | Peak-hour demand |
|---|---|---|
| Couple: two showers | 2 × 38 + 15 | 91 L |
| Family of four: three showers and the dishwasher | 3 × 38 + 23 + 15 | 152 L |
| Family of four: four showers, dishwasher and a warm wash | 4 × 38 + 23 + 27 + 15 | 217 L |
| Busy household: five showers, a bath, dishwasher and warm wash | 5 × 38 + 75 + 23 + 27 + 15 | 330 L |
03 /Step 2: How do you match demand to a tank's first-hour rating?
Pick the smallest tank whose first-hour rating meets or beats your peak-hour demand. The calculator uses these typical FHR values; the actual figure for any model is printed on its label or spec sheet, so check it before you buy.
| Fuel | Size (US gal / litres) | Typical FHR |
|---|---|---|
| Gas | 40 / 151 L | about 260 L |
| Gas | 50 / 189 L | about 310 L |
| Gas | 60 / 227 L | about 360 L |
| Gas | 75 / 284 L | about 450 L |
| Electric or heat pump | 40 | about 200 L |
| Electric or heat pump | 50 | about 235 L |
| Electric or heat pump | 65 | about 270 L |
| Electric or heat pump | 80 | about 330 L |
Run the worked examples through that table. The 152 L family morning is covered by a 40-gallon gas tank (about 260 L) or a 40-gallon electric tank (about 200 L). The 217 L morning still fits a 40-gallon gas tank, but on electric it needs the 50-gallon size. The 330 L household needs a 60-gallon gas tank, or an 80-gallon electric or heat pump tank, which meets it exactly with no margin. To see how a shortlisted tank copes with the timing of your morning rather than its total, try the Hot Water Morning Simulator.
Federal efficiency rules group tanks the same way. Natural Resources Canada's standard for household gas storage water heaters sorts them into draw patterns by first-hour rating: very small below 68 L, low from 68 to 193 L, medium from 193 to 284 L and high above 284 L. Compare efficiency only between tanks in the same group.
04 /Why not just buy the biggest tank?
Oversizing costs you three times: a higher purchase price, a heavier and harder install, and more standby loss every day for the life of the tank. Natural Resources Canada advises against oversizing a tank in anticipation of an infrequent high hot water need, noting that larger tanks mean higher standby losses and increased energy costs. It also observes that most households use much less hot water per day than their heater can produce.
Before sizing up, try the cheaper levers that effectively add capacity:
- A mixing valve. Storing at 60 °C and blending down to 49 °C at the taps means each litre of stored water becomes about 1.26 litres of 49 °C water when the inlet is 7 °C. The temperature setting guide explains the safety side.
- Low-flow showerheads. Cutting shower flow cuts hot water per shower in the same proportion.
- Staggering loads. Run the dishwasher and laundry outside the shower hour.
- Fixing the tank you have. A tank that used to cope and now runs out early may have a broken dip tube, a failed lower element or heavy sediment. Check the not enough hot water guide before replacing on size alone.
05 /How do you size a heat pump water heater?
Size a heat pump water heater by first-hour rating as above, then lean towards the next size up. In heat pump mode it reheats slowly, and the bigger store lets the compressor carry more of the load without switching on the backup elements. The heat pump water heaters guide covers the operating modes that decide how it behaves at peak times.
06 /How do you size a tankless water heater?
Size a tankless unit by adding up the flow of every fixture you run at the same time, then checking the unit can heat that flow through your winter temperature rise. The calculator uses 7.6 L/min for a shower or clothes washer and 5.7 L/min for a sink or dishwasher, and a target of 49 °C at the tap.
- Add the simultaneous flow
Two showers plus a kitchen sink: 7.6 + 7.6 + 5.7 = 20.9 L/min.
- Find the temperature rise
49 °C minus your coldest inlet temperature. With the default 7 °C inlet, the rise is 42 °C.
- Convert to heat
kW = L/min × rise × 4.186 ÷ 60. Here: 20.9 × 42 × 4.186 ÷ 60 = about 61 kW.
- Convert to the unit's rating
Gas input in BTU/h = kW × 3,412 ÷ 0.95, about 220,000 BTU/h here, beyond most single residential units, so you would size for two showers (about 160,000 BTU/h) and accept a lower flow at the sink, or use two units. Electric amps at 240 V = kW × 1,000 ÷ 240.
Natural Resources Canada suggests multiplying a tankless unit's flow in gallons per minute by 60 to compare it with a tank's first-hour rating. By that measure tankless units deliver far more per hour than tanks, but the fair comparison is simultaneous use in February. The tankless vs tank guide explains why winter inlet temperature dominates, and tankless water heater sizing adds the gas line, venting and electrical limits.
07 /What size water heater suits my household? A quick guide
Use the table as a starting point, then confirm with your own peak hour in the calculator. It assumes ordinary showerheads, no large soaker tub and one peak hour per day.
| Household | Typical peak hour | Gas tank | Electric or heat pump tank | Gas tankless (winter) |
|---|---|---|---|---|
| 1–2 people | about 90 L | 40 gal | 40 gal | One shower plus a sink, about 140,000 BTU/h |
| 3–4 people | 150–220 L | 40–50 gal | 50–65 gal | Two showers, about 160,000 BTU/h |
| 5+ people or soaker tub | 300 L or more | 60–75 gal | 80 gal | Two showers plus more, or two units |
When your size is settled, the choosing a water heater checklist covers the remaining decisions, and a quote request that states your target first-hour rating keeps installers quoting like for like.
FAQQuestions people ask
What size water heater do I need for a family of four?
Most families of four have a peak hour of about 150 to 220 litres. That is usually covered by a 40- or 50-gallon gas tank, or a 50- or 65-gallon electric or heat pump tank. Check your own morning in the Water Heater Size Calculator.
Is a 40-gallon water heater enough?
A 40-gallon gas tank with a first-hour rating near 260 litres suits many households of up to four people. A 40-gallon electric tank delivers less, about 200 litres in the first hour, because its elements recover more slowly.
Does a bigger water heater cost more to run?
Yes, somewhat. A larger tank has more surface area and more stored water losing heat all day, so its standby losses are higher. Natural Resources Canada advises against oversizing for rare peaks.
What tankless size equals a 50-gallon tank?
Compare flow, not volume. A 50-gallon gas tank delivers about 310 litres in its first hour, while a whole-house gas tankless unit sized for two winter showers delivers about 15 L/min continuously. The tankless unit wins on duration; the tank wins on brief high simultaneous flow.
How much hot water does a person use per day?
Natural Resources Canada's Water Heater Guide estimated about 75 litres per person per day and 225 litres for an average household. The Energy Cost Comparer defaults to 60 litres per person, which you can edit.
Where do I find the first-hour rating?
It is printed on the rating label or the manufacturer's spec sheet for gas and electric tanks, and the EnerGuide rating groups models by capacity based on it.