Size an on-demand water heater the right way — by the peak flow you draw and how far the water has to be heated, not by gallons of storage. Tick the fixtures that run at the same time, set your incoming and target temperatures, and get the gas BTU/hr and electric kW the unit must deliver, with a cold-climate reality check. A planning estimate, not a substitute for a load assessment by a qualified installer.
Fixtures running at the same time
Set how many of each could be open during your busiest moment. Flow rates are typical published figures — edit any to match your own fixtures.
FixtureFlow eachHow many
Flow is shown in GPM. Switch units below to work in litres per minute.
GPM
The total flow of every hot-water fixture you expect to run at once, added together.
Units
Temperature rise
Incoming (cold) water
Cold ≈ 40°F, moderate ≈ 55°F, warm ≈ 70°F. Size for your coldest water, not the yearly average.
°F
°F
120°F (49°C) is a common safe delivery temperature.
Fuel & efficiency
Fuel type
Gas condensing units run roughly 0.90–0.98; non-condensing nearer 0.80. Electric resistance is about 0.98–1.0.
A planning estimate. Confirm the final unit, and especially the gas-line and electrical-service capacity, with a qualified installer and your local code before buying or fitting anything.
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Tank or tankless — and can the lines feed it?
Comparing against a storage tank instead? The water heater sizing calculator sizes a tank by first-hour rating and recovery rate. Before fitting a gas unit, check the supply pipe can carry the BTU/hr with the gas pipe sizing calculator, and weigh running costs by fuel with the heating fuel cost calculator.
How an on-demand water heater is sized
A tankless, or on-demand, water heater stores nothing. It heats water as it flows through, so its capacity is not a number of gallons but a flow rate it can raise to temperature. Two things set that: how much water you draw at once, measured in gallons (or litres) per minute, and how far that water has to be warmed — the temperature rise, or ΔT, between the cold water coming in and the hot water you want out. Get either wrong and the unit either keeps up or it doesn’t.
The energy needed follows directly from the water itself. Raising a gallon of water one degree Fahrenheit takes about 8.33 BTU, and a gallon-per-minute flow runs for 60 minutes in an hour, so the output a unit must deliver is:
The headline GPM printed on a unit is quoted at a small temperature rise, which is exactly why a heater that looks generous on the box can disappoint in practice. In a cold-water month the same unit has to do far more work per gallon, so it delivers fewer gallons per minute. Size to your coldest incoming water.
Add the fixtures that overlap, not one shower
The single most common sizing mistake is planning around one fixture — usually a shower — and forgetting that real mornings stack uses on top of each other. Someone showers while the dishwasher fills and the kitchen tap runs. Peak demand is the sum of every hot fixture that might be open at the same time, so the builder above lets you tick and quantify them. Typical published flow rates, all editable, are:
Fixture
Typical flow (GPM)
L/min
Shower
2.0
7.6
Bathroom faucet
1.0
3.8
Kitchen faucet
1.5
5.7
Tub fill
4.0
15.1
Dishwasher
1.5
5.7
Washing machine
1.7
6.4
Showers commonly fall in the 1.5–2.5 GPM range depending on the head; a low-flow head trims peak demand and shrinks the unit you need. The point of summing them is that a heater happy with one shower can run lukewarm the instant a second tap opens, the symptom owners notice first.
Temperature rise and your climate
Temperature rise is target minus inlet. With a default target of 120 °F, a cold northern inlet near 40 °F is an 80 °F rise; a moderate 55 °F inlet is 65 °F; a warm 70 °F inlet is only 50 °F. The presets above are generic cold, moderate and warm starting points — if you can measure your incoming water at a tap that has run a while, enter it directly. There is no built-in temperature map here; the inlet is whatever you tell it.
Worked example
Say two showers and a kitchen faucet might run together: 2.0 + 2.0 + 1.5 = 5.5 GPM. In a cold climate the inlet is 40 °F and the target 120 °F, an 80 °F rise.
That 64.5 kW figure is the tell-tale: at 240 volts it is over 250 amps of dedicated draw, well past any normal home service. In a cold climate that demand points firmly at gas, or at splitting the load across point-of-use units, rather than a single whole-home electric heater.
Gas line and electrical reality
A whole-home gas unit can call for 150,000–200,000 BTU/hr of input — far more than the pipe feeding an old tank was sized for, which is why an undersized gas line is a frequent and frustrating install problem. Electric whole-home units in a cold climate run into the amperage wall above. This calculator flags when the electric load crosses what a typical residential service can supply, but the supply pipe sizing, the breaker and the service capacity all need confirming against local code by a qualified installer — this matters more here than for a small tank because the loads are large.
Whole-home or point-of-use?
A single whole-home unit is simplest when the climate is mild and the peak demand is modest. As the rise climbs or fixtures multiply, point-of-use units — a small heater dedicated to a distant bathroom or the kitchen — cut the peak each unit has to meet, shorten the wait for hot water, and can sidestep the need for one enormous heater and the service upgrade it would demand. The builder lets you test a smaller simultaneous set to see how the required size falls.
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Frequently asked questions
What size tankless water heater do I need?
A tankless unit is sized by flow rate and temperature rise, not by stored gallons. Add up the flow of every fixture that might run at the same time — a shower is about 2 GPM, a kitchen faucet about 1.5 — to get peak demand, then find the temperature rise by subtracting your incoming water temperature from your target (usually 120 °F). Multiply: required output BTU/hr = 500 × GPM × temperature rise. A 4 GPM peak with an 80 °F rise needs about 160,000 BTU/hr, or roughly 47 kW of electric. Size to the busiest moment, not the daily total.
Why does my tankless heater go lukewarm when another tap opens?
That is the classic sign of an undersized unit, and it is the most common complaint with tankless heaters. The headline GPM on the box is quoted at a small temperature rise; the moment a second fixture opens, the unit has to heat more water per minute, and if the combined flow exceeds what it can raise to temperature it either reduces flow or lets the temperature sag. The fix is to size for simultaneous fixtures from the start — add the flows that overlap rather than assuming only one shower runs at a time.
Why does cold incoming water need a bigger unit?
An on-demand heater raises the water by a fixed temperature difference, so colder inlet water means a larger rise and more energy for the same flow. At a 40 °F winter inlet, reaching 120 °F is an 80 °F rise; at a 70 °F summer inlet it is only 50 °F. The same unit therefore delivers far fewer gallons per minute in winter than the warm-climate flow rate printed on its spec sheet. Always size for your coldest incoming water, not the annual average.
Can a whole-home electric tankless heater work in a cold climate?
Often it cannot, practically speaking. Heating several gallons a minute through a large winter temperature rise can demand 27 kW or more, which at 240 volts is over 100 amps of dedicated draw — frequently beyond what a typical residential electrical service can spare without an upgrade. In cold regions a gas unit or a couple of point-of-use electric units near the fixtures is usually more realistic than one whole-home electric heater. This tool flags when the electric load crosses into that territory.
What incoming water temperature should I use?
Use the coldest temperature your water reaches, because that is when the unit struggles. As a rough guide, a cold northern climate runs near 40 °F in winter, a moderate climate around 55 °F, and a warm southern climate near 70 °F. If you know your actual inlet temperature — measure it at a tap that has run a while — enter it directly for a more accurate rise. The default target of 120 °F is a common safe delivery temperature; some homes run a little lower.
Does a gas tankless heater need a bigger gas line?
Frequently, yes. A whole-home gas tankless unit can call for 150,000 to 200,000 BTU/hr, far more than the line feeding an old tank heater was sized for, so an undersized gas line is a common installation problem that leaves the unit unable to reach full output. Check the line capacity against the unit’s input rating before buying. A gas line sizing calculator can tell you whether your existing pipe diameter and length can carry the required BTU/hr, and gas and electrical work should be confirmed against local code by a qualified installer.