How to Size a Tankless Water Heater
A tankless heater is not sized in gallons — it has no tank. It is sized by two things happening at the same instant: how many gallons per minute you draw at once, and how far it has to raise that water’s temperature. Get either wrong and the shower turns cold in January.
The two numbers that size a tankless unit
Because a tankless heater makes hot water on demand, its job is defined by a single worst-case moment. You need the required flow rate — the total GPM of every fixture that might run together — and the temperature rise, the gap between your incoming cold water and your target hot temperature. A unit is big enough only if it can deliver your flow at your rise; a “7 GPM” heater is not 7 GPM at every temperature, it is 7 GPM at some easy rise and much less on a cold day.
Step 1: add up simultaneous flow
List the fixtures that realistically run at the same time and add their flow rates. A modern shower is about 2.0 GPM, a bathroom faucet 1.5 GPM, a kitchen faucet 1.5 GPM. Two showers plus a faucet is roughly 4 to 5 GPM. Do not add up every fixture in the house — size for plausible overlap, not for everything open at once. If you are unsure of a fixture’s real flow, measure it with the bucket-test flow calculator.
Step 2: find your temperature rise
Temperature rise is target minus incoming. Most people want hot water delivered near 105–120°F. Incoming (ground) water temperature is the variable that bites: it might be 70°F in summer in the south and 40°F in winter in the north. Always size for your coldest incoming water, because that is when the rise — and the load — is largest. Sizing for a mild summer rise is the classic mistake that leaves a unit gasping in December.
Step 3: convert flow and rise to a load
The tankless sizing calculator turns those two numbers into the capacity the unit must deliver:
gas BTU/hr = GPM × 500 × temperature rise
electric kW = GPM × temperature rise × 0.1465
Worked example. Say you need 4 GPM at a 45°F rise (100°F target from 55°F incoming). Gas load is 4 × 500 × 45 = 90,000 BTU/hr. As electric, it is 4 × 45 × 0.1465 ≈ 26.4 kW. That electric figure is why whole-house electric tankless units are demanding: 26 kW needs a heavy electrical service, whereas a 90,000 BTU/hr gas unit is unremarkable on the gas side. The 500 factor is not magic — it comes from water weighing 8.34 lb/gallon times 60 minutes, so a gallon-per-minute raised one degree takes about 500 BTU/hr.
Why cold climates change everything
A unit rated “up to 5 GPM” is quoting an easy rise. Double the rise and you roughly halve the deliverable flow, because the load is fixed by the burner. In a cold-inlet region, a single tankless unit may comfortably run one shower but stumble on two — which is why some homes install two units or a larger one. If your winter rise is 70°F rather than 45°F, redo the math with that number before you buy.
Gas line and electrical service
A 90,000–199,000 BTU/hr gas tankless draws far more gas than a storage tank, so a retrofit frequently needs a larger gas line. Before assuming a straight swap, total your appliance load and size the pipe with the appliance load and gas pipe size tools. For electric units, confirm the panel and circuit can carry the kW — 26 kW is well over 100 amps at 240 V.
One unit or two?
When the calculated load is more than a single residential unit can deliver at your winter rise, you have two paths: a larger single unit, or two units in parallel. Two smaller units let you serve simultaneous demand and give redundancy — if one fails, you still have some hot water — at the cost of more gas piping, more venting and a higher install. A common pattern is one unit dedicated to the primary bathrooms and another to a distant kitchen or master suite, which also shortens the run and cuts the wait for hot water. Before assuming a single big unit is simplest, redo the flow total for your true simultaneous peak: if it is driven by two showers plus a tub-fill that rarely coincide, sizing for the honest overlap may keep you on one unit; if the household genuinely runs three fixtures at once in winter, plan for the capacity that requires.
Minimum flow and the cold-water sandwich
Tankless units have a quirk worth knowing before you buy: a minimum activation flow. Below a threshold — often around 0.5 GPM — the unit will not fire, so a barely-cracked faucet runs cold. Very low-flow fixtures can occasionally sit near that threshold. Related is the “cold-water sandwich”: a brief slug of cool water between draws, when the unit cycles off and back on, that some users notice when they turn a shower on and off quickly. Neither is a sizing error — they are characteristics of on-demand heating — but they shape expectations, and a recirculation option or a small buffer tank can soften them. Weigh these against the endless-hot-water upside when you decide whether tankless fits how your household actually uses hot water.
Put it together
Size the flow for realistic overlap, size the rise for your coldest month, and confirm the unit’s rated capacity meets the resulting BTU/hr or kW — then check that your gas line or electrical service can feed it. These are planning figures; a tankless install is licensed, permitted work, so confirm the final sizing and the utilities with a professional.