Tankless water heater sizing calculator
Every tankless heater is sold on a flow rate measured at a small temperature rise. This works it out at yours.
Tankless water heater sizing calculator
Pick the fixtures you need running at once and your state. The result updates as you change it, and the numbers below are already worked out if you have JavaScript turned off.
What you need
In Ohio, winter groundwater arrives at about 48°F. Reaching 120°F means a 72°F rise at 6.5 GPM.
| Fuel | Size needed | Unit class | What that means |
|---|---|---|---|
| Gas | 286,000 BTU | Two units in parallel | Beyond a single residential heater |
| Electric | 68.5 kW | Maximum residential electric | 286 A across 8 × 40 A breakers |
The electric figure is the one worth staring at. 286 amps is 143% of a 200 A residential service before you have turned on anything else — which is why most whole-home tankless installs end up on gas.
On this page
How this tankless water heater size calculator works
Three inputs, one equation.
The equation is the same one every plumbing engineer uses:
BTU required = GPM × temperature rise × 500
kW required = (GPM × temperature rise) ÷ 6.83
The 500 comes from physics rather than convention: one BTU raises one pound of water by one degree Fahrenheit, a gallon of water weighs 8.34 pounds, and there are 60 minutes in an hour. Multiply those and you get 500.4, and the figure already carries a working allowance for a non-condensing gas unit's thermal efficiency.
The electric equation has no efficiency term because electric resistance heating is effectively 100% efficient at the appliance. Every watt becomes heat in the water.
Why the state selector exists
This is the input that makes the calculator worth using, and it is the one every fixture-count chart leaves out.
Temperature rise is your setpoint minus the temperature of the water arriving at your house. Setpoint is a choice — almost always 120 °F. Incoming temperature is geography.
Groundwater temperature tracks mean annual air temperature closely, because the earth acts as a thermal flywheel. Across the continental United States that means winter mains temperatures from about 70 °F in south Florida to 40 °F across the northern tier. The dataset behind the selector is built on that relationship, using cold-month figures rather than annual averages.
The consequence is large. Two showers at 5 GPM:
- Florida, 70 °F incoming, 50 °F rise: 125,000 BTU
- Ohio, 48 °F incoming, 72 °F rise: 180,000 BTU
- Minnesota, 40 °F incoming, 80 °F rise: 200,000 BTU
Same household, same fixtures, and the Minnesota requirement is at the ceiling of what a single residential gas unit can deliver. This is why "what size tankless water heater for a family of four" has no answer without a location attached.
Why we ask for fixtures rather than bathrooms
Bathroom counts are a proxy for simultaneous demand, and a poor one. A four-bathroom house where two people live has the same peak demand as a one-bathroom flat, because the peak is set by what runs at once rather than what exists.
Be honest and specific about the worst realistic morning. Most households peak at two showers plus a kitchen tap. Households with a bath filler running while someone showers peak considerably higher, because a tub filler wide open is 4 GPM on its own.
What you should not do is add every fixture in the house. That is how people specify a 36 kW electric unit, discover it needs 150 amps, and end up quoting a service upgrade for a scenario that has never occurred.
Reading the electric row
The electric result is usually the surprise, and it is the most useful thing this calculator tells you.
A 27 kW unit draws 112.5 amps at 240 V. On a 200 A service that is over half your total capacity dedicated to one appliance, before the range, the dryer and the air conditioning. Whether it fits is decided by a load calculation under NEC Article 220, and in a typical all-electric house it frequently does not.
That is why whole-home tankless is overwhelmingly a gas product in the United States, and why the electric option is genuinely excellent at point of use — a 6 to 11 kW unit serving one remote bathroom is one breaker, no venting, and no long pipe wait.
The full breakdown of breakers, conductors and panel spaces is in the electrical requirements for electric tankless units.
What the calculator does not account for
Three things worth knowing.
Minimum activation flow. Tankless heaters need a minimum flow before the burner or elements will fire, typically 0.4 to 0.8 GPM. Below that you get cold water regardless of how the unit is sized. This is the source of most "cold water sandwich" complaints, and a unit with a 0.5 GPM threshold behaves considerably better on older, low-pressure plumbing than one at 0.8.
Altitude. Gas appliances derate above about 2,000 feet — commonly 4% per 1,000 feet — because there is less oxygen for combustion. If you are in Denver or higher, size up accordingly.
Recirculation. A recirculation loop keeps hot water near the fixtures and eliminates the wait, and it also adds continuous demand that the sizing above does not include.
For the method behind all of this, how to size a tankless water heater works through it step by step, and the ten-year cost comparison against a tank covers whether tankless is the right decision before you size anything.
Questions people actually ask
Why does my state matter for water heater sizing?
Because incoming water temperature sets your temperature rise, and temperature rise determines how much of a heater's rated capacity you actually get. Winter groundwater runs about 70 °F in Florida and 40 °F in Minnesota. Reaching a 120 °F setpoint therefore takes a 50 °F rise in one and an 80 °F rise in the other, and the same heater delivers nearly twice the flow in the first case.
Should I size for winter or the annual average?
Winter, always. Groundwater bottoms out in late February or early March, two months after most people install a heater and decide it is fine. A heater sized against a June measurement will disappoint in the coldest week of the year, which is exactly when you least want it to.
Why is the electric number so much higher than the gas one?
They are measuring the same energy in different units, and electric resistance heating needs a great deal of electrical current to deliver it. 27 kW is about 92,000 BTU per hour, and it draws 112.5 amps at 240 V — more than half a typical 200 A residential service. Gas delivers the same energy through a pipe rather than a conductor, which is why whole-home tankless is overwhelmingly a gas product.
Should I add every fixture in the house?
No. Size for the worst realistic simultaneous demand, not the theoretical maximum. Most households peak at two showers plus a tap. Sizing for every fixture at once is how people end up specifying a 36 kW electric unit and then a service upgrade to feed it.
Does the calculator account for condensing units?
The gas figure assumes a non-condensing unit at about 82% thermal efficiency, which is the conservative case. A condensing unit at 94% needs roughly 13% less input for the same output, so if you are specifying condensing you can size down slightly from the number shown.
Can I share my result?
Yes — the URL updates as you change the inputs, so copying the address bar shares your exact configuration. The canonical URL for search engines stays the clean one without parameters, so sharing a result does not create duplicate pages.