Tankless Water Heater Size Calculator

GPM from your fixtures, temperature rise from your groundwater, then gas BTU or electric kW, amps and breakers.

Project Details
Enter your measurements and requirements
Results
Calculated estimates for your project
Peak hot water demand6.5 GPM
Temperature rise70F (50F in, 120F out)
Gas: output needed227,500 BTU/hr
Gas: input rating239,474 BTU/hr
Gas unit2 x 199k BTU, cascaded
Electric: power66.6 kW, 278 A at 240 V
Electric unitOver 36 kW: one whole-house electric unit cannot keep up; use gas or two point-of-use units
Panel checkOver 150 A: more than most 200 A services can spare; plan on gas or a service upgrade
Installed cost$5,000 to $9,000 with venting and gas line
Yearly running cost (gas)$220 vs $341 for a 50 gal tank
Yearly running cost (electric)$693 vs $738 for a 50 gal tank

Turn these numbers into a build plan.

  • 6.5 GPM peak demand
  • 70F temp rise
  • 239,474 BTU/hr gas input

Your home needs about 6.5 GPM at a 70F rise: a pair of 199k BTU gas units.

Demand 6.5 GPM2 x 199k BTU: 10.8 GPM at 70F rise

How we got this: output BTU = GPM x rise x 500, input = output / 0.95efficiency; electric kW = GPM x rise / 6.83. Every unit is rated at a specific rise, so check the maker's flow chart at 70F, not the headline GPM.

Remind me to descale it every year

Scale is what kills tankless heaters early. A yearly flush through the isolation valves keeps the warranty and the flow rate.

The calendar file repeats every year. In Blueprint the descale becomes a recurring maintenance task next to the heater's model and install date.

Save this as a Water Heater Replacement project in Blueprint for a plan with permit and venting checks and a parts list sized to this result. Going electric? Check the panel with the electrical load calculator first.

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Project plan · built from your numbers

Tankless water heater swap, 6.5 GPM

Your fixtures can pull 6.5 GPM at once, so plan for a 2 x 199k BTU gas (cascaded) unit that delivers 6.5 GPM at a 70F rise (239,474 BTU/hr input). You size, prep and coordinate; a licensed plumber handles gas, venting and electrical under a permit.

Materials
$3,125
Time
2 weekends
Level
Easy
8 steps 7 tools 5 materials
  1. 1

    Confirm your sizing

    Measure cold-water temperature at a tap in winter; your plan assumes 50F in and 120F out. Count what really runs at once: 2 standard showers, 1 kitchen sink, for 6.5 GPM peak.

    • Winter inlet temp measured
    • Simultaneous fixtures counted
    • Target 6.5 GPM at 70F rise
  2. 2

    Check the gas, power and vent path

    A 239,474 BTU/hr unit often needs a bigger gas line and meter than a tank heater. Photograph the meter, the gas line size at the old heater, the electrical panel and the wall where a vent could exit. A licensed plumber or gas fitter must assess gas sizing; do not alter gas lines yourself.

    • Meter and gas line photographed
    • Panel space photographed
    • Possible vent route measured
  3. 3

    Pick the unit

    Shortlist condensing models that deliver 6.5 GPM at a 70F rise on the maker's flow chart. Your load is above what one residential unit delivers, so ask about two cascaded units. Note the vent type (PVC for condensing) and whether it needs a 120V outlet.

    • Flow at your rise confirmed on the chart
    • Vent type noted
    • Electrical requirement noted
01 · Your numbers
Peak demand
6.5 GPM
Temp rise
70F
Gas input
239,474 BTU/hr
Est. cost
$3,125
02 · The plan

8 steps · 7 tools · 5 materials

5 more steps, the full tool list and a shopping list you can check off as you go.

03 · Finished

Your plan is 60% built. Save it.

3/5

Tankless water heater swap, 6.5 GPM

Finished in 2 weekends · $3,125 DIY

Built for your peak demand of 6.5 GPM. Your first plan saves free. No card needed.

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How to size a tankless water heater in 3 steps

  1. Add up the peak flow. Add the GPM of every fixture that could run at the same time: two showers and the kitchen sink is 6.5 GPM.
  2. Find the temperature rise. Subtract the coldest incoming water temperature (from 35F in the north to 70F in the south) from the output temperature, usually 120F.
  3. Match the rating at that rise. Pick a unit whose flow chart shows your GPM at your rise. Gas: input BTU = GPM x rise x 500 / efficiency. Electric: kW = GPM x rise / 6.83.

Worked example, a family of 4 running two showers and the kitchen sink (6.5 GPM): in Minneapolis winter water is about 37F, a 83F rise, so the unit needs 269,750 BTU of output and 283,947 BTU of input at 95% efficiency. In Miami the water is about 66F, a 54F rise, and 184,737 BTU of input is enough.

Tankless flow rate by temperature rise

Approximate GPM delivered; gas at 95% efficiency
Unit35F rise45F rise55F rise65F rise75F rise
120k BTU gas6.55.14.13.53
160k BTU gas8.76.85.54.74.1
180k BTU gas9.87.66.25.34.6
199k BTU gas10.88.46.95.85
18 kW electric3.52.72.21.91.6
27 kW electric5.34.13.42.82.5
36 kW electric75.54.53.83.3

Hot water flow by fixture

FixtureGPM
Low-flow shower1.8
Standard shower2.5
Bathtub4
Kitchen sink1.5
Bathroom sink0.5
Dishwasher1.2
Clothes washer1.8

Incoming water temperature by state

Typical yearly average groundwater; subtract about 8F for winter
StateGroundwater
Minnesota45F
North Dakota43F
Maine45F
Michigan47F
New York48F
Massachusetts49F
Washington50F
Colorado50F
Ohio52F
Pennsylvania52F
Illinois54F
Missouri57F
Virginia57F
North Carolina60F
California60F
Georgia64F
Arizona68F
Texas70F
Louisiana70F
Florida74F

Gas vs electric tankless

Gas units reach 199,000 BTU, enough for two or three fixtures in a cold climate, but need a gas line sized for that load and a PVC vent (condensing) or stainless vent (non-condensing). Electric units need no vent but draw 75 to 150 A, so they suit warm climates, small homes or point-of-use. Check the panel with the electrical load calculator, and follow the water heater installation cheat sheet and the permit and inspection guide.

Signs a tankless heater is undersized

Water goes lukewarm when a second shower or the dishwasher starts, temperature swings in winter but not summer, or the unit cycles off at low flow. The fix is a bigger unit, a second unit in cascade, or low-flow (1.8 GPM) shower heads that cut demand by a quarter.

Tankless water heater sizing FAQ

What size tankless water heater do I need for a family of 4?

Two showers and a sink at once is about 6.5 GPM. In Minnesota (83F rise in winter) that needs 283,947 BTU of gas input, two cascaded units; in Florida (54F rise) a 199k BTU unit is enough.

Is 199,000 BTU enough?

A 199k BTU condensing unit delivers about 5.4 GPM at a 70F rise and 8.4 GPM at 45F, enough for two showers and a sink in most of the country. Colder water or a soaking tub may need two units.

Can one tankless unit serve 3 bathrooms?

Only if they are not all in use at once, or the incoming water is warm. Three showers at 2.5 GPM plus a sink is 9 GPM; that is two cascaded gas units in the north.

What size breaker does an electric tankless need?

Whole-house units run on several 40 A double-pole breakers: 2 for 18 kW (75 A), 3 for 27 kW (113 A) and 4 for 36 kW (150 A), each on 8 AWG copper. That is often more than a 200 A panel can spare.

How do I find my incoming water temperature?

Run the cold tap for a minute in winter and read it with a kitchen thermometer, or enter your ZIP and the calculator uses typical groundwater for your state, minus 8F for winter.

Gas vs electric tankless: which is better?

Gas handles whole-house flow in cold climates and runs cheaper where gas is cheap. Electric is simpler to vent and cheaper to buy, but at a 70F rise even 36 kW only delivers about 3.5 GPM, so it suits warm climates or point-of-use.

Why does my tankless water heater run out of hot water?

It is undersized for the rise: too many fixtures at once, colder winter water, or scale on the heat exchanger. Signs are water that turns lukewarm when a second shower starts, or a unit that shuts off at low flow.

Tankless vs tank cost?

A gas tankless unit costs $2,500 to $4,500 installed versus about $1,500 for a 50 gal tank, and saves roughly $143 a year on gas for a family of 4 in the north, so payback is long. The case is endless hot water and a 20 year life.

Does a recirculation pump change sizing?

No, size for the same peak flow. A recirculation loop adds a little standby loss and may need a unit with a built-in pump or a small buffer tank.

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