A tankless water heater is sized by two numbers: how many gallons per minute (GPM) you need at the same moment, and how many degrees the water must be heated. Add up the fixtures that could run together, find your cold water temperature, and pick a unit whose rated flow covers that load at that temperature rise.
Most households land between a unit that serves one bathroom plus a kitchen tap and one that serves three or four fixtures at once. The steps below show how to find your own number, and where gas line capacity can quietly cap an otherwise correct choice.
Find your peak demand in four moves
Sizing starts with a worst-case morning, not an average day. You are looking for the most hot water fixtures that could plausibly run at the same time.
- 1
List the fixtures that overlap
Think of a shower, a dishwasher and a kitchen tap running together. A second shower while the washing machine fills is the next level up.
- 2
Assign a flow rate to each
Use the label on the fixture or the typical values in the table below. Low-flow heads draw less than older ones.
- 3
Add them up
The total is your peak GPM. Be honest about the busiest hour, because the heater has to meet that moment.
- 4
Check it against the unit's chart
Every tankless model lists the GPM it can deliver at several temperature rises. Match your peak to the right column.
Typical hot water draw by fixture
These are common planning values, and the label on your own fixture always wins. Treat the table as a starting point for the addition.
| Fixture | Typical draw (GPM) | Hot share of the flow |
|---|---|---|
| Shower | About 2 | Most of it |
| Bathroom sink | About 1 | About half |
| Kitchen sink | About 1.5 | About half |
| Dishwasher | About 1.5 | All of it while filling |
| Clothes washer | About 2 | Depends on cycle |
| Tub filler | About 4 | Most of it |
A tub with a high-flow filler can be the one fixture that rules out a small unit, even if the household rarely runs two showers at once.
Why temperature rise changes the answer
Temperature rise is the gap between the cold water entering the heater and the hot water you want at the tap. A tankless unit has a fixed amount of heating power, so a bigger rise means a lower flow it can deliver. The same machine that supplies a generous flow on a mild day may fall short when the incoming water is colder.
The U.S. Department of Energy suggests a 120 degree Fahrenheit thermostat setting for household hot water. Subtract your incoming water temperature from that target and you have the rise to look up in the manufacturer's chart.
Do not guess the incoming temperature. Run a cold tap for a minute, catch the water in a cup, and read it with a kitchen thermometer in more than one season. A plumber can also use a probe at the heater inlet during a sizing visit.
Housing eras, permits and gas lines in Erlanger
Erlanger grew fastest in the 1950s, 1960s and 1970s, and housing units rose from 6,865 in 2000 to 8,249 in 2020. Many houses from the boom years were built around a tank heater and a gas line sized for it. If you own one, ask for the line to be checked against any tankless model before you commit to a unit.
Under Kentucky's 815 KAR 20:050, installing or replacing a water heater needs a plumbing permit, and a licensed master plumber can pull it. Ask the plumber who sizes your unit to explain who is handling the permit, so the paperwork is not left to chance.
Neighbors in Florence and Fort Mitchell face the same sizing math, so a unit that worked well down the road is only a clue. Your fixtures, your cold water temperature and your gas line are what count.
Source type: City of Erlanger history page and Census housing data, Kentucky 815 KAR 20:050
Sizing mistakes that cause cold-water complaints
Most disappointment with tankless heaters traces to a short list of avoidable errors made before the unit was ever bought.
Sizing for the average day
A unit chosen for typical use runs short during the busiest hour, when everyone is awake and washing.
Ignoring the coldest water
Using a warm-season temperature rise in the math leaves you with a unit that struggles when incoming water is coldest.
Counting the low-flow rating only
Aerators clog and old fixtures draw more than their label. Measure a few with a bucket and a timer.
Forgetting a future bathroom
If a remodel is planned, size for the finished house, not the current one.
Skipping the gas line check
A burner needs enough fuel at full fire. A line sized for a tank heater can starve a larger tankless unit.
One large unit or two smaller ones
When the peak load is high, the choice often becomes one big heater or a pair working together. Each route has honest trade-offs.
One large unit
- Simpler venting and a single gas connection
- One unit to flush and service
- Needs a bigger gas supply line
- A failure leaves the house with no hot water
Two smaller units
- Load is shared, so each runs less hard
- One can keep working if the other is down
- Two vent runs and more wall space
- Two units to flush and maintain
Neither is automatically better. A plumber can compare both against your layout, gas supply and a standard tank alternative if a tankless setup turns out to be a poor fit.
The gas supply that sets the real ceiling
A gas tankless heater is rated in BTUs per hour, and a large model can draw several times what a tank heater does. The gas meter, the pipe size and the pipe length all have to deliver that fuel at the moment the burner runs flat out. If any of them fall short, the unit throttles back and your hot water suffers.
That is why a sizing quote that never mentions the gas line is incomplete. Upgrading the pipe is sometimes the largest cost factor in the job, along with the venting material and run. Electric tankless models skip the gas question but draw heavy electrical current, so panel capacity becomes the check instead.
If you ever smell gas near a heater, leave the building and call the gas utility or 911 from outside before calling a plumber. Do not flip switches or relight anything.
Quick Answers on What size tankless water heater do i need
It depends on how many fixtures run together, not on head count alone. Add the flow of the busiest overlap, such as two showers and a dishwasher, then match that total to a unit at your temperature rise.
No. An oversized unit costs more to buy and may need a bigger gas line and vent for no benefit. The goal is enough capacity for your peak demand with a small margin.
Often yes, if the unit's rated flow at your temperature rise covers both showers plus anything else running. Low-flow shower heads help a lot. A plumber can confirm using the model's chart.
Sometimes. A gas tankless heater can demand much more fuel than a tank heater, so a tankless installation often includes checking pipe size and length. Your plumber will measure it rather than assume.
The thing that matters is the temperature of the water entering your heater, which you can measure at the tap. Rather than assume, test it in more than one season and size to the colder reading.