What Size Tankless Water Heater Do I Need?
Tankless water heater size comes down to two numbers: how many gallons per minute (gpm) of hot water you run at the same time, and how many degrees the heater has to raise the incoming water. Add up the flow of the fixtures you use together at your busiest moment, work out the temperature rise, and pick a unit rated for that flow at that rise.
Tank size in gallons doesn't apply here. A tankless unit is sized by flow, and the same heater that fills two showers in Phoenix may struggle with one in Minneapolis.
Step 1: add up your peak flow
Think about the busiest few minutes in your house. Two showers in the morning? A shower plus the dishwasher? Write down every hot water fixture that runs at that moment.
The Department of Energy's own example uses a hot faucet at 0.75 gpm and a shower head at 2.5 gpm, which adds up to 3.25 gpm when both run together. Your fixtures may use less. Stiebel Eltron's sizing guide assumes 1.5 gpm at 105°F for a shower, 1 gpm at 105°F for a kitchen sink and 0.5 gpm at 90°F for a bathroom faucet. Check the flow rating stamped on your shower heads and faucet aerators; a newer low-flow head changes the math a lot.
Step 2: work out the temperature rise
Temperature rise is your target temperature minus the temperature of the water coming into the house. The DOE says to assume 50°F incoming water unless you know otherwise, and that most uses need 120°F, which means a 70°F rise.
Ground water runs warmer in the South and colder in the North. Eemax's sizing guide shows inlet zones running from 37°F to 77°F, and Stiebel notes the zone temperatures are averages that shift with the seasons. Size for winter. A heater that's fine in August is the one that leaves you shivering in February.
Step 3: match a unit to flow and rise
For gas, the DOE says a 70°F rise at 5 gpm is typical for gas-fired demand heaters, and electric units manage far less flow at that rise. Every manufacturer publishes a chart of gpm at each rise. Find your rise on the chart and read off the flow. That flow has to cover the peak you added up in the first step.
For electric, Eemax gives the formula directly: kW required = gpm × (temperature rise ÷ 6.83).
| Scenario | Peak flow | Rise | Electric kW needed |
|---|---|---|---|
| One shower plus a faucet, cold region | 3.25 gpm | 70°F | About 33 kW |
| Two showers, cold region | 5 gpm | 70°F | About 51 kW |
| One low-flow shower plus a kitchen sink | 2.5 gpm | 70°F | About 26 kW |
That second row is the one that catches people. The biggest model in Stiebel Eltron's Tempra line is 36 kW, so two full showers in a cold climate are past what one whole-house electric unit delivers. That's gas territory, or two electric units. See electric tankless water heaters for what the electric models really do, and how many breakers they need.
Sizing an electric tankless unit
Electric sizing is stricter because the kilowatts are fixed by your breakers. Stiebel's 36 kW unit gives an 82°F rise at 3.0 gpm but only 55°F at 4.5 gpm on 240 volts. On a 208-volt supply the same unit drops to 27 kW, and Stiebel says to go up one model size for 208-volt installs.
Before you shop, have an electrician confirm the panel can carry the load. A big electric unit can need three separate 50-amp double-pole breakers.
Sizing a gas tankless unit and its gas line
Gas tankless units are rated by input in BTUs per hour and output in gpm at a given rise. The higher the input, the more gas the line has to carry. Many older homes have a gas line sized for a tank heater, which uses far less gas at once.
Gas line size depends on the unit's input, the length of the run, the other gas appliances on the line and the gas pressure. The installation manual for the model you choose has the pipe sizing table. HomeGuide lists upgrading to a larger gas supply line at $350 to $2,000, so get that number into your quote before you commit.
A quick tankless water heater size chart
| Household | Typical peak | What to look for |
|---|---|---|
| One bath, warm region | One shower plus a sink | A mid-size electric or a small gas unit |
| Two baths, cold region | Two showers at once, 5 gpm | A gas unit rated near 5 gpm at a 70°F rise |
| Big family, three baths | Three fixtures or more at once | Large gas, or two units; the DOE notes large households may need two or more |
If the numbers point to two units, step back and compare with a tank. Our tankless vs tank water heater page covers which one fits which house. For tank sizing by first-hour rating, see what size water heater do I need.
FAQ
What size tankless water heater do I need for a family of 4?
It depends on how many showers run at once, not on how many people live there. If two showers overlap in a cold climate, plan for about 5 gpm at a 70°F rise, using the DOE's showerhead and temperature figures. If showers happen one at a time, a smaller unit will do.
What size electric tankless water heater do I need?
Use kW = gpm × (rise ÷ 6.83). A shower and a faucet at 3.25 gpm with a 70°F rise needs about 33 kW, which is near the top of what one residential unit offers. Check your panel capacity before you buy.
How many gpm do I need for two showers?
At the DOE's 2.5 gpm per shower head, two showers need 5 gpm. With low-flow heads at 1.5 gpm, Stiebel's assumption, it drops to about 3 gpm.
What tankless water heater works for two bathrooms?
If both bathrooms' showers run at once in a cold climate, that's about 5 gpm at a 70°F rise, which points to a gas unit. In warm regions with low-flow fixtures, a large electric unit can work.
What does it cost to install a tankless water heater?
HomeGuide puts tankless units at $1,400 to $5,600 installed. Gas line, venting and electrical upgrades push it toward the top of that range. See tankless water heater cost for the breakdown.
Sources
- U.S. Department of Energy, Energy SaverSizing a New Water Heater (opens in a new tab)
- Stiebel EltronTempra Trend and Tempra Plus Sizing Guide (opens in a new tab)
- Eemax (Rheem)Eemax Sizing Guide (opens in a new tab)
- Stiebel EltronTempra Trend and Tempra Plus Submittal Sheet (opens in a new tab)
- HomeGuideWater Heater Installation and Replacement Cost (opens in a new tab)
- U.S. Department of Energy, Energy SaverTankless or Demand-Type Water Heaters (opens in a new tab)