What size tankless water heater do I need?
By Dan Whitlock, Home Projects Editor · Updated August 2026
Most homes need a tankless water heater rated for 5 to 10 GPM (gallons per minute). Size it by adding up the fixtures that run at once, then check that the unit delivers that flow at your local temperature rise: colder incoming water means you need a bigger unit. A single bathroom needs about 3.5 GPM; a family of four with two showers running needs roughly 7 to 9 GPM.
Tankless water heaters are rated in gallons per minute, not gallons of storage, so “bigger” means more simultaneous flow rather than a larger tank. Two numbers decide the size you need: your peak flow rate (how much hot water you draw when the most fixtures run at once) and your temperature rise (how far the unit must heat your incoming groundwater). Get both right and a tankless unit never runs out. Get them wrong and it runs lukewarm the moment someone starts the dishwasher.
Quick answer: size by household
This table is the fast starting point. It assumes a typical 120°F setpoint and moderate groundwater temperatures; cold-climate homes should size up one row, and warm-climate homes can often size down.
| Household | Bathrooms | Typical peak demand | Recommended size (GPM) |
|---|---|---|---|
| 1–2 people | 1 bathroom | One shower plus a sink | 3.5–5 GPM |
| 2–3 people | 1–2 bathrooms | One shower plus a dishwasher | 5–7 GPM |
| 3–4 people | 2 bathrooms | Two showers at once | 7–9 GPM |
| 5+ people | 3+ bathrooms | Two showers plus a faucet or appliance | 9–11 GPM |
These are gas-unit figures. A single whole-home electric unit tops out lower (see gas vs. electric below), so larger households on electric often split the load across two units or a point-of-use unit at a far fixture.
The accurate way: add up your fixtures
The household table is a shortcut. The real method is to decide how many fixtures could run at the same time on a busy morning, then add their flow rates. That total is the minimum GPM your unit must deliver at your temperature rise.
| Fixture | Typical flow (GPM) |
|---|---|
| Shower (standard) | 1.5–2.5 |
| Bathroom or kitchen faucet | 1.0–1.5 |
| Dishwasher | 1.0–1.5 |
| Washing machine | 1.5–2.0 |
| Tub filler | 2.0–4.0 |
Example: two showers (about 4 GPM combined) plus a kitchen faucet (about 1.5 GPM) is a 5.5 GPM peak. Round up and you are shopping for a 6 GPM or larger unit at your temperature rise, not a 6 GPM unit at its best-case rating.
Temperature rise: why climate changes the size
A tankless unit’s GPM rating always assumes a temperature rise, and its real output drops as that rise grows. Temperature rise is simply your target temperature minus the incoming groundwater temperature. The U.S. Department of Energy suggests assuming 50°F incoming water and a 120°F setpoint unless you know otherwise, which is a 70°F rise. At that rise a typical gas unit delivers about 5 GPM and an electric unit about 2 GPM.
| Region | Approx. groundwater temp | Rise to reach 120°F | Effect on sizing |
|---|---|---|---|
| Northern states | ~40°F | ~80°F | Size up: the unit delivers fewer GPM |
| Central states | ~50°F | ~70°F | Use the rated GPM as published |
| Southern states | ~60°F | ~60°F | The unit delivers closer to its top GPM |
This is why the same model is “enough” in Florida and undersized in Minnesota. Always read a unit’s flow-rate chart at the rise that matches your groundwater, not the headline number on the box.
Gas vs. electric sizing
Fuel type sets your ceiling. Gas units heat far more water per minute, which is why nearly every whole-home recommendation above is a gas figure.
| Fuel | Typical whole-home flow | Sizing note |
|---|---|---|
| Gas | ~5–11 GPM | Handles two-plus showers; may need a larger gas line and new venting |
| Electric | ~2–8 GPM | Best for small homes or warm climates; large units need 150–200A service |
Electric makes sense for a small household, a warm-groundwater region, or a single far-away fixture. For a cold-climate family that showers on the same schedule, gas is usually the only fuel that keeps up at the temperature rise involved. The fuel decision also drives the install cost, since gas often means a line or venting upgrade and whole-home electric can mean a panel upgrade. See what a tankless water heater costs installed in 2026 to price the size you land on.
How we built this guide
The GPM ranges and temperature-rise figures here are compiled from published U.S. Department of Energy guidance, manufacturer sizing documentation, and 2026 cost guides (sources listed below), then reconciled into the household and fixture tables above. These are planning figures to help you shortlist a size before you shop. They are not lab-tested and are not a substitute for a licensed plumber’s load calculation for your specific home, fixtures, and groundwater temperature. Always confirm the final size against the manufacturer’s flow-rate chart at your local temperature rise.
Frequently asked questions
What size tankless water heater do I need for a family of four?
Most four-person households run well on a 7 to 9 GPM gas tankless water heater, which covers two showers plus a sink at the same time in a warmer climate. In a cold-groundwater region, size up toward 9 to 10 GPM because the same unit delivers fewer gallons per minute at a higher temperature rise.
How many GPM do I need for two showers at once?
Two standard showers draw about 3 to 5 GPM combined, so a unit rated for at least 5 GPM at your local temperature rise handles them. Add another 1 to 2 GPM if a faucet, dishwasher, or washing machine may run at the same time.
Can an electric tankless water heater supply a whole house?
A whole-home electric tankless unit can, but only if your electrical panel supports it. Electric units typically deliver about 2 GPM at a 70 degree temperature rise, and reaching 6 to 8 GPM in a cold climate can require 150 to 200 amps of dedicated service. In warm-groundwater regions the same unit stretches further because the temperature rise is smaller.
What is temperature rise and why does it change the size I need?
Temperature rise is the desired hot-water temperature minus the incoming groundwater temperature, usually 120 degrees minus a groundwater figure that runs roughly 40 degrees in the North to 60 degrees in the South. Every tankless unit delivers fewer gallons per minute as the temperature rise grows, so a colder climate needs a larger unit for the same flow.
Is it better to oversize or undersize a tankless water heater?
Size to your realistic peak demand, then round up rather than down. An undersized unit runs cold when several fixtures draw at once, while a modestly oversized unit simply modulates down and costs a little more up front. Oversizing by a huge margin wastes money without adding usable capacity.
Have a size in mind? The next question is what it costs to put it in. Our calculator prices gas vs. electric, gas-line and panel upgrades, and your region.
Tankless water heater cost calculator →Sources
Sizing and cost ranges cross-referenced against these 2026 guides and manufacturer documentation: