Sizing a tankless water heater starts with flow rate, not storage volume. A storage tank is rated in gallons it holds, while a tankless unit is rated in gallons per minute (GPM) it can heat to the temperature you need. Choose wrong in either direction and the unit either runs out of hot water during the evening rush or burns fuel heating water nobody uses. The basics of water heater selection and installation begin with this distinction, because every downstream decision, from the gas line to the breaker panel, follows from the flow rate you pick.
Two numbers drive the choice. The first is your household’s peak hot water demand, which is the combined flow of every fixture running at once. The second is the unit’s output at the temperature rise your location requires. Compare the two and the answer appears: buy the smallest unit whose output at your incoming water temperature meets or beats peak demand.
How Tankless Water Heater Sizing Works
A tankless water heater raises water temperature as it flows through a heat exchanger, so output depends on two variables: flow rate in GPM and temperature rise, the difference between incoming ground water temperature and the outlet temperature you set. A unit rated at 8 GPM at a 77 degree Fahrenheit rise delivers less flow at a larger rise. In northern states, where ground water runs colder, the same unit produces less usable hot water than it would in the South.
What GPM Actually Measures
Manufacturers rate gas units in GPM at a standard temperature rise, commonly 77 degrees F, which assumes 50 degree incoming water heated to 127 degrees at the tap. If your incoming water runs colder, subtract roughly 0.5 GPM for every 10 degrees of extra rise. This is why the same 8 GPM unit serves a three-bathroom house in Georgia but struggles in Minnesota during January.
Why the Rating Label Can Mislead
Labels advertise an average flow range, and average is not peak. A unit that averages 6 GPM may hold 8 GPM for a minute, then throttle back as the heat exchanger reaches its limit. Design for sustained peak demand, not the marketing number, or the first long shower after the dishwasher starts ends cold. The same discipline applies when a home adds a second bathroom or a deep soaking tub: demand grows faster than the label suggests.
Peak Demand Timing
Most households concentrate hot water use between 9 p.m. and 11 p.m., when showers, tooth brushing, and kitchen cleanup overlap. That window defines the peak the unit must satisfy. A household that never runs more than one fixture at a time can drop a full GPM from its requirement, while a household with two teenagers cannot.
Some homes skip a separate boiler and heat space and water with one appliance. Combined hydronic heat and hot water systems use a tankless unit to feed both baseboard loops and domestic taps, and they need the top of the sizing range because the heating load stacks on top of the domestic load in cold weather. If that describes your project, size the unit for the combined demand, not the water heater load alone.
Fixture Flow Rates and Peak Demand
The practical starting point is a fixture-by-fixture flow table. Each fixture in the house draws a known range of GPM when fully open, and peak demand is the sum of the fixtures that run at the same time. Plumbers use these ranges to estimate demand before any unit is purchased.
| Fixture | Typical Flow Rate (GPM) |
|---|---|
| Shower | 2.0 – 3.0 |
| Kitchen or bathroom faucet | 1.0 – 2.0 |
| Washbasin | 1.5 – 2.0 |
| Washing machine | 2.0 – 2.5 |
| Dishwasher | 1.5 – 2.5 |
A Sample Peak Demand Calculation
Add up the fixtures you expect to run together. One person showering while the kitchen faucet fills a pot and the washing machine runs draws roughly 2.5 plus 1.5 plus 2.25, about 6 GPM at the midpoint of each range. That number, not the bathroom count, is what the unit has to deliver.
Run the same math on a shower at 2.5 GPM with two faucets at 1.5 GPM each and the requirement lands near 5.5 GPM, which is why a 5 GPM unit is the practical floor for a typical one-bathroom home. A 4 GPM unit that serves a studio apartment will throttle during that same shower.
When Demand Exceeds Flow
The choice between a tankless water heater vs hot water tank comes down to how your peak demand is shaped. A tank absorbs a short burst of high flow because it stores heat; a tankless unit must generate the entire peak instantly. Homes with two showers running at once, a soaker tub, or frequent simultaneous appliance use often discover their first price quote is 2 to 3 GPM short of the demand calculation.
Sizing by Number of Bathrooms
Bathroom count is a blunt but useful proxy when the fixture list is unknown. These ranges assume standard shower heads and typical simultaneous use:
| Bathrooms | Recommended Unit Size |
|---|---|
| Studio or small one-bathroom home | 4 – 5 GPM |
| One bathroom | 6 – 7 GPM |
| Two bathrooms | 8 – 9 GPM |
| Three or more bathrooms | 9 – 11 GPM |
A family of four or five with two bathrooms should plan on 8 to 9 GPM. A family of six in a three-bathroom house belongs at the 9 to 11 GPM level. These figures assume standard 2.5 GPM shower heads; low-flow fixtures let you drop to the bottom of the range, while rain heads and body sprays push you to the top.
Replacing a 50-Gallon Tank
Most 50-gallon storage tanks serve two-bathroom homes, and the usual replacement is an 8 to 9 GPM gas unit. The tank carried the house through brief peak events that a tankless unit must handle live, so err toward the top of the range unless every fixture is low-flow. Planning an advanced water heater replacement means checking the existing gas line size, venting route, and electrical service before the old unit comes out, because those limits set the ceiling on what the new unit can be.
Undersizing Symptoms
An undersized unit announces itself in predictable ways:
- A water temperature drop the moment a second shower starts.
- Rapid on-off cycling that wastes fuel.
- Lukewarm faucets after 60 to 90 seconds of use.
- Error codes when the unit cannot hold its outlet temperature.
Gas vs Electric Sizing
Fuel type changes the sizing language. Gas units are rated in GPM and cover the full residential range from 2 to 12 GPM, with 5 to 10 GPM models serving most families. Electric units are rated in kilowatts and work best at low demand: an electric tankless heater can serve fixtures totaling up to about 8 GPM, but once demand passes that point a gas unit is the practical choice, because the electrical service needed to heat 8 or more GPM becomes prohibitive.
Electric Ratings in Kilowatts
An electric unit’s kilowatt rating determines its output. As a rule of thumb, a 240-volt electric unit needs roughly 3 kW of capacity for each GPM of output at a 77 degree rise: a 6 GPM home lands near 18 kW, and a full-house 8 GPM installation approaches 27 to 36 kW. That translates into a dedicated 150 to 200 amp panel capacity in the worst cases, which is why electric tankless units are common in small apartments and mild climates and rare in large northern homes.
Breakers and Wire
The electrical side is non-negotiable. A 27 kW unit at 240 volts draws about 112 amps, so installers split it across two or three dedicated double-pole breakers, typically 40 to 50 amps each, with copper wire sized for the run. The engineering inside instantaneous hot water systems makes the trade-off clear: electric units gain efficiency at the exchanger but pay for it in service capacity, while gas units need only a vent and a fuel line.
Service Upgrade Costs
Adding breakers and wire for a large electric unit often forces a service upgrade from 100 to 200 amps, which can add $1,500 to $3,000 before the heater itself is bought. Include that line item when comparing electric and gas pricing, or the budget comes up short at the panel.
Installation, Cost, and Whole-Home Demand
Sizing stops at the spec sheet, but the project ends at the permit. Sizing, connections, safety, and code compliance all have to line up before a gas unit can be commissioned: the vent must meet the manufacturer clearance table, the gas line must deliver the full BTU load at the farthest appliance, and a drip leg and sediment trap belong at the inlet.
Fuel Supply and Venting
Gas units need a line sized for their BTU input, typically 3/4-inch pipe for most residential units with short runs, stepping up to 1 inch for long distances or high-input models. Propane-fired units burn roughly 1.5 gallons per hour at full fire, and the tank must be sized so it does not freeze during a long shower. Condensing models vent through plastic pipe and run cooler, which simplifies clearances in tight mechanical rooms.
Costs and Payback
- Gas tankless unit: $800 to $2,500 for the appliance.
- Electric whole-home unit: $900 to $1,800.
- Gas installation with venting and gas line: $1,200 to $3,500.
- Electric installation: $400 to $1,200.
Energy savings offset part of that outlay. A gas tankless unit runs 24 to 34 percent more efficient than a storage tank because it eliminates standby losses, and the savings show up fastest in homes with low daily usage, where the tank spent most of the day holding heat for one or two showers. The units last 20 years or more against a tank’s 10 to 15, so a second tank replacement is often avoided.
Putting It Together
Final sizing works like this: total the fixture demand, add a 10 to 15 percent margin for cold snaps, pick the fuel type your service can support, and verify the unit’s GPM at your local temperature rise. Homes that also use the heater for space heating need the margin more than most: a tankless water heater for space heating setup draws the largest continuous load in the house, and undersizing it guarantees cold rooms on the worst winter nights. Work through the fixture math, keep the margin, and the unit you install will be the one your household actually needs.