Tankless Water Heater Sizing Calculator

Use this tankless water heater sizing calculator to work out the flow and heat that your fixtures need at the same time. Start with measured flow and a cold inlet range. The result is a planning estimate. It does not prove that a named heater will meet your demand or that your home can supply it.

Size a tankless heater for simultaneous hot-water flow at the coldest inlet temperature you expect. Mixed shower flow includes cold water, so only part of it passes through the heater. Compare the resulting hot-side GPM with the model's flow chart at the same temperature rise. Then check gas or electric supply, low-flow activation and the installation manual.

How the tankless water heater sizing calculator uses GPM

GPM means US gallons per minute. It measures flow, not stored water and not the number of rooms in your home. A heater that suits one house may fail to meet a second house's peak use, even when both houses have the same floor area. The difference may be two showers running together, a bath fill, or cold winter inlet water.

Make a list of the uses that overlap. Count the shower that runs while someone washes dishes. Leave out a tap that never runs during that period. If you have two separate morning routines, calculate each as its own case.

A total that assumes every fixture runs at once may describe a demand that you never create.

Enter low and high flow for each row. Use the same value in both boxes when you know the flow. Use a range when a measured draw varies. Do not turn a missing measurement into zero.

Unknown flow means the estimate is incomplete until you measure it or select a clearly assumed example.

For a safe bucket measurement, collect water in a container of known volume and time the fill. Flow equals gallons collected divided by minutes. A one-gallon fill in half a minute is 2 GPM. Avoid hot-water contact and spills; do not lift heavy containers.

A fixture's rated flow can also help, though the real pressure may affect what it delivers.

Hot-side and mixed fixture flow at 120°F

Hot-side flow is water that comes from the heater. Mixed flow is the combined stream after hot and cold water meet at a tap or shower valve. These are different inputs. A 2 GPM shower set to 105°F does not need 2 GPM from a heater set to 120°F when some of its water comes from the cold supply.

The worksheet uses a simple heat balance for mixed rows. Hot fraction equals mixed target minus cold inlet, divided by heater setpoint minus cold inlet. At a 50°F inlet, a 105°F mixed target and a 120°F heater setting, that fraction is 55 divided by 70. The 2 GPM shower therefore draws about 1.57 GPM from the heater.

This assumes both streams mix without heat loss. It does not model a long pipe run or the detailed behavior of a shower valve. A measured hot-side flow avoids that mixing assumption. If you only know total shower flow, use the mixed option and state the target rather than silently treating it as all hot.

The starting 120°F setting is an editable example. CPSC's tap-water scald guidance addresses the danger from hot water. That setting is not a guarantee against burns or a universal sanitation instruction. Do not raise a heater setting just to improve a sizing result.

Follow the appliance instructions and the scald controls needed for your household.

Temperature rise and winter demand in °F

Temperature rise is heater outlet temperature minus cold inlet temperature. A 120°F outlet and 50°F inlet require a 70°F rise. A 60°F inlet requires a 60°F rise. The colder case needs more heat for the same hot-side flow.

Enter a low and high inlet value that you have measured or that your system design supports. The tool does not guess temperature from a ZIP code. A summer tap reading may not capture the winter supply. If winter conditions are unknown, keep the result labelled as a case and gather better inlet evidence before choosing equipment.

Take care about where the reading comes from. Water standing in indoor pipes can warm toward room temperature. A reading from that initial water may differ from the cold supply after the pipe has cleared. Record the reading conditions and time of year so that someone checking a model curve can understand the input.

The tool recomputes mixed demand at both inlet endpoints. It does not take the largest hot flow from one temperature and multiply it by a rise from another. For a fixed mixed temperature, hotter cold water lowers the hot fraction. That pairing matters when several mixed and hot-only rows share one estimate.

Worked 3 GPM heat-output example

Suppose two assumed hot-side draws run together at 2 GPM and 1 GPM. Their total is 3 GPM. Use a 50 to 60°F inlet range and a 120°F setpoint. These are math assumptions.

They do not describe typical fixture flow or household use.

Input or result	Assumed value
Combined hot-side flow	3 GPM
Cold inlet	50 to 60°F
Outlet setpoint	120°F
Required temperature rise	60 to 70°F
Thermal output	90,072 to 105,084 Btu/h
Thermal power	About 26.4 to 30.8 kW

The equation is 500.4 × hot-side GPM × temperature rise. The constant combines about 8.34 pounds of water per US gallon, 60 minutes per hour, and water's approximate heat capacity. Divide Btu/h by 3412.142 to convert to thermal kW. Small rounding differences do not show extra capacity.

These figures describe heat delivered to water. They are not gas burner input. To estimate burner input, divide thermal output by a stated steady-state efficiency. When that efficiency is unknown, burner input remains unknown.

UEF measures performance under a defined test pattern and cannot simply stand in for that efficiency.

Check a manufacturer's GPM curve before choosing a heater

Find the exact US model and the correct fuel or voltage version. Read its flow chart at your winter rise. Compare hot-side demand at that same rise, then check the other inlet endpoint too. A product's large maximum-flow headline may refer to a much smaller rise than your house requires.

Capacity is only one check. Look for lowest flow needed to start heating and any flow reduction under heavy demand. Stiebel Eltron's flow table is an example of a maker document that connects flow with rise. It is not a recommendation of a model for your home.

Installation fit needs separate evidence. Gas supply, approved exhaust route, drains, water pressure and electric service can limit the options. A thermal number cannot select a gas pipe diameter, vent material, breaker or wire. Use the exact current manual for those rules, with installation performed by the right licensed trades.

If a model cannot meet the case, do not round the demand down to its headline rating. Revisit which uses truly overlap, measure uncertain flow, or compare another system type. A lower-demand routine is a choice to make explicit. It is not an invisible adjustment that turns an inadequate model into a fit.

Frequently asked questions

Does floor area determine tankless size?

Floor area alone does not tell you how much hot water runs at once. Two homes of equal size can have different fixtures, routines and winter inlet temperatures. Use simultaneous flow and rise for tankless capacity. Include the planned use of large baths or extra showers when that use is real.

Is the result a gas burner rating?

No. The main result is heat output into water. Gas input also depends on the heater's steady-state heat efficiency. Enter that only when you have supporting evidence.

An efficiency label from a seasonal or draw-pattern test answers a different question and should not be substituted just to fill the optional field.

Can the tool select a breaker?

No. The electric utility estimates power and aggregate current under stated assumptions. An exact heater may split its load across several circuits. Breaker sizes, conductor selection and the home's service calculation follow its manual and adopted rules.

A current estimate does not give permission to add that load to an existing panel.

Why is a measured range useful?

A range makes input uncertainty visible. It lets you compare a lower and higher flow at both inlet temperatures. If the resulting capacity range crosses a model's limit, improve the measurement or investigate a different option. Hiding the range behind a single rounded number can conceal that decision.

Tankless water heater sizing calculator verdict and next steps

Keep the fixture list, inlet range and chosen setpoint with your result. Use the electric sizing utility when you need a power case. Read the tank-versus-tankless comparison before committing to a technology. That comparison also leads to the outdoor, point-of-use and whole-house planning guides.

This tankless water heater sizing calculator does not store your household entries or send them to a service. They stay in page memory while you use it. Printing preserves the useful worksheet and assumptions. Leaving or refreshing the page clears the entries.

Keep a copy yourself if you need to revisit the same case later.

Sources checked October 14, 2026: DOE's historical transition guideline, CPSC scald guidance, and the linked maker flow table. Current model instructions take priority over the older guideline. The methodology records the equations and checks; contact us to report a source or calculation error.

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