Sizing Tankless Water Heater Calculator

Calculator inputs

Enter the expected simultaneous fixture use, incoming cold water temperature, desired outlet temperature, and project design factors. The calculator then estimates flow demand, required heating input, altitude adjustment, and likely unit count.

Incoming water temperature (°F)

Target hot water temperature (°F)

Thermal efficiency (0 to 1)

Fuel type

Project altitude (ft)

Gas and propane sizing is derated above 2,000 ft.

Peak design runtime per day (minutes)

Diversity factor

Use 1.00 for strict simultaneous demand.

Safety factor (%)

Max single unit input (BTU/hr)

Used to estimate how many units may be needed in parallel.

Showers count

Shower flow each (GPM)

Kitchen faucet count

Kitchen faucet flow each (GPM)

Bathroom faucet count

Bathroom faucet flow each (GPM)

Dishwasher count

Dishwasher demand each (GPM)

Clothes washer count

Clothes washer demand each (GPM)

Tub filler count

Tub filler flow each (GPM)

Utility sink count

Utility sink flow each (GPM)

Custom fixture label

Custom fixture count

Custom fixture flow each (GPM)

Example data table

Scenario	Incoming temp (°F)	Target temp (°F)	Fixture demand	Diversity	Safety	Approx. design flow	Approx. required input
Small house	55	120	1 shower, 1 kitchen, 1 bath faucet	0.90	10%	4.16 GPM	152,000 BTU/hr
Family home	50	120	2 showers, kitchen, bath faucet, washer	0.90	15%	7.25 GPM	270,000 BTU/hr
Large demand	45	125	3 showers, kitchen, tub filler, washer	0.95	20%	13.34 GPM	568,000 BTU/hr

Formula used

1) Raw simultaneous flow
Raw Flow (GPM) = Sum of every fixture count × its flow rate

2) Diversified flow
Diversified Flow = Raw Flow × Diversity Factor

3) Design flow with safety
Design Flow = Diversified Flow × (1 + Safety Factor)

4) Temperature rise
Temperature Rise (°F) = Target Hot Water Temperature − Incoming Water Temperature

5) Heat required by water
Water Heating Load (BTU/hr) = Design Flow × Temperature Rise × 500.4

6) Heater input requirement
Heater Input (BTU/hr) = Water Heating Load ÷ Thermal Efficiency

7) Altitude adjustment for gas or propane
Adjusted Input = Heater Input ÷ (1 − Altitude Derate)

8) Parallel unit estimate
Units Required = Ceiling of Adjusted Input ÷ Maximum Single Unit Input

How to use this calculator

Enter the local incoming water temperature for the project location.
Enter the desired outlet temperature, often 120°F for domestic use.
Choose the heater fuel type and set the expected efficiency.
Input fixture counts and realistic flow rates for simultaneous use.
Apply a diversity factor if not all fixtures will run together.
Add a safety factor to cover uncertainty or future demand growth.
Enter the maximum input for one unit to estimate parallel units.
Press calculate and review design flow, input load, graph, and recommendation.

FAQs

1) What does this calculator estimate?

It estimates simultaneous hot water flow, required temperature rise, heater input, altitude adjustment, and the probable number of units needed for a project.

2) Why is temperature rise so important?

Tankless heaters are sized by both flow and temperature rise. Colder incoming water requires far more heating input for the same delivered flow.

3) What diversity factor should I use?

Use 1.00 for strict peak simultaneity. Use lower values, such as 0.80 to 0.95, when fixture overlap is likely but not constant.

4) Should I add a safety factor?

Yes, many designers add 10% to 20% for occupancy changes, fixture uncertainty, cold-weather variation, and equipment selection flexibility.

5) Does altitude affect sizing?

Yes, gas and propane appliances commonly lose available input at higher elevations. This calculator adds a simple derate above 2,000 feet.

6) Can one unit handle a large home?

Sometimes, but large homes often need cascading units. A high design flow or high temperature rise can exceed common single-unit capacities.

7) Are fixture flow rates always exact?

No. Manufacturer data, aerators, pressure conditions, mixing behavior, and user habits can change actual demand, so field judgment still matters.

8) Is this enough for final equipment selection?

It is a strong preliminary sizing tool. Final selection should still verify manufacturer performance tables, venting, gas supply, electrical service, and code requirements.