Determining the flow and temperature rise a tankless heater must deliver
A tankless water heater is selected for the fixtures that can demand hot water at the same time, not simply by how many bathrooms appear on a floor plan. Two showers operating together create a different peak demand from a shower used after the dishwasher has stopped. Add the flow rates of fixtures that realistically operate simultaneously, using measured flow or fixture specifications rather than the sum of every outlet in the building. The calculator groups showers, taps and other concurrent appliances separately so that the assumption can be checked against an actual household routine.
Supply-water temperature is equally important. A heater delivering five gallons per minute with a small temperature increase may be unable to sustain that same flow during winter when incoming water is considerably colder. Enter the lowest plausible inlet temperature and the intended hot outlet setting to obtain a temperature rise. The result compares the required flow, including an explicitly chosen allowance, against the manufacturer's rated flow at that same rise. A published maximum GPM figure obtained under different test conditions is not an equivalent comparison.
Simultaneous-demand and heating-duty formulas
Design comparison flow = concurrent demand × (1 + allowance ÷ 100)
Temperature rise (°F) = outlet temperature − cold inlet temperature
Approximate delivered heat (BTU/hr) = design flow × temperature rise × 500
Approximate input heat = delivered heat ÷ (thermal efficiency ÷ 100)
The factor of 500 is a rounded conversion for water flow in GPM and temperature rise in degrees Fahrenheit. It gives a useful thermal-demand comparison, not a gas-piping, venting, electrical-service or appliance-compliance design. The displayed kW figure expresses the entered input heat in energy-equivalent units and must not be treated as proof that an electric heating system or existing panel can supply it. A qualified installer needs to verify manufacturer performance curves, installation conditions and applicable safety requirements.
Worked example: simultaneous fixtures during winter
Suppose two showers together use 2.0 GPM, a tap uses 1.5 GPM and another hot-water appliance uses 1.0 GPM. Simultaneous demand is 4.5 GPM; a 10% allowance makes the comparison flow 4.95 GPM. With incoming water at 50°F and the target at 120°F, the temperature rise is 70°F. Required delivered heat is approximately 4.95 × 70 × 500 = 173,250 BTU/hr.
At an assumed 95% thermal efficiency, the corresponding input-heat estimate is approximately 182,368 BTU/hr. A manufacturer rating of 5.5 GPM at the same 70°F rise exceeds the comparison demand by 0.55 GPM; a 5.5 GPM marketing maximum at some other rise does not establish that margin.
Interpreting a manufacturer's flow chart before choosing equipment
Measure cold-inlet temperature under the conditions that matter for the property. Groundwater and municipal supply temperatures may vary seasonally, and a warm-weather observation can exaggerate available winter flow. Check fixture specifications at realistic supply pressure and distinguish mixed shower-water flow from the hot-water share actually passing through the heater. The calculator treats entered flow as hot-water demand; entering a shower's total mixed flow may deliberately produce a conservative comparison, but the assumption should be understood.
Many households can reduce simultaneous demand through usage patterns or efficient fixtures, but a smaller GPM figure should not be selected solely to make the estimated heater look adequate. Recirculation systems, long pipe runs, flow activation thresholds and fuel delivery constraints may influence actual performance. Verify the heater's certification and manufacturer installation requirements before ordering, especially when replacing a storage tank in a home with existing plumbing or electrical limitations.
Tankless sizing questions
Should I add the flow of every shower and faucet in the house?
No. Add fixtures that could plausibly draw hot water at the same time during peak usage. If occupants regularly use multiple bathrooms while laundry runs, that peak demand belongs in the estimate. Adding every faucet regardless of actual usage produces a theoretical maximum that may be much larger than the intended design load.
Why does cold inlet temperature change the required unit?
The heater must raise the incoming water to the selected outlet temperature. Lower winter inlet temperatures increase the heat required for each gallon, so the same appliance may deliver fewer gallons per minute. Compare the required flow against the manufacturer's performance curve at the entered temperature rise.
Does this calculation specify electrical or gas installation size?
No. The BTU/hr and kW-equivalent values are thermal comparisons, not fuel-pipe, electrical-panel, exhaust-vent or permitting specifications. The appropriate equipment and connections depend on manufacturer requirements, local code and qualified installation assessment.