Example Data Table
| Scenario |
Flow |
Temperature rise |
Efficiency |
Approximate input |
| One shower |
2.0 GPM |
70 °F |
95% |
73,684 BTU/hr |
| Two showers |
4.0 GPM |
70 °F |
95% |
147,368 BTU/hr |
| Shower and sink |
3.5 GPM |
60 °F |
90% |
116,667 BTU/hr |
Formula Used
Total flow demand = Σ(quantity × fixture flow) × simultaneous factor × (1 + safety margin).
Temperature rise = desired outlet temperature − incoming water temperature.
Output BTU/hr = 500 × total GPM × temperature rise in °F.
Input BTU/hr = output BTU/hr ÷ heater efficiency.
Input kW = input BTU/hr ÷ 3412.142.
Maximum GPM = net heater output ÷ (500 × temperature rise in °F).
Operating cost = energy used per time period × entered energy price.
How To Use This Calculator
Choose flow and temperature units first. Enter the incoming water temperature and your target outlet temperature.
Add only fixtures that may run at the same time. Enter each fixture count and flow rate.
Set efficiency, safety margin, derate, and heater count. Enter the model rating if you want a capacity check.
Use the result to compare required input, available output, maximum flow, and estimated energy cost.
Why Tankless Sizing Matters
A tankless water heater heats water only when fixtures run. That makes sizing different from tank storage. The key question is not tank volume. It is flow at the needed temperature rise. A small unit may work for one shower. It can fail when a washer and faucet run together. Correct sizing prevents cold bursts, slow flow, and wasted money.
What This Calculator Checks
This calculator combines fixture flow, inlet temperature, outlet temperature, fuel efficiency, and safety margin. It estimates total demand in gallons per minute. It also converts the heat load into BTU per hour and kilowatts. These units help compare gas, propane, and electric models. The tool also checks available capacity when you enter a heater rating. That check is useful during replacement planning.
Important Physics Behind The Result
Water needs heat to rise from the incoming temperature to the desired delivery temperature. Colder climates need more energy because the inlet water is colder. A heater serving 5 gallons per minute with a 70 degree Fahrenheit rise needs much more capacity than the same flow with a 40 degree rise. Efficiency also matters. A ninety five percent unit wastes less input energy than an eighty percent unit.
Practical Buying Notes
Use realistic simultaneous fixtures. Do not add every fixture in the house unless they can run together. Add a safety margin for winter water, aging, scale, and flow variation. Check manufacturer tables before final selection. Real output changes with altitude, venting, gas pressure, voltage, and installation quality. Electric tankless units may need large breakers and service upgrades. Gas units may need larger gas lines.
Using The Result Wisely
The result gives a technical estimate. It is not a permit drawing. Use it to compare models and discuss options with a licensed installer. The capacity status shows whether the entered heater rating covers your adjusted demand. If the margin is small, choose a larger model or reduce simultaneous flow. Good sizing gives steady hot water and avoids overspending. Record both summer and winter inlet temperatures when possible. Many homes see wide seasonal swings. A winter design case is safer. It protects comfort when demand peaks and incoming water is coldest for the busiest household hour.
FAQs
What size tankless heater do I need?
You need enough capacity for simultaneous flow and temperature rise. Add the fixtures that run together. Then size the heater for that adjusted flow, not for total home fixture count.
Why does inlet water temperature matter?
Cold inlet water needs more heat. The same heater delivers less hot water in winter if the incoming water temperature drops. Use winter inlet temperature for safer sizing.
What does GPM mean?
GPM means gallons per minute. It measures how much water moves through fixtures. Tankless heaters are often rated by GPM at a specific temperature rise.
Why is BTU per hour important?
BTU per hour shows heat delivery rate. Gas and propane tankless heaters are commonly rated by input BTU per hour. Higher demand needs a higher rating.
How do I size an electric tankless heater?
Use the required input kW result. Then compare it with the model rating, voltage, and available amperage. Electric units often need dedicated circuits and service checks.
Should I include every fixture?
No. Include fixtures that can run at the same time. Adding every fixture may oversize the unit and raise purchase cost without improving normal comfort.
What safety margin should I use?
A 10% to 20% margin is common for estimates. Use more margin for cold climates, high demand homes, uncertain flow rates, or future fixture upgrades.
Is this result a final installation design?
No. It is an estimate for planning. Always confirm model charts, fuel supply, venting, wiring, code rules, and permits with a qualified installer.