Formula Used
The calculator uses a simplified heat loss method. Temperature difference is found first.
Delta T = Indoor design temperature - Outdoor design temperature
Transmission loss = U value x Surface area x Delta T
Air leakage CFM = ACH x Room volume / 60
Air leakage loss = 1.08 x CFM x Delta T
Total BTU/h = Wall loss + Window loss + Ceiling loss + Floor loss + Air leakage loss
Final BTU/h = Total BTU/h x (1 + Safety margin / 100)
Watts = Final BTU/h / 3.412141633
Amps = Watts / Voltage
Continuous sizing current = Amps x 1.25
How To Use This Calculator
Enter the room length, width, and ceiling height. Add the indoor design temperature and the outdoor design temperature. Choose an insulation profile. Add outside walls, window area, voltage, heater wattage, and safety margin. Press Calculate. The result appears above the form. Use the CSV or PDF buttons to save the same calculation.
Example Data Table
| Room |
Area |
Insulation |
Delta T |
Estimated watts |
Use case |
| Bedroom |
180 sq ft |
Good |
60 F |
900 to 1,500 W |
Small room heater |
| Workshop |
400 sq ft |
Average |
65 F |
3,000 to 5,000 W |
Wall heater |
| Garage |
600 sq ft |
Poor |
70 F |
6,000 W or more |
Unit heater |
Understanding Electric Heat Sizing
Electric heat works best when the heater matches the room load. A small unit runs constantly and may never reach the set temperature. A large unit can cycle often, waste money, and feel uneven. This calculator estimates the design load by combining transmission loss through walls, windows, ceiling, and floor with air leakage loss. It then converts that load into watts, kilowatts, BTU per hour, amperage, and heater count.
Why Inputs Matter
Room size gives the heated volume. Ceiling height matters because tall rooms hold more air. Indoor and outdoor design temperatures create the temperature difference. A colder design day needs more heat. Insulation quality changes the default U values and leakage rate. Better insulation reduces losses. Window area matters because glass usually loses heat faster than insulated walls. Voltage affects current, so the same wattage can draw different amps on different circuits.
Using The Result
Use the calculated wattage as a planning value. Add a margin when the space has many doors, cold floors, large glass, or frequent drafts. The heater count shows how many units of your chosen size may be required. The amperage result helps compare the load with a circuit. Continuous electric heat should be reviewed carefully because it can run for long periods. Local electrical codes and equipment labels should always guide final installation.
Practical Sizing Tips
Measure the room before guessing. Use the coldest normal outdoor temperature for your area, not a mild day. Select poor insulation when the room has gaps, unsealed doors, or thin walls. Select excellent insulation only for modern sealed construction. If the result is close to a heater size, choose the next practical size only when the circuit and thermostat can support it. For comfort, placement also matters. Baseboard and wall heaters work better when air can move freely. Keep furniture away from the unit. Avoid blocking thermostats with curtains or shelves.
Planning Notes
This tool is a simplified heat loss estimator. It is useful for rooms, offices, cabins, garages, workshops, and additions. It does not replace a full Manual J style load study. Still, it gives a clear starting point. With careful inputs, it can reduce undersizing, oversizing, and unsafe assumptions during early planning.
FAQs
What does electric heat sizing mean?
It means estimating the heater capacity needed to offset heat loss on a cold design day. The result is usually shown in watts, kilowatts, or BTU per hour.
Is this calculator for baseboard heaters?
Yes. It can help size baseboard, wall, fan-forced, radiant panel, and unit heaters. Match the final choice with product instructions and local electrical rules.
Why is outdoor design temperature important?
It controls the temperature difference. A lower outdoor value creates a larger heating load. Use a realistic winter design value for your location.
Should I add a safety margin?
A margin is useful for drafts, doors, cold floors, and uncertain inputs. Too much margin can oversize the heater, so keep it reasonable.
What is ACH?
ACH means air changes per hour. It estimates how often room air is replaced by leakage or ventilation. Drafty rooms have higher ACH values.
Can I use watts per square foot only?
Watts per square foot is a quick rule. This calculator is better because it also considers temperature, insulation, glass, leakage, and voltage.
Does the breaker estimate choose my wiring?
No. It is only a planning guide. Circuit size, conductor size, thermostat rating, and protection must follow local code and equipment labels.
Why does voltage change amperage?
Current equals watts divided by voltage. A heater with the same wattage draws less current at higher voltage, but installation rules still matter.