Room Heating & Cooling Load Calculator
Enter room dimensions, insulation quality, window area, and climate zone to get the BTU/hr and kW required for heating and air conditioning. Nothing uploaded.
Room Settings
Load Results
Simplified estimate. Oversizing heaters or AC units by more than 25% causes short-cycling and humidity issues. Consult an HVAC professional for system selection.
Learn more: heating and cooling load calculations
Understanding heating and cooling loads - BTU and kilowatt basics
Heating and cooling load is the rate of energy needed to maintain a room temperature. Measured in BTU per hour (British Thermal Units) or kilowatts (kW), it depends on room size, insulation quality, window area, ceiling height, and climate zone. A room that needs 12,000 BTU per hour to heat in winter might need 15,000 BTU per hour to cool in summer due to solar gain through windows.
The simple rule of thumb and why it breaks down
A quick estimate: 40-60 BTU per square foot for average insulation in temperate climate. A 200 sq ft room would need 8,000-12,000 BTU per hour. But this rule ignores ceiling height, insulation quality, window orientation, and outdoor temperature. A poorly insulated room with tall ceilings and north-facing windows will need much more. The calculator accounts for all these factors to give you an accurate estimate.
Oversizing and short-cycling - why you don't want a too-powerful heater or AC
Oversizing a heater by more than 25-30 percent causes short-cycling - the unit reaches setpoint quickly, shuts off, then starts again frequently. This wastes energy, causes uncomfortable temperature swings, and wears out equipment faster. For air conditioners, oversizing prevents adequate run-time to dehumidify the air, leaving rooms feeling clammy even though the temperature is right.
FAQ
Should I size my heater for the coldest day of the year?
Yes - the unit needs to handle the worst-case outdoor temperature. But you do not want to oversize beyond that. Know your local winter design temperature (usually 99th percentile low) and size within 10-15 percent of the calculated load.
Do I need to account for appliances and people generating heat?
Not usually for residential heating. For cooling, yes - interior heat from people, lights, and equipment can add 5-15 percent to the load. Commercial spaces need this factored in.
What's the difference between heating load and air conditioning load?
Heating is driven by outdoor temperature difference. Cooling includes outdoor temperature plus solar gain through windows. Cooling loads are typically 10-30 percent higher than heating loads in the same space and climate.