How to Calculate HVAC Capacity

Your HVAC unit exerts a certain amount of thermal energy as it runs, which should reflect on the size of your house and other factors that might call for an increase in thermal energy output.

You must calculate what you need from an HVAC unit before buying one.You must calculate what you need from an HVAC unit before buying one.
If you get an HVAC unit that doesn't have enough power to heat or cool your home, it runs continuously, eating a bunch of electricity. An HVAC unit with overwhelming amounts of power doesn't dehumidify a house properly either.

Measure every room's length and width. Multiply each value to get the area of each room in square feet. Write the areas down for each room.

Add all the areas together to get the full area you intend to cool.

Multiply the full area by 25. This gives you the raw cooling capacity necessary for the house, not accounting for other things that might affect your calculation.

Count all the windows in your house and add 1000 to your previous number for each window.

Determine the average number of occupants your house should hold. If you live alone, you count as one occupant. Add 400 for each occupant. The final number determines the HVAC capacity you need, measured in British thermal units (BTUs).


  • Don't confuse BTU capacity with horsepower. The horsepower measurement points to how much power the blower motor has.

About the Author

Mikhail Polenin has been working with computers since 1997. His experience also expands to astrophysics, masonry, electricity and general appliance repair. He's written about various different subjects regarding astrophysics and electrical circuits for various online publications. Polenin attended the New World School of the Arts and the University of Florida.