Cheat Sheet: How to Calculate HVAC Square Footage

When installing new heating and cooling systems, it’s critical that you correctly size the equipment. System size will have a great impact on your clients, from comfort to cost, installation and ongoing. Size them wrong, and you’ll create big problems for your business and your clients.


To determine the residential home or commercial building’s heating and cooling loads, you’ll need to incorporate many figures. The first is the structure’s square footage. In our HVAC calculator square footage cheat sheet, we’ll walk you through how to find the precise square footage of the space you’re working with. This figure will be a solid foundation for your load calculations.

HVAC Calculator Square Footage

For heating and cooling purposes, figuring the square footage of a building is more than length x width. Heating and cooling load calculations consider the entire structure three-dimensionally.

  • Take room measurements of all conditioned areas. Add these areas to determine the total square footage. By using room measurements, you’ll eliminate the dead space throughout the home, such as wall thickness and closets, which may throw off system sizing.
  • You must also note the amount of square footage by ceiling, as well as the ceiling’s grade. Mark portions of the ceiling set under attic space, as well as the area under roof joists.
  • Note the area by floor type. Is a portion of the home on a crawl space and the remainder is on a slab? Mark this area.
  • Consider walls above and below grade. Note the square footage of each, as well as partitions, as they will affect the building’s loads.

HVAC Calculator Square Footage for Window Area

Glass throughout the home or building has a great impact on heating and cooling loads. Total area, or square footage, of glass such as windows and sliding patio doors is used in Manual J calculations. Your HVAC calculator square footage must also include these elements for accuracy.

  • Measure each window, skylight, and glass sliding door in the building. Note the thickness of the glass (single pane, double pane, etc). Also, note its directional orientation. From the exterior, measure the overhang of the roof eave extending over the window, if any.
  • Measure doors. Find the square footage of all door area. Also, note the door’s materials, which affect its insulation.

These are the square footages you’ll need to use when working to find the home or building’s heating and cooling loads. Once you’ve calculated the square footage of the residential or commercial space, move forward performing Manual J load calculations to accurately size the new heating and cooling system.

To make the task easier, members have access to our own HVAC calculator – square footage and other factors are input to quickly determine Manual J!

Our online HVAC calculator square footage is just one of the many tools we offer members to make their businesses better!