Floor plans are drawn to scale, in comparison to the space being created by the plan. Typically, it states the square footage for the total space and provides a breakdown of that square footage by living area within the building or home. A room that is 10-feet by 12-feet, for example, provides 120 square feet of living space.
Most homeowners, when having a home constructed, will be told what the square footage is for the home. This eliminates the need to personally come up with the square footage calculation for the floor plan. However, if the need should arise for you to have to calculate your own square footage, here are the tools, materials, and steps needed to accomplish this task.
Obtain a Copy of Floor Plan
To calculate the floor plan’s square footage, you will need to obtain a current copy of the floor plan. This can be obtained from the builder directly or the building inspector’s office where they were filed. You probably have a copy of the home’s plans buried in the paperwork that the builder gave you when the house building process began as well.
Make sure that the plan reflects any additions or changes that have been made since the construction process began for the home.
Look for the Scale
At or near the bottom of the plan, there will be a scale for the plan’s measurements, relative to the home’s actual size. This scale should be denoted in inches or fractions of an inch relative to a square foot.
Obtain Square Foot Calculation Standard
The American National Standards Institute (ANSI) is an organization that creates measurement standards for various industries. They have developed a standard method that can be used for calculating floor areas in a home called, "Square Footage—Method for Calculating." You can obtain a copy of this standard by contacting the National Association of Home Builders (NAHB). Note that this standard is only useful for single-family homes.
Check to see that your home builder follows this standard put out by ANSI. The standards are voluntary guidelines supported by NAHB but are not necessarily adopted by all home builders. If your home builder does not use the ANSI standard, contact them for a copy of the standard that they use to calculate floor plan square footage instead.
Calculate the Square Footage in the Floor Plan
Using the ANSI standards for calculating floor space, take a ruler and measure the livable or useable areas within the floor plan. This includes closets, attics, basements, and any other space that can be occupied. Using the scale and a calculator you can determine the square foot breakdown by room/space and also calculate the total floor plan square footage.