How Do I Find the Square Footage of a Building?

Building Area

Building Area is defined by building codes and is the allowable size of the building based on fire hazard and type of construction. The measurement should not be used in lease agreements or for calculating rent payments. Always be sure to check your local code for the proper definition in your jurisdiction.

According to the 2018 International Building Code, Building Area is defined as:

The area included within surrounding exterior walls, or exterior and fire walls, exclusive of vent shafts and courts. Areas of the building not provided with surrounding walls shall be included in the building area if such areas are included within the horizontal projection of the roof or floor above.

Building Area is the footprint of the building measured to the inside face of the exterior walls. All enclosed spaces are included and deductions are not made for mechanical shafts, vertical circulation, interior walls, or structural elements. Internal courtyards that are open to the sky above are excluded from Building Area.

What Is Included In The Square Footage?

In measuring the square footage of a house, it is crucial to know what can and can’t be included in the calculations. Not every foot of your home enclosed by walls will count towards total square footage. Instead, you are trying to determine the gross living area — or the livable parts of your home. Keep reading to learn more about the specifications for measuring square footage:

Height Requirements

There is one measurement far too many inexperienced “appraisers” forget about: ceiling height. That’s not to say you measure the area as a three-dimensional space, but rather that the ceiling is one of the criteria I already alluded to. You see, for an area’s square footage to count in the home’s overall square footage, the ceiling above it must be a certain height. According to ANSI’s American National Standard For Single-Family Residential Buildings, finished areas must have a ceiling height of at least seven feet, “except under beams, ducts, and other obstructions where the height maybe six feet and four inches.” On the other hand, Angled ceilings must rest at the previously discussed seven feet for at least half of the room’s total floor area. If the ceiling is at least seven feet for at least half of the room’s floor area, total square foot calculations should include every area where the ceiling is at least five feet tall.

Garages, Protrusions, and Unfinished Areas

No matter how much you may wish your garage was included in the total square footage of your house, it’s not. I repeat, garages are not included in the total square footage of a property, even if they are finished — that’s because they are not the same level as the home itself. Similarly, chimneys and window areas are not included in a home’s square footage; not only are they not finished, but they are not on the same level.

Finished Home Connections

If you have a finished area connected to the house by a finished hallway or stairway, the subsequent area may be included in the home’s total square footage. However, finished areas connected in any other way (like by an unfinished hallway or staircase, for instance) won’t be included in the home’s total square footage.

Basements & Attics

Basements do not typically count towards a home’s gross living area regardless of whether they are finished. Since they are below the rest of the home, basements can’t be included in the total square footage. That said, homeowners may note the size of a finished basement in a respective listing elsewhere. On the other hand, attics may be counted in a home’s total square footage if they are finished and meet the height requirements stated above.

Covered, Enclosed Porches

Covered, enclosed porches may be included in a home’s gross living area if they are finished, and they are heated using the same system as the rest of the house.


Net Assignable Area measured as Net Assignable Square Feet (NASF)

Net Assignable Area is used when calculating areas in post-secondary educational institutions (colleges, universities, related research buildings, etc.) using the FICM. Simply stated, NASF is the sum of spaces that can be assigned to people or programs within the ten major space use categories (classrooms, laboratories, offices, study areas, special use space, general use areas, support rooms, healthcare, residential, and unclassified space.) Excluded from NASF are the three non-assignable space categories (building service area, circulation area, and mechanical area) that are required for the operation of the building.

Net Assignable Area is measured to the inside face of each individual space, so wall thicknesses are excluded from the calculations – wall thicknesses are considered part of the Structural Area of the building or space.

Do not make deductions for structural columns or projections. It would take more effort than it is worth to exclude them from the calculations since these elements are such a small percentage of the overall space in the building.

The following images depict Net Assignable Area in purple:

How to Calculate Square Footage

Square footage is area expressed in square feet. Likewise, square yardage is area expressed in square yards.  Square meters is also a common measure of area.

Assume you have a rectangular area such as a room and, for example, you want to calculate the square footage area for flooring or carpet.

The way to calculate a rectangular area is by measuring the length and width of your area then multiplying those two numbers together to get the area in feet squared (ft2). If you have on oddly shaped area, such as an L-shape, split it into square or rectanglualar sections and treat them as two separate areas. Calculate the area of each section then add them together for your total. If your measurements are in different units, say feet and inches, you can first convert those values to feet, then multiply them together to get the square footage of the area.

Convert all of your measurements to feet

  • If you measured in feet skip to “Calculate the Area as Square Footage”
  • If you measured in feet & inches, divide inches by 12 and add that to your feet measure to get total feet
  • If you measured in another unit of measure, do the following to convert to feet – inches: divide by 12 and that is your measurement in feet – yards: multiply by 3 and that is your measurement in feet – centimeters: multiply by 0.03281 to convert to feet – meters: multiply by 3.281 to convert to feet

Calculate the Area as Square Footage

  • If you are measuring a square or rectangle area, multiply length times width; Length x Width = Area.
  • For other area shapes, see formulas below to calculate Area (ft2) = Square Footage.

What Is Included in the Total Square Footage Calculation?

What is the total square footage of a home’s living space? This answer is often a misrepresentation of what the actual home size is. Your calculation may include the finished areas of your home, such as the bedrooms, kitchen, and living rooms, but what about the unfinished areas, such as the basement or attic?

You need to consider what areas of the home you plan to include in the total square footage calculation and what areas you leave out. A space is unlivable if the homeowner will not be spending a regular amount of time there due to restrictive environments, such as an attic with a sloping ceiling or an unfinished basement with exposed concrete walls, framing, or wiring.

The finished space in a basement or attic is considered much less expensive than the finished footage on the main floor of the home. Why, you may ask? The foundation, walls, and roof are already set in place for a basement and attic. It is also assumed the people living in the home will get more use out of the main living areas. If the price per square foot of the home was calculated based on “finished” square footage, that figure is typically lower for homes with finished basements or attic spaces since homeowners will not be spending

What about a home’s patio enclosure or three-season room? This might include a screened-in porch or deck. Those areas may be considered livable areas of the home, especially if the roof covers those outdoor hybrid spaces. However, square footage metrics often leave these areas out, which dilutes the total value of the home in appraisals.

Typically, garage space is not counted towards the home’s livable square footage metric, either. This is troubling since garages are not free to build. In theory, they should be reported in a home’s square footage calculation to estimate the total value of a home. A four-car garage should impact the price per square foot as compared to a one-car garage since the value is higher, right? You would think, but unfortunately this is not the case for many builders.

What to leave out

A good rule of thumb to ensure you’re taking proper measurements is to exclude space you can’t walk on or live in. These types of spaces do not count as “gross living area.”

“Someone might think, ‘If I get the measurement of my first floor and I have a two-story house, I just multiply that by two,’” Day says. However, if that first floor includes a two-story foyer, you can’t count the non-usable space.

Basements and garages, even if they are finished, don’t generally count toward total square footage. Basements are typically excluded because they are built below grade, meaning below ground level. If your state does allow basements to be included in the total square footage of a home, though, you’ll likely need an ingress and egress, or a safe way to enter and exit the basement to the outside.

Finished attic spaces — with some regulations, including ceiling heights — can count toward the total square footage of your home. If you are planning to sell your home, work with a real estate agent to craft a listing that accurately reflects your property.

3. What is Net Assignable Square Feet?

Net Assignable Square Feet (“NASF”) is the sum of

Net Assignable Square Feet (“NASF”) is the sum of all areas that are assigned to (or available for assignment to) an occupant for specific use. Examples of assignable space include classrooms, laboratories, offices, study areas, residential areas, general use rooms and special use rooms. These rooms are areas where people gather to accomplish a task.

A rule of thumb is if an area allows occupants to accomplish part of their institution’s mission, then it is most likely included in net assignable square feet. Examples of areas that would not be included in net assignable square feet are stairwells, hallways, elevator shafts and closets. These areas are not included in NASF because they are not assigned for occupant use.

Having an accurate measurement of NASF can help to determine the best spaces to allocate to a department. NASF can also help facility managers assess revenue for leased spaces and determine staffing needed to keep the area functioning correctly.

How to Calculate Net Assignable Square Feet

Determine your building’s net square feet. Then, subtract areas that do not have a specific, assigned use for occupants (i.e. stairs, hallways, closets etc.). This number is your net assignable square feet, because it measures all areas in your facility that people can actually use for activities on a day-to-day basis.

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