I was recently on site with one of my clients looking at a space that his company was thinking of leasing.  As we walked the site, he gave me the run down that the realtor had given him: the overall square footage, what kind of use was there prior, when the latest remodel took place, etc.  Once we had finished our walk-through, I got out my laser measure and began to create the As-Builts.  An As-Built is a document that is created by taking the existing measurements of a building, or space, and drawing out the floor plan and/or the elevations.  Architects create these As-Builts when no plans exist and these become the background upon which the project can move forward.  After a while of measuring he asked me if I thought that there was roughly 8,000 sf (the space listed at 7,948 sf.)  Footprints of this size are too big to make assumptions and I told him that I felt like we were close but I could not be certain until I had time to enter the measurements into my program and create the plan.  As it turns out, there was only 7,110 sf.  So why do we have 2 different numbers?  Well, there are 2 different ways of measuring a space (or building) and each are dictated by standards.  One is the IBC (International Building Code) and the other is BOMA International (Building Owners and Managers Association) where one deals with Life Safety and the other with Rentable Area.


Figure 1 – IBC Code Plan

International Building Code

The IBC is the building code.  This manual sets forth the rules, restrictions, and exemptions that an architect must design by and a contractor must build by.  If you are dealing with either one of those who is not using this, I highly recommend that stop work immediately and pursue one who does.  Obtaining the square footage for the code is quite simple – measurements are taken from wall to wall, utilizing the greatest overall spans first, then back filling as necessary.  Architects should be taking note of door, window and column locations along with the walls.  And that’s it – the square footage is then determined by the area enclosed.  That’s the how – now for the why.  The IBC’s parameters for square footage deals specifically with life safety and how an individual will occupy the space.  The space I was walking with my client was an Office Space and it is classified in the IBC as a “B” Use, or occupancy, and allows for 100 square feet per occupant.  Using the 7,110 measured square feet from above, I got an occupant load of 71.1 – rounded up – becomes 72 occupants in the space.  Knowing this occupant load, the architect can now figure out how many exits are required, how wide corridors, stairwells, ramps and exits need to be along with several more requirements.  This numbers tells us the maximum number of people who can safely occupy a space with the code minimum prerequisites.  Having this information will now allow an architect to correctly design the space and the contractor to build the space but it still does not fully explain why we have the 2 different numbers.


Figure 2 – BOMA Floor Plan

Building Owners and Managers Association

The second type of measurement comes from the BOMA standards and its primary objective has to do with obtaining Rentable Square Feet (RSF).  Basically it is looking at the building as a whole and how to allocate square footage of building and floor common areas that are shared by the tenants to each tenant’s lease.  This method employs the same measurements as the IBC but then takes it a step further.  BOMA measurements are taken from the centerline of demising walls and the face of glazing at exterior windows – just to name a few.  Even the restrooms, mechanical rooms, and janitor’s closet are all taken into consideration to obtain square footage.

Notice the Square Footage of Office (Figure 1) and Tenant 102 (Figure 2).  This is the same building, same space yet we have a difference of roughly 240 square feet.  And this is only the beginning!  Once you have identified these square footages, you then take these numbers and, through a long process of various calculations, you apply factors to areas in order to get to your Rentable Square Foot number.

These are the 2 main area calculations that are used in Commercial construction and leasing.  If you would like more information regarding these calculations, please click here