Speed Estimating Office Square Footage, by Corinne Maddox, CCM, CFM
One simple way to calculate square footage is to divide the existing number of office occupants
by the total number of occupied square feet. The average range will probably be 300-400 SF per
person for a private office layout, and 200-300 for an open plan layout. Multiply the projected
headcount by the SF per person for each year to determine total SF needed by year.
There are many factors that can affect the square footage requirements, not the least of which is
how square footage is defined. A precise space program can be built by multiplying the sum
square footage of the individual office and support spaces by a Circulation Factor to find the
Usable Square Footage, and multiplying this by the building Rentable to Usable Ratio to find the
Rentable Square Footage. Another factor must be applied to find building Gross Square Footage.
A spreadsheet can be easily set up to test all of the variables and space alternatives.
When calculating the office square footage, it will be necessary to project future employees by
office type/size, and estimate support space growth such as additional file and storage space and
additional conference rooms, copy rooms, coffee rooms, etc. A thorough inventory and careful
consideration must be made to accurately estimate the space requirements; however, a rough
estimate can be made by applying a percentage to the total office area. If space reduction is
critical, be sure to take into account space saving strategies such as high density files, off-site
conferencing and storage, and alternate officing solutions. Learn as much as possible about
corporate business plans that may affect requirements such as centralization/decentralization,
new markets or products, and economic trends. As facility manager, you need to take the lead in
extracting the necessary information for a good facility plan.
The circulation factor normally falls in the range of 1.25 to 1.4 depending on size of corridors and
amount of unusable space. A non-rectangular building or an irregular space plan will have a
higher circulation factor, as will a plan with smaller offices or cubicles. The Rentable to Usable
Ratio will usually range from about 1.1 to 1.2. This multiplier accounts for the tenant's share of
the common building spaces such as restrooms, mechanical and electrical rooms, common
corridors and lobbies. Rentable Square Footage of the building increases to the Gross Square
Footage by adding another 10 to 20% for vertical circulation elements. High rise buildings have
the largest Gross Square Footage due to the large amount of space required for the elevator
banks and stairwells.