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.