Two spaces quoted at the same price per square foot are not necessarily the same price per square foot of space you can use. The difference is the way the area is measured. This article explains the two measures, how they relate, and how to compare quotes fairly.
Usable area is, broadly, the space inside your suite that you occupy. Rentable area is usable area plus a share of shared building space such as lobbies, corridors, and common restrooms. Landlords bill rent on rentable area, which is why the rentable figure is nearly always larger than the usable one for a multi-tenant space.
The difference is often expressed in one of two ways. Some people quote an add-on percentage: rentable area is usable area plus a percentage. Others quote a ratio: rentable divided by usable. A 15% add-on is the same as a ratio of 1.15, but terminology varies, so ask exactly how the number is defined before you compare two buildings.
The Building Owners and Managers Association (BOMA) publishes floor measurement standards, including ANSI/BOMA Z65.1 for office buildings. BOMA's site lists the 2017 and 2024 editions, and its materials refer to different methods of allocating common areas. Older buildings may have been measured under older standards or under a landlord's own method. Whether any standard applies to your space is a matter of the lease, so ask what the lease says. We do not apply a standard in the calculator.
Suppose your suite has 2,000 usable square feet, and the building uses a 15% add-on. Rentable area is 2,000 × 1.15 = 2,300 square feet. At a quote of $30.00 per rentable square foot per year, annual base rent is $30.00 × 2,300 = $69,000. Per usable square foot that is $69,000 / 2,000 = $34.50. If another building offers $32.00 per rentable foot with a 25% add-on, rentable area is 2,500, annual rent is $80,000, and the cost per usable foot is $40.00. The building with the lower headline rate per rentable foot costs more per foot you can use. The figures are illustrations.
Enter the area on which rent is billed: the rentable square footage. If you want to compare cost per usable foot, divide the annual cost shown by your usable area by hand. Operating expenses are usually allocated on the same rentable basis, so keep the area consistent.
The BOMA office standard is written for office buildings, and other property types such as retail, industrial, and mixed-use are measured under their own conventions or the landlord's method. Retail, industrial, and other property types do not necessarily follow the office convention, so ask how any particular quote was measured. Because conventions differ, treat any assumption from one property type as a question to ask on another.
To compare a rate quoted per rentable foot with one quoted per usable foot, convert one of them. The rate per usable foot equals the rate per rentable foot multiplied by the ratio of rentable to usable area. With a rate of $30.00 per rentable foot and a ratio of 1.15, the rate per usable foot is $30.00 × 1.15 = $34.50, the same figure we found above. So a quote of $34.50 per usable foot in a different building is equivalent in rent per usable foot, even though the headline numbers look five dollars apart. Convert in the other direction by dividing by the ratio: $34.50 / 1.15 = $30.00.
If you plan around headcount or equipment, usable area is what fits your desks, shelves, and workflow. If you plan around the budget, rentable area is what you pay for. Knowing both numbers keeps your space plan and your cost plan from drifting apart. When a landlord offers to add or remove space later, ask whether the change is measured on the same basis as the original lease.
Try these three questions in order. First: "Is this area rentable or usable, and what is the load factor?" Second: "How was it measured, and can I see the floor plan with dimensions?" Third: "Is rent adjusted if the measured area changes, and who pays for a re-measurement?" The answers tell you whether the headline per-foot rate is comparable with the next quote.