Usable SF = Rentable SF ÷ (1 + load factor). A 5,750 RSF suite in a building with a 15% load factor gives you 5,000 usable SF. The other 750 SF is your share of the building's common areas, and you pay rent on all of it.
The load factor is also shown as an R/U ratio. A 15% load factor is an R/U ratio of 1.15: every usable square foot is billed as 1.15 rentable square feet.
This is why two suites with the same RSF can feel like different sizes. When you're comparing options, compare usable square feet and rent per usable square foot, not just the number on the LOI.
What's a normal load factor?
It depends on the building. Single-story and single-tenant buildings tend to run low; multi-story office buildings with big lobbies and shared amenities run higher. Many multi-tenant office buildings land somewhere around 10–20%. Ask the landlord for their number and how it was measured.
Can the load factor change during my lease?
It can if the landlord remeasures the building or switches measurement standards. Your lease should say whether a remeasurement can change your RSF and your rent. That's worth negotiating up front.
Is this the same as BOMA measurement?
The calculator uses the same RSF and USF concepts as the BOMA Office Standard (ANSI/BOMA Z65.1), but every building's measurement is different. Treat it as a planning estimate, not a substitute for the landlord's measurement.