Usable versus rentable area
Usable area is the space a tenant can actually occupy for offices, workstations, rooms, and internal circulation. Rentable area includes the usable area plus an allocation of common areas such as lobbies, corridors, restrooms, mechanical rooms, and other shared building spaces under the measurement method being used.
BOMA standards provide recognized ways to measure and allocate space, but a lease should state which standard and version applies. The same suite can have different rentable areas under different methods, building configurations, or landlord measurement practices.
- Usable square feet describes the tenant's physical premises.
- Rentable square feet is the billing area used for rent.
- Load factor connects the two values.
How to calculate
The basic formulas are load factor = rentable area / usable area, rentable area = usable area x load factor, and add-on factor = load factor - 1. If a tenant has 10,000 usable square feet and the building load factor is 1.18, rentable area is 10,000 x 1.18 = 11,800 rentable square feet.
If rent is quoted at $42 per rentable square foot per year, annual base rent is 11,800 x $42 = $495,600. The effective rent per usable square foot is $495,600 / 10,000 = $49.56, which is often the better number for comparing two buildings.
Why load factor changes deal economics
A lower asking rent can be more expensive if the load factor is high. For example, Building A at $40 with a 1.25 load factor costs $50 per usable square foot. Building B at $43 with a 1.12 load factor costs $48.16 per usable square foot.
Tenant improvement allowances, operating expense stops, parking ratios, and cleaning charges may also be tied to rentable area. That means load factor can affect more than base rent; it can influence the whole occupancy-cost model.
Who needs this calculation
Tenants use the calculation to compare proposals, check expansion options, and understand why a suite plan differs from a rent schedule. Brokers use it to normalize options across buildings. Landlords use it to explain building measurement and allocate shared area consistently.
Architects and space planners also need the distinction because programming is usually based on usable space, while budgets and leases often use rentable space. A planning error can make a space look affordable on paper but too small in practice.
Common mistakes
Do not call the add-on percentage the load factor unless you define it. A 20 percent add-on corresponds to a 1.20 load factor, not a load factor of 20. Also avoid comparing buildings using rentable square feet alone when the tenant's actual usable area differs.
Another mistake is assuming a quoted load factor applies to every suite. Floor common areas, building common areas, multi-tenant floors, single-tenant floors, and modified gross lease structures can all change the practical result.
Frequently asked questions
Is load factor the same as add-on factor?
They are related but not identical. Load factor is rentable area divided by usable area, while add-on factor is usually load factor minus 1, expressed as a percentage.
What is a good office load factor?
It depends on building type, floor plate, amenities, and market practice. The useful question is whether the effective cost per usable square foot is competitive for the space quality.
Can BOMA rentable area change during a lease?
It can if the lease permits remeasurement or if expansion, contraction, or building changes trigger a new calculation. The lease language controls the commercial result.
Should I compare rent on usable or rentable square feet?
Use both. Rentable square feet determines the invoice, but usable square feet shows what you actually occupy and helps compare alternatives.