Two offices are both advertised at 1,500 square feet. You visit them and one feels noticeably bigger. Nobody is necessarily lying: the figure on a listing or a lease can be measured in several different ways, and the difference between them is often 10 to 20 percent of the space you pay for. Knowing which number you're looking at helps you compare properly and negotiate from a stronger position.
The three numbers you'll meet
Gross area is the total floor area of a building or unit measured to the outside face of the external walls. It includes the walls themselves, stairwells, lift shafts, plant rooms and everything else. It's mainly used by developers, valuers and planners.
Rentable area (sometimes called net lettable or net internal area, depending on the country and the measurement standard) is the space a landlord charges rent on. It usually includes your private space plus a share of common areas such as lobbies, corridors and shared toilets.
Usable area is the space you can actually occupy: your offices, meeting rooms and kitchen, measured inside your own walls. It's the number that matters for fitting in desks, shelving or equipment.
The load factor
The gap between usable and rentable area is expressed as a load factor (or add-on factor):
Load factor = rentable area ÷ usable area
If a suite has 1,300 usable square feet and the landlord bills 1,500 rentable square feet, the load factor is 1,500 ÷ 1,300 ≈ 1.15. In other words, you pay for about 15% more space than you can put furniture on. Some people quote the same idea as a percentage: a 15% "common area factor".
Load factors vary with building design. A single-tenant building with few shared spaces may be close to 1.0, while a large multi-tenant tower with generous lobbies can be much higher. Neither is automatically bad. What matters is knowing the figure so you can compare cost per usable square foot.
Comparing two spaces fairly
Suppose you're choosing between:
- Office A: 1,500 rentable sq ft, load factor 1.20, rent 30 per sq ft per year
- Office B: 1,450 rentable sq ft, load factor 1.08, rent 32 per sq ft per year
First work out usable area:
- A: 1,500 ÷ 1.20 = 1,250 usable sq ft
- B: 1,450 ÷ 1.08 ≈ 1,343 usable sq ft
Then annual rent:
- A: 1,500 × 30 = 45,000
- B: 1,450 × 32 = 46,400
Finally, cost per usable square foot:
- A: 45,000 ÷ 1,250 = 36.00
- B: 46,400 ÷ 1,343 ≈ 34.55
Office B has the higher headline rent, yet it's cheaper per square foot you can actually use, and it gives you roughly 90 more usable square feet.
Homes and apartments
Residential listings have their own quirks. In some markets an apartment's advertised size includes a share of hallways or balconies; in others it's the internal floor area only. Some listings include a garage, a basement or a loft conversion without head-height limits, while others exclude them. Rules and conventions differ by country and even by city, so check what the agent or listing platform includes.
If the size really matters to you, for example because you're paying rent per square foot or checking whether furniture will fit, measure it yourself.
Measuring usable area yourself
- Sketch the floor plan and label each room.
- Measure inside walls, wall to wall, at floor level. Ignore skirting boards.
- Split awkward rooms into rectangles, work out each area, then add them up.
- Decide what to exclude: pillars, built-in cupboards and service risers usually aren't usable.
- Total the rooms and compare the result with the figure on the lease or listing.
For irregular layouts, running the room measurements through a square footage calculator saves time and cuts down on arithmetic mistakes, especially when you're converting between feet, metres and inches.
Questions to ask before signing
- Which measurement standard was used, and who measured it?
- What is the load factor, and which shared areas does it cover?
- Is the rent quoted per rentable or per usable square foot?
- If the building is remeasured later, can the rentable area (and your rent) change?
- Are balconies, terraces or storage areas included in the total, and are they charged at the same rate?
Key takeaways
A square-footage figure without context is only half an answer. Ask whether it's gross, rentable or usable; find the load factor; and compare spaces on cost per usable square foot. A few minutes with a tape measure and a calculator can reveal differences that a glossy listing hides.