Rentable vs Usable Square Feet

The distinction between rentable square feet and usable square feet is the most under-explained number in a commercial lease. The tenant pays rent on rentable square feet. The tenant actually occupies usable square feet. The difference — the load factor — can be 10% to 25% in modern office buildings, and it directly affects how much money changes hands.

The two measurements

Usable square feet (USF) is the area inside the leased premises — the space the tenant can actually furnish, occupy, and use. Measured to the inside face of demising walls. It excludes everything outside the demised space.

Rentable square feet (RSF) is the usable square feet plus the tenant's pro rata share of certain common areas — typically lobbies, lift lobbies, common corridors, central restrooms, plant rooms, and (depending on the standard) some shared amenity space. Measured per a defined standard, of which the dominant ones in international-grade buildings are BOMA (Building Owners and Managers Association) in the US, IPMS (International Property Measurement Standards) globally, and various local equivalents.

The ratio RSF / USF is the load factor (sometimes called the "add-on" or "core factor"). A 15% load factor means rent is paid on 1.15 square feet for every 1 square foot the tenant uses.

Why the load factor varies

Three reasons:

Building configuration. A single-tenant floor has a low load factor — corridors and floor restrooms are dedicated to that tenant, so they fold cleanly into usable area. A multi-tenant floor has a high load factor because corridors, lift lobbies, and shared restrooms are pro-rated across multiple tenants. Older buildings with central core configurations and large mechanical floors carry higher load factors than efficient newer designs.

Measurement standard. BOMA 2017 (the current US office standard) treats certain shared amenity spaces (rooftop gardens, conference centres) as common area to be loaded across tenants. BOMA 1996 — still used in older leases — has a narrower common-area definition. IPMS 3 — used in many international markets — treats some areas differently from BOMA. A switch from BOMA 1996 to BOMA 2017 in the same building can shift rentable area by 3% to 6% with no physical change.

Negotiation. The landlord defines what counts as common area; the tenant audits it. If the landlord has loaded an unusual amount of amenity space onto the rentable area, the load factor balloons and the rent-per-usable-foot rises.

How load factor changes the economics

A simple example. A tenant signing for 10,000 USF in a building with a 15% load factor pays rent on 11,500 RSF. At a face rent of US$100 per RSF per year, that is US$1.15 million per year — but per USF the tenant is actually paying US$115. The "real" rent per usable square foot is what the tenant should compare across competing offers, not the headline RSF number.

Tenants comparing two buildings with very different load factors (say 12% and 20%) can be deceived by the headline rent per RSF. The lower-RSF-rent building may actually be more expensive per USF.

What to verify in a lease

Three concrete steps before signing:

Get the building's measurement standard in writing, including the year and version. "BOMA 2017" is precise. "Industry standard" is not.

Get the actual measurement of both USF and RSF, with the load factor stated. This may seem basic but landlords sometimes quote only RSF.

Reserve audit rights. The tenant should have a defined right to commission a measurement (at the tenant's cost, refundable if the landlord's number is materially overstated) within a defined window after lease commencement. A 2% to 3% measurement discrepancy is not unusual; anything above that is worth a rent adjustment.

APAC measurement quirks

Hong Kong uses gross floor area (GFA), constructible floor area, saleable area, and other official terms with regulatory roots in the Buildings Ordinance. Office leases historically quoted gross area which included a substantial efficiency loss — load factors of 25% to 35% were common in older Grade-A stock. Newer buildings in Central, Causeway Bay, and Kowloon East have moved closer to international standards. The Lands Department's Code of Measuring Practice provides a baseline.

Singapore office leases typically quote a "lettable area" approximate to RSF, with measurement standards close to BOMA. The Urban Redevelopment Authority's gross floor area concept is regulatory rather than commercial.

Japan uses tsubo (1 tsubo ≈ 3.31 m² ≈ 35.6 sq ft) as the dominant unit, which is independent of the rentable / usable distinction. Tokyo office leases historically had inefficient core configurations and higher effective load factors than US equivalents; international-grade towers in Marunouchi, Otemachi, and Roppongi are now closer to global norms. Reading "rentable" vs "actual occupiable" tsubo is the equivalent of the US RSF / USF check.

When this matters most

The rentable / usable distinction matters most in three scenarios:

A comparative analysis of two competing leases — always normalise to USF rent.

A portfolio aggregation across markets, where leases use different measurement standards. Convert everything to a single standard (USF, or a normalised IPMS metric) before computing portfolio-level metrics.

A midterm audit of a long-running lease — buildings sometimes re-measure after a major renovation, and the new RSF can change rent if the lease references the building's "as-measured" area.

If you are aggregating leases across HK, SG, JP, and US markets, the RSF / USF / tsubo conversions are the silent killer of portfolio comparability. LeaseTrace extracts the measurement standard, USF, RSF, and load factor per lease so the conversion math is at least visible on a single line.