Direct answer
Usable, gross, internal, external, and rentable floor areas are not interchangeable labels. An efficiency ratio is meaningful only when both numerator and denominator follow documented definitions from the same applicable measurement basis.
What this calculation tells you
The usable-to-gross comparison shows how much of an entered gross area is represented by an entered usable area. It can help compare layouts or reconcile schedules when both figures are prepared on compatible definitions.
It cannot decide whether walls, columns, balconies, voids, common areas, amenities, parking, plant, or vertical penetrations count. Nor can it transform a marketing area into a standards-compliant measured area.
Where it is used
Leasing
Understand the area basis behind rent and service-charge allocations.
Development design
Track efficiency as layouts change without confusing gross and usable scope.
Valuation and transactions
Check that comparable price-per-area evidence uses a compatible denominator.
Asset records
Reconcile drawings, surveys, tenant schedules, and management systems.
Common situations
- A brochure area differs from a survey or lease schedule.
- Two buildings report similar gross area but different usable area.
- Price per square metre is compared across incompatible area definitions.
- A conversion from square feet to square metres appears to create a discrepancy.
State the measurement instruction first
Record the purpose, property extent, adopted standard, measurement date, methodology, plan source, unit, rounding, and responsible party. If another basis is required by a lease, planning authority, valuation instruction, or local practice, name it explicitly.
RICS currently directs members and regulated firms to continue using the sixth edition of its Code of Measuring Practice and/or IPMS: All Buildings until the next final edition, adopting the appropriate basis for the instruction and documenting that basis. The broader lesson is universal: a number without its basis is incomplete.
Build an inspectable area schedule
Organize each floor and component rather than publishing one total. Show included and excluded spaces, deductions, common areas, vertical penetrations, external areas, and any conversion or apportionment. Cross-reference the schedule to plans.
When an area changes, the schedule should reveal whether geometry changed, a classification changed, or an error was corrected. That distinction matters for rent, density, valuation, service charges, and development reporting.
Use ratios as comparisons, not classifications
Usable-to-gross efficiency can help compare designs, but a higher ratio is not automatically better. Circulation, structure, amenities, plant, accessibility, fire strategy, daylight, servicing, and user needs may justify space outside the usable numerator.
Similarly, rentable area and load factors depend on the governing convention. Never create a rentable area by applying an unexplained market factor to usable area.
Reconcile units and bases separately
Unit conversion is mathematical: square feet and square metres can represent the same area. Measurement-basis reconciliation is definitional: two correctly converted figures can still differ because they include different spaces.
Investigate discrepancies in this order—property extent, standard, inclusion rules, plan version, field measurement, unit conversion, then rounding.
- Do not compare price per area across unlike denominators.
- Do not describe gross area as usable area after a unit conversion.
- Do not use an efficiency ratio as a planning or code check.
- Do not hide excluded spaces or unexplained schedule differences.
Practical questions
Frequently asked questions
Can I convert gross area into usable area with one percentage?
Only as an explicitly labeled scenario. The actual relationship depends on the building and the adopted definitions; it should be measured or scheduled rather than assumed.
Why do two professional reports show different areas?
They may use different standards, purposes, property extents, dates, plans, inclusion rules, or measurement evidence. Reconcile those items before deciding one is wrong.
Is rentable area the same as usable area?
Not necessarily. Some leasing conventions allocate common areas or use other rules. Use the definition stated in the applicable lease or measurement standard.
Further reading
Authoritative sources
Use these primary and professional resources to check definitions, conventions, or requirements that may extend beyond this guide.