Calculating Pro-Rata Share (And Why the Denominator Matters)

A tenant pro-rata share is the percentage of recoverable expenses it pays. The standard calculation is straightforward; the arguments are all about what goes in the denominator.

The basic calculation

share = tenant rentable sq ft / building rentable sq ft

A 12,400 square foot suite in a 148,000 square foot building carries an 8.38% share.

Both figures must be rentable, not usable, and both must use the same measurement standard.

Three ways leases express it

Pro-rata by area is the standard calculation above.

Fixed percentage is a negotiated figure stated in the lease that does not move even if areas are remeasured. Common for anchor tenants and in buildings with unusual configurations. It protects the tenant from denominator changes entirely.

Fixed amount is a flat dollar recovery, sometimes with its own escalation. Technically not a share, but it occupies the same position in the calculation.

Record which method applies per lease. The three produce materially different results, and the choice is rarely revisited once signed.

The denominator problem

Whether the denominator is total building area, total leasable area, or total leased area changes every tenant share — and the third one is a genuine trap.

Using leased area as the denominator means vacancy is redistributed onto sitting tenants. At 80% occupancy, a tenant with a nominal 8.38% share suddenly carries 10.48%. Their charge rises purely because the landlord has vacancy.

This is the problem gross-up is designed to solve, and it solves it more transparently — by normalizing costs rather than inflating shares. A lease that both uses leased-area denominators and grosses up is double-counting vacancy, and a tenant auditor will find it.

Use total leasable area as the denominator, and handle vacancy through gross-up.

State the denominator in the lease

The building rentable area should be a stated figure in the lease, not something recomputed later.

A denominator that quietly shrinks — because a portion was reclassified as non-leasable, or a new measurement standard was adopted — raises every tenant share without anyone renegotiating anything. This is among the most common findings in a tenant CAM audit, and it is usually inadvertent rather than deliberate.

If the building is remeasured, the lease should say what happens: whether shares adjust, whether they are frozen, and whether the tenant has any right to object.

Different shares in different pools

A tenant can hold different shares in different recovery pools.

A ground-floor retail tenant in a mixed-use building might carry a full share of site maintenance and parking lot costs, but no share of elevator maintenance or upper-floor HVAC. An anchor tenant might be excluded from certain pools entirely.

Modeling shares per pool rather than per lease is what makes this work without side calculations. A single share applied to a single undifferentiated pool cannot express it, which is why those portfolios end up with a spreadsheet alongside the software.

Verify against the rent roll

A useful periodic check: sum the pro-rata shares of all leases in a pool. If they exceed 100%, something is wrong — usually a stale denominator or a lease whose area was updated without the building total being updated.

If they fall well short of 100%, that gap is vacancy, and it should be consistent with what gross-up is normalizing. Run the numbers in the CAM reconciliation calculator.