Why Did My CAM Share Change? Understanding Pro-Rata Denominator Shifts

Your CAM bill jumped $8,000 from last year. Your space is the same. Your lease terms haven't changed. The total expense pool didn't grow by anything close to that. So what happened?

The answer is almost always the denominator.

In commercial CAM reconciliation, your share is calculated as:

Your RSF ÷ Total Leasable SF = Pro-Rata Share %

The "Total Leasable SF" is the denominator. When it decreases — even slightly — every remaining tenant's share increases, even if the expense pool and your square footage stayed identical. On a $200,000 annual CAM pool, a denominator that shrinks by 5% adds $10,526 to the total amount allocated to remaining tenants. Each tenant pays more with no change in their space or the actual expenses.

This is the most common reason tenants see unexpected CAM increases. It is also one of the most frequently overlooked errors in CAM reconciliation reviews.

1. Anchor tenant vacancy

When a large tenant — a department store, grocery anchor, or major office occupant — vacates, the remaining tenants in the property absorb a larger percentage of the total CAM pool. A tenant previously at 8% of the building might jump to 11% after an anchor departure.

If the anchor's space is excluded from the denominator after departure but the anchor-area expenses remain in the CAM pool, remaining tenants pay for space they do not benefit from. Some leases protect against this through "anchor exclusion" provisions that require the anchor's expenses to be removed from the pool if the anchor is removed from the denominator.

Anchor vacancy increases are often contractually legitimate — the lease may not protect against them. The key question is whether the denominator and expense pool are being handled consistently. If the anchor is excluded from the denominator but not from the expense pool, that is a potential overcharge.

To see the math: a 200,000 RSF retail center has one anchor occupying 60,000 RSF. A secondary tenant at 10,000 RSF has a 5% share. The anchor vacates. If the denominator drops to 140,000 RSF (occupied only), the secondary tenant's share jumps to 7.14% — a 43% increase in their allocation percentage with no change in their space.

2. Building remeasurement — BOMA standard change

When a building is remeasured under a new or updated BOMA standard, the total rentable SF changes — which changes every tenant's denominator. BOMA 2024 added outdoor amenities, reclassified balconies and rooftop terraces, and simplified circulation allocation in ways that typically increase total building RSF by 2–5%.

A larger denominator means a smaller pro-rata share for existing tenants, assuming the expense pool and individual space measurements stay the same. But if only the building total was remeasured while individual tenant spaces remain at their original lease measurements, the shares no longer sum to 100%.

A landlord adopting BOMA 2024 when your lease specifies BOMA 2017 may not be entitled to change your denominator without a lease amendment. Check what measurement standard your lease references. For a detailed breakdown, see BOMA 2024 vs 2017: how measurement changes affect your CAM charges.

3. Lease amendment — your space changed

If you recently expanded your space or an adjacent tenant expanded into adjacent vacant space, the RSF figures in the denominator and possibly the numerator have changed. This is a legitimate change — but the mid-year adjustment must be time-weighted correctly.

Yes, if the SF change reflects a signed lease amendment and the time-weighting is correct. An amendment executed July 1 should affect only the second half of the year's allocation, not the full year.

4. Lease expiration — tenant rollover

When tenants leave and new tenants arrive, the composition of the denominator changes. Some landlords use "occupied" SF rather than "total leasable" SF in the denominator, meaning vacant space does not dilute shares. If a neighboring tenant's lease expires and the space sits vacant, and your lease uses an occupied-only denominator, your share increases.

It depends on your lease's denominator definition. If the lease says "total leasable area," vacant space should remain in the denominator. If it says "occupied" or "leased" area, vacancy legitimately increases your share. This is one of the most important denominator definitions to negotiate before signing a lease.

5. Building expansion or renovation

If new leasable space was added to the property — through construction, conversion of common areas, or building expansion — the denominator increases. This dilutes each existing tenant's share.

Expansions generally increase the denominator legitimately, but the timing matters. New space should be added to the denominator only when it is rentable, not when it is still under construction.

The formula is simple. Getting the right inputs is the work:

Your RSF ÷ Total Building RSF (correct denominator) = Pro-Rata Share %

Pull your lease and find the rentable square footage of your space. This should match the figure in the reconciliation's numerator. If it does not, that is a finding.
Read your lease's pro-rata share definition carefully. It should specify what goes in the denominator — total leasable area, occupied area, net rentable area per BOMA, or something else.
Ask the landlord for the current tenant roster with each tenant's RSF. Verify that the sum of tenant RSF figures plus any common area classifications equals the denominator used in the reconciliation.
Calculate your percentage using steps 1–3 and compare to the percentage shown on the reconciliation. A discrepancy of more than 1% on a large expense pool is material and worth disputing.

When a denominator change is legitimate vs. an error

Scenario	Legitimate?	What to check
Anchor vacancy, denominator reduced	Depends on lease	Is the expense pool also reduced proportionally?
BOMA remeasurement, denominator increased	Depends on lease	Does lease authorize BOMA standard change?
New tenant added mid-year	Yes, if time-weighted	Is the SF addition time-weighted to the correct effective date?
Space expanded per lease amendment	Yes	Is the time-weighting correct for the amendment date?
Vacant space removed from denominator	Depends on lease	What does the lease say — "total leasable" or "occupied"?
Denominator reduced with no explanation	Possibly error	Request documentation of what changed and why

What to do if you suspect a denominator error

Ask the landlord in writing for the denominator figure used in the reconciliation, the basis for that figure (measurement certificate, rent roll, or other source), and an explanation of any change from the prior year's denominator.
Determine what your share would have been under the correct denominator and multiply by total CAM expenses to quantify the overcharge.
Dispute in writing within your dispute window. A denominator error affects every line item in the reconciliation — it is one of the most impactful errors to identify and one of the clearest to document.
Request prior-year reconciliations if the denominator has been wrong for multiple years. The overcharges compound, and your lease's audit rights clause determines how far back you can go.

For the full verification process, see the CAM reconciliation review checklist. For the CPA audit framework that covers pro-rata share errors systematically, see CAM reconciliation for CPAs.