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 have not changed. The total expense pool did not 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.
When your CAM share jumps with no change in your space or total expenses, the denominator moved. Five scenarios drive most of these shifts: anchor vacancy, building remeasurement, lease amendments, tenant rollover, and building expansion. Only some of these are legitimate under your lease language.
40% of commercial CAM reconciliations contain material billing errors, with pro-rata denominator changes among the most common undetected causes (Tango Analytics, 2023)
The 5 reasons your CAM share changes
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 to 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.
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.
An amendment executed July 1 should affect only the second half of the year's allocation, not the full year. If the landlord applied the new SF for the full year, the overcharge covers the first six months.
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.
The legitimacy depends entirely 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.
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.
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 |
30% of CAM statements contain errors that cost tenants money, with denominator shifts among the least-challenged error types (Springbord Research, 2024)
How to verify your current pro-rata share
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 through 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.
For the broader context of denominator types and their dollar impacts, see the pro-rata share calculation error guide. For the complete set of formulas used in a full CAM audit, see CAM overcharge detection formulas.
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 related error types that often appear alongside denominator changes, see CAM proration errors and admin fee overcharges in CAM statements. For recovery guidance once the error is documented, see CAM recovery.
Frequently Asked Questions
Why did my CAM charges increase when my space did not change?
The most common cause is a denominator change: the total rentable SF used to calculate your pro-rata share decreased, which automatically increases your percentage share. This can result from another tenant vacating, a building remeasurement, or a configuration change in the property management system. The second most common cause is a genuine increase in total expenses. Check the denominator first.
How much can a pro-rata share change from a denominator shift?
Significantly. On a 200,000 RSF building, a 10,000 SF anchor vacancy with an occupied-only denominator moves every remaining tenant's share by approximately 5.6%. For a tenant paying $40,000 in CAM annually, that is a $2,240 increase with no change in expenses or their own space. Anchor departures are the single largest source of unexpected CAM share increases.
Is it legal for my landlord to change the denominator without telling me?
A landlord does not typically need to notify you before changing the denominator, but the change must be permitted by your lease. If your lease uses 'total leasable area' as the denominator, a landlord cannot switch to 'occupied area' without a lease amendment. If your lease specifies a BOMA measurement standard, the landlord generally cannot apply a different standard unilaterally. Whether a particular change is permitted depends on your specific lease language.
How do I find out what denominator my landlord used?
The reconciliation statement should show the denominator used to calculate your pro-rata share. If it does not show the total building SF, request it from the landlord in writing. Also request the rent roll or tenant roster showing each tenant's RSF so you can verify the total independently.
Can I dispute a pro-rata share change?
Yes. If you believe the denominator used in your reconciliation is inconsistent with your lease's pro-rata share definition, you can dispute it in writing within your lease's dispute window. Document the correct denominator per your lease, calculate the correct share, and quantify the overcharge. Send the dispute letter before the deadline specified in your lease.
When should I worry about anchor vacancy changing my CAM share?
Any time an anchor tenant leaves your shopping center or office building and your CAM share increases, check whether your lease's denominator definition allows that increase. If your lease uses 'total leasable area' as the denominator, the anchor's vacant space should remain in the denominator. If the landlord dropped the anchor from the denominator while keeping anchor-area costs in the expense pool, that is a recoverable overcharge.