Every number on your CAM reconciliation statement is multiplied by one figure: your pro-rata share percentage. Management fees, janitorial costs, insurance, landscaping, taxes. All of it runs through that single percentage before becoming your bill.
Get that percentage wrong, and every line item is wrong. Not by a fixed dollar amount. By a fixed percentage applied to every cost in the building's operating expense pool, every year.
I built CAMAudit because this error type appeared repeatedly in the audit cases I tested during development. It is one of the highest-value catches in the detection engine because the error is structural: it does not affect one line item, it affects every line item simultaneously. A 1% pro-rata share overstatement on a $200,000 CAM pool is $2,000 per year. Over a five-year lease, $10,000 from one input being wrong.
40% of CAM reconciliations contain material errors (Tango Analytics / PredictAP, 2023)
How pro-rata share is calculated
The formula is simple: your rentable square footage divided by the total rentable square footage of the building (or the defined CAM area under your lease).
Example: You lease 10,000 square feet in a 100,000 square foot building.
Pro-rata share = 10,000 ÷ 100,000 = 10.0%.
If total CAM expenses are $150,000, your share is $15,000.
Everything else being equal, any change to either the numerator or denominator changes your bill. The errors that matter most are the ones that make the denominator smaller or the numerator larger, because both of those increase your percentage.
Error Type 1: Wrong Denominator (Occupied vs. Total)
This is the most common pro-rata share error, and it tends to be the most expensive.
Your denominator should be the total rentable area of the building, regardless of whether all that space is currently occupied. Some landlords use the occupied area instead of the total area. This shrinks the denominator, increases your percentage, and increases your bill.
The math: Your space is 10,000 square feet. The building has 100,000 square feet total but is only 90% occupied (90,000 square feet leased).
If the landlord uses total area: 10,000 ÷ 100,000 = 10.0% If the landlord uses occupied area: 10,000 ÷ 90,000 = 11.1%
On a $150,000 CAM pool, that 1.1% difference costs you $1,650 per year. Over five years, $8,250. Over a ten-year lease, $16,500.
The error is more severe when the building has significant vacancy. A building at 70% occupancy that uses the occupied denominator produces a denominator-based pro-rata share that is 43% higher than it should be.
Your lease will specify which denominator applies. Look for language like "total rentable area of the building" or "total leasable area." If the lease says "occupied" or "leased" area, that is a different structure and needs to be evaluated on its own terms. But if the lease says "total" and the landlord is using "occupied," you have an overcharge.
Error Type 2: Wrong Tenant Square Footage
The numerator can be wrong too. If the landlord's billing system records your square footage incorrectly, your pro-rata share will be overstated by the same ratio.
Example: Your lease specifies 10,000 square feet. The landlord's system records 10,500 square feet.
Pro-rata share (correct): 10,000 ÷ 100,000 = 10.0% Pro-rata share (incorrect): 10,500 ÷ 100,000 = 10.5%
On a $150,000 CAM pool: overcharge of $750 per year.
This sounds like a small error but numerator errors often trace back to a data entry mistake that has never been corrected. The lease specifies one square footage and the billing system uses another. Nobody checks because the reconciliation gets paid, not audited.
Cross-check your pro-rata share percentage against your lease. Take the percentage on your reconciliation, multiply it by the denominator you calculate independently, and see whether you get your lease square footage back. If the number does not match your lease, something is wrong in the numerator.
Error Type 3: Failure to Update After Expansion or Contraction
Commercial tenants sometimes expand into additional space or give back space during their lease term. When that happens, the numerator changes. If the billing system is not updated to reflect the change, the pro-rata share continues to be calculated on the old square footage.
Expansion errors go both ways: if your space grew and the numerator was not updated, you are being undercharged (good for you, but creates a reconciliation liability). If your space contracted and the numerator was not updated, you are being overcharged.
The contraction error is more common in tenant-initiated right-size negotiations. The landlord agrees to take back 2,000 square feet, a lease amendment is signed, rent is adjusted, but the CAM billing system does not get updated. The tenant pays CAM on 12,000 square feet when they should be paying on 10,000.
To check this: look at the effective date of any lease amendments. Compare the pro-rata share percentage used in reconciliations before and after that date. If the percentage did not change when the square footage changed, the billing system was not updated.
Error Type 4: BOMA Measurement Inconsistency
BOMA (Building Owners and Managers Association) measurement standards define how rentable area is calculated. BOMA 1996 and BOMA 2017 use different methodologies for calculating rentable square footage, particularly for common areas that are allocated to tenants through "load factors."
If your lease specifies one measurement standard and the landlord calculates the denominator using a different standard, the square footage totals will not match. This is a less common error, but it shows up in older buildings where the original measurement was done under BOMA 1996 and the landlord has since recalculated under BOMA 2017 without renegotiating the lease.
This error is harder to catch without access to the floor plans and measurement methodology. The signature is a pro-rata share percentage that does not reconcile with the square footages you can independently verify.
Running the math on your own statement
Run this check on your current reconciliation:
Step 1: Find the pro-rata share percentage on your reconciliation statement. Note the exact number.
Step 2: Find your leased square footage in your lease. Note the exact number.
Step 3: Find the total building area your lease uses as the denominator. This is usually stated in the CAM section of the lease or in an exhibit. Note the exact number.
Step 4: Divide your lease square footage by the lease denominator. This is your lease-compliant pro-rata share.
Step 5: Compare to the percentage on your reconciliation. If they do not match, identify which number is different (numerator, denominator, or both).
Step 6: Calculate the overcharge. Subtract your lease-compliant percentage from the billed percentage. Multiply the difference by the total CAM expenses for the year. That is your annual overcharge.
Example walkthrough: Your lease says 10,000 sq ft. Your lease defines the denominator as total building area of 100,000 sq ft. Lease-compliant share = 10.0%.
Your reconciliation shows 11.1% pro-rata share. Total CAM = $150,000.
Lease-compliant charge: $150,000 × 10.0% = $15,000. Billed charge: $150,000 × 11.1% = $16,650. Overcharge: $1,650.
That is this year. Multiply by the number of years the same percentage has been used. If the denominator error has existed since lease commencement, the total is that annual figure times the number of years since you moved in.
How CAMAudit checks this (Rule 4)
CAMAudit's Rule 4 extracts three data points from your uploaded documents: your square footage from the lease, the denominator definition from the lease, and the pro-rata share percentage from the reconciliation statement.
It then recalculates the lease-compliant percentage and compares it to what was billed. If there is a variance beyond a de minimis rounding tolerance (typically 0.01%), Rule 4 flags it as a potential overcharge.
The system also checks whether the pro-rata share is consistent across multiple years if you upload multiple reconciliations. A percentage that changes between years without a corresponding lease amendment is a signal worth investigating. It could mean the landlord corrected an error partway through (which would mean you were overcharged in earlier years) or introduced a new error (which means you are overcharged now).
Rule 4 outputs the lease-extracted square footage, the denominator used, the correct percentage, the billed percentage, and the resulting annual overcharge calculation. That output feeds directly into the dispute letter draft.
"The pro-rata share error is the multiplier error. Every dollar of CAM expense runs through it. A small denominator mistake produces overcharges across every single line item, and most tenants have no idea because they look at individual expense amounts rather than the share percentage itself." — Angel Campa, Founder of CAMAudit
The denominator change problem
Some landlords recalculate the building's rentable area mid-lease, often as part of a BOMA remeasurement. If the remeasurement produces a smaller total building area (which happens when common areas are reclassified), your percentage goes up even though nothing changed about your space.
Whether the landlord can change the denominator mid-lease without your consent depends on your lease language. If the lease defines the denominator as a specific number (e.g., "100,000 rentable square feet"), that number is fixed. If the lease defines it dynamically ("total rentable area of the building as measured from time to time"), the landlord has more flexibility.
A mid-lease denominator change that increases your pro-rata share is worth challenging if your lease pegged the denominator to a fixed figure. The audit rights clause gives you the right to request the measurement methodology and the floor plans supporting the calculation.
Checking multiple years at once
Look at the pro-rata share percentage across all years you have been billed. If the percentage changed in a year when you did not sign a lease amendment, that is the first thing to investigate.
If the percentage has been consistent but wrong from day one, you need the full history to calculate your total exposure. Upload each year's CAM reconciliation to CAMAudit separately. Rule 4 runs on each one, and the findings include the year-specific overcharge. Add them up to see the total recovery potential.
The statute of limitations on written contracts in most states runs 3 to 10 years. For a structural error that has been running since lease commencement, the recovery window may be substantial.
The denominator is one number. One wrong number. But it is the number that everything else depends on.