How to Calculate Your Pro-Rata Share (And Verify What Your Landlord Charged)
Pro-rata share is the percentage of building-wide CAM costs you owe. In its simplest form it's just your square footage divided by the building's total leasable area. But that "simple" calculation has two variables — your numerator and your denominator — and landlords make errors in both.
The most common error is the denominator. Your lease defines exactly which square footage belongs in the bottom of that fraction. Some leases use total leasable area. Others use only occupied space. A few exclude anchor tenants who negotiated their own CAM terms. Use the wrong denominator and your percentage goes up — sometimes by a full percentage point or more — and you overpay every year without knowing it.
40% of CAM reconciliations contain material errors (Tango Analytics/PredictAP, 2023) A wrong denominator is one of the most common sources of those errors, and it compounds: if you sign a 5-year lease and never check, you may overpay on every annual reconciliation.
7 Steps to Calculate and Verify Your Pro-Rata Share
1. Find your rentable square footage in the lease
Open your lease and locate the section that defines your premises. It will state your rentable square feet (RSF) — for example, 4,800 RSF. This is your numerator. Do not use usable square feet (USF), which excludes common area load. The lease will be explicit about which measurement applies to CAM calculations.
2. Find the denominator your lease specifies
This step is critical. Scan your CAM exhibit and the definitions section of your lease for language describing the denominator. You're looking for one of these:
- Total leasable area (TLA): All rentable square footage in the building, whether occupied or vacant. Most favorable to tenants in partially occupied buildings.
- Occupied area: Only the square footage currently occupied by paying tenants. Shrinks when occupancy drops, which raises your percentage — this is the version that hurts tenants.
- Defined denominator: Some leases hard-code a number (e.g., "the Building contains 48,200 rentable square feet for purposes of this calculation"). This is the most predictable version — it doesn't fluctuate.
If your lease specifies an occupied-area denominator, check also for a gross-up provision, which may partially offset the impact.
3. Pull the property's total square footage
Your lease should state the building's total rentable area. If it doesn't, or if you suspect the number is wrong, cross-reference with the county assessor's property record or the building's certificate of occupancy. A discrepancy between the lease-stated total and the actual building size is a legitimate basis for dispute.
4. Run the calculation
Formula: Your RSF ÷ Denominator RSF = Pro-Rata Percentage
Worked example:
- Your space: 4,800 RSF
- Building total leasable area: 48,200 RSF
- Correct pro-rata share: 4,800 ÷ 48,200 = 9.96%
Now apply the wrong denominator — an occupied-area figure of 42,500 RSF:
- 4,800 ÷ 42,500 = 11.29%
On $140,000 in annual CAM costs, that difference is:
- Correct charge: $140,000 × 9.96% = $13,944
- Inflated charge: $140,000 × 11.29% = $15,806
- Annual overcharge: $1,862
Over a 5-year lease, that's $9,310 in excess CAM payments — from a single denominator error.
5. Compare your calculation to the statement
Pull your CAM reconciliation statement and find the percentage the landlord applied. It should match the percentage you just calculated within rounding — typically no more than 0.1%.
If the stated percentage is higher than your calculation, that's your target. Before disputing, check whether your lease has any modifier provisions (see Step 7) that might explain the difference.
6. Investigate differences greater than 0.1%
A gap of 0.1% or less may be rounding. A gap larger than that requires explanation. Common causes:
- Landlord used an occupied-area denominator when your lease requires TLA
- The building's total area was understated in the landlord's records
- A tenant buyout or remeasurement changed the denominator mid-year without notice
- Calculation error in the reconciliation spreadsheet
Request the landlord's pro-rata share calculation worksheet as part of any audit rights request. It should show the denominator used for each month of the year.
7. Check for modifier provisions
Some leases contain provisions that intentionally change how pro-rata share works:
Anchor exclusions: If anchor tenants (grocery stores, big-box retailers) have their own CAM arrangements, they may be excluded from both the numerator and denominator. This is standard and doesn't inflate your share, but confirm the exclusion is applied consistently.
Occupied-area clauses: If your lease has an occupied-area denominator, verify that the landlord is using actual occupancy data, not a fixed percentage. The actual number should vary month to month and be supported by the leasing records.
Cap on pro-rata share: A few leases cap how high your percentage can go even if occupancy falls. This is rare but worth checking if you have a high-occupancy-risk building.
"I built CAMAudit because verifying pro-rata share manually requires pulling occupancy data month by month. Our tool extracts the denominator from your lease and flags any month where the applied percentage doesn't match." — Angel Campa, Founder of CAMAudit
Common Mistakes
Trusting the percentage on the statement. The reconciliation statement shows you a percentage but doesn't show you how it was calculated. Always derive it independently from the lease terms.
Confusing usable and rentable square footage. CAM is almost always calculated on rentable square footage. Using your usable square footage as the numerator will make your share look lower than it is — and you won't catch the error because the statement looks right.
Checking only the current year. If you discovered the wrong denominator this year, check prior years. Your lease's audit rights window typically allows you to go back 12–24 months.
FAQ
What if the landlord's percentage looks right but the dollar amount is wrong? The overcharge could be in the total CAM pool rather than the percentage. Run the multiplication yourself: stated percentage × stated total CAM pool = your charge. If the math doesn't match what was billed, the error is in the billing arithmetic, not the percentage.
My lease uses an occupied-area denominator. Is that illegal? No — it's a negotiated lease term. It's unfavorable to tenants, but it's enforceable if you agreed to it. The goal at this stage is to confirm the landlord is applying the term correctly, not to challenge the term itself. For future leases, push for a total leasable area denominator or a CAM cap.
Can I use the tool to verify this without doing the math manually? Yes. Upload your lease and reconciliation statement at /tools/cam-overcharge-estimator. CAMAudit extracts your square footage, identifies your denominator language, and checks the applied percentage against what the lease requires.