How to Calculate CAM Pro-Rata Share by Square Footage
The foundational formula for allocating common area maintenance expenses among tenants in a multi-tenant commercial building.
Pro-rata allocation by square footage is the default method for distributing CAM expenses among tenants in most commercial leases. Each tenant pays a fraction of the total CAM pool equal to their rentable square footage divided by the total rentable square footage of the building (or a defined denominator). While conceptually simple, the calculation is frequently disputed due to ambiguity in three areas: (1) the definition of the denominator (total building RSF vs. leasable RSF vs. occupied RSF vs. a fixed contractual SF), (2) the tenant's RSF measurement (which BOMA standard, whether it includes pro-rata common areas), and (3) partial-year adjustments when tenants commence or expire mid-year. Getting these three elements right is the foundation of an accurate CAM reconciliation.
Formula
Tenant Share = Total CAM Expenses × (Tenant RSF / Denominator SF)
Variables
| Name | Symbol | Definition | Example |
|---|---|---|---|
| Total CAM Expenses | TCE | The total recoverable operating expenses in the CAM pool for the reconciliation period, after applying any required adjustments (exclusions, gross-up, management fee caps). | $1,200,000 total recoverable CAM pool |
| Tenant Rentable Square Footage | TRSF | The tenant's premises as measured in rentable square feet per the lease. Typically measured per BOMA standards and specified as a fixed number in the lease. Disputes arise when landlords re-measure or when the lease is ambiguous about add-on factors. | 8,000 RSF |
| Denominator Square Footage | DSF | The total square footage against which the tenant's share is calculated. The lease must specify whether this is total gross building area, total leasable area, total occupied area, or a fixed number. The denominator definition has the largest impact on tenant billing after total expenses. | 120,000 RSF total leasable area (as defined in lease) |
| Pro-Rata Share | PRS | The percentage fraction representing the tenant's proportionate share, calculated as TRSF / DSF. Some leases specify a fixed pro-rata share percentage rather than computing it dynamically. | 8,000 / 120,000 = 6.667% |
| Proration Period | PP | For tenants who did not occupy the premises for the full reconciliation year, the fraction of the year during which they were obligated to pay CAM. Calculated as days occupied / total days in the year. | Full year = 1.0; tenant who commenced April 1 = 275/365 = 0.7534 |
Step-by-Step Process (4 steps)
Confirm the Denominator Definition
Read the lease's CAM provision carefully to identify which square footage figure is used as the denominator. Common definitions: (a) Total gross building RSF — largest denominator, lowest tenant share; (b) Total leasable RSF (excludes common areas measured separately) — most common; (c) Occupied RSF — smaller denominator, higher tenant share, often used in retail; (d) Fixed contractual denominator — a specific number written into the lease that never changes regardless of building modifications.
Example:
Lease states: 'Tenant's proportionate share shall be a fraction, the numerator of which is the RSF of the Premises and the denominator of which is the total leasable area of the Building.' Denominator = 120,000 leasable RSF.
Confirm Tenant's Rentable Square Footage
Verify the tenant's RSF against the lease exhibit. If the lease specifies a fixed RSF, use that number regardless of remeasurement. If the lease references BOMA standards without specifying a fixed RSF, confirm measurement methodology. Flag any discrepancy between the lease exhibit RSF and what the building management system shows.
Example:
Lease Exhibit A specifies 8,000 RSF. Property management system also shows 8,000 RSF. Confirmed.
Calculate the Pro-Rata Fraction
Divide the tenant's RSF by the denominator RSF to obtain the pro-rata fraction. Express as both a decimal and a percentage for the reconciliation workpaper.
Pro-Rata Share = Tenant RSF / Denominator SF
Example:
8,000 / 120,000 = 0.06667 = 6.667%
Adjust for Partial-Year Occupancy
If the tenant commenced or terminated their lease mid-year, prorate the annual CAM charge by the fraction of the year they were in occupancy. Use the actual commencement and expiration dates from the lease. For partial first or last months, calculate the exact number of days.
Adjusted Tenant Share = Full-Year Share × (Days Occupied / Days in Year)
Example:
Tenant commenced April 1: 275 days occupied / 365 days = 75.34% of year. Full-year share = $80,000. Adjusted share = $80,000 × 0.7534 = $60,274.
Worked Example
Scenario
Retail tenant in a 120,000 SF shopping center occupying 8,000 SF. Total CAM pool for 2025: $1,200,000. Tenant occupied for the full calendar year.
Inputs
| Variable | Value |
|---|---|
| Total CAM Pool | $1,200,000 |
| Tenant RSF | 8,000 SF |
| Total Leasable Area (Denominator) | 120,000 SF |
| Pro-Rata Share | 6.667% |
| Occupancy Period | Full year (365/365) |
Calculation
Pro-Rata Share = 8,000 / 120,000 = 6.667% Annual Tenant CAM = $1,200,000 × 6.667% = $80,000 Monthly Estimate = $80,000 / 12 = $6,667/month Per-SF Rate = $80,000 / 8,000 SF = $10.00/SF
Result:
Tenant's annual CAM obligation: $80,000/year ($10.00/SF). Monthly estimates should be set at $6,667/month. This is the tenant's gross CAM before any caps, exclusions, or gross-up adjustments apply.
Common Mistakes
Using the wrong denominator — a landlord using occupied RSF instead of total leasable RSF effectively forces existing tenants to subsidize vacant space, which is a common overbilling claim.
Using the gross building area (including non-leasable space like mechanical rooms) as the denominator when the lease specifies leasable area, understating each tenant's share.
Failing to apply the partial-year proration to the first and last year of occupancy, overbilling a new tenant for the full year.
Not updating the denominator when the landlord adds square footage through building expansion, when the lease requires the denominator to reflect actual total leasable area.
Using a fixed pro-rata percentage stated in the lease without verifying it was calculated correctly against the correct denominator when the lease was executed.
Applying different denominators to different tenant pools (e.g., one denominator for CAM, a different denominator for taxes), creating allocation inconsistencies.
When to Use This Calculation
- For every multi-tenant commercial property CAM reconciliation — pro-rata by SF is the default allocation method in the vast majority of commercial leases.
- When auditing a CAM statement to verify that the landlord used the correct denominator and tenant SF per the lease terms.
- When preparing monthly CAM estimates for new tenants, to set the correct initial billing amount.
- When a tenant is expiring or commencing mid-year and a partial-year adjustment is required for the final reconciliation.
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