Calculate Tenant Pro-Rata Share for CAM and Expenses
Pro-rata share determines what percentage of shared building expenses a tenant is responsible for. It is calculated by dividing the tenant's leased square footage by the total rentable area of the building. Most NNN and gross leases use pro-rata share to allocate operating expenses.
By Angel Campa, Founder · Updated March 2026
The Formula
Pro-Rata Share = Tenant Leased Area ÷ Total Rentable Building Area
Some leases use "occupied area" or "leasable area" instead of "rentable area." Confirm which denominator your lease uses — it significantly affects the calculation, especially in buildings with vacant space.
Worked Example
Example Inputs
| Tenant Leased Area | 4,500 sq ft |
| Total Rentable Building Area | 65,000 sq ft |
| Anchor Tenant Exclusion | 20,000 sq ft (excluded from CAM pool) |
| Denominator Per Lease | 45,000 sq ft (excluding anchor) |
Result
10.00% pro-rata share (using denominator excluding anchor)
Step-by-Step Breakdown
| Line Item | Value |
|---|---|
| Standard Pro-Rata (full building) | 6.92% |
| Adjusted Pro-Rata (excluding anchor) | 10.00% |
| Annual CAM at $4.00/sq ft total | $18,000 (standard) / $26,000 (adjusted) |
| Impact Over 5-Year Term | $40,000 difference |
Frequently Asked Questions
What is pro-rata share in a commercial lease?
Pro-rata share is the percentage of shared building expenses (such as CAM, property taxes, and insurance) that a tenant is responsible for. It is calculated as the tenant's leased area divided by the total rentable area of the building. A tenant leasing 5,000 sq ft in a 50,000 sq ft building has a 10% pro-rata share.
What denominator should be used for pro-rata share?
Leases specify whether pro-rata share is calculated against total rentable area, occupied area, leasable area, or a modified denominator that excludes anchor tenants. The denominator choice significantly affects the calculation. Tenants should negotiate to use total rentable area (not occupied area) so their share does not increase during high-vacancy periods.
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