How much CAM do you owe based on square footage?
Common-area-maintenance share is a straight ratio: your square footage divided by the building's total rentable square footage, multiplied by the building's annual CAM expenses. Here is that arithmetic worked out for six suite sizes in the same building, holding total area and annual expenses at the calculator's own defaults.
Modeled CAM by suite size in a 10,000 sq ft building
| Suite size | Area share | Modeled annual CAM | Modeled monthly CAM |
|---|---|---|---|
| 250 sq ft | 2.50% | $3,000.00 | $250.00 |
| 500 sq ft | 5.00% | $6,000.00 | $500.00 |
| 1,000 sq ft | 10.00% | $12,000.00 | $1,000.00 |
| 2,500 sq ft | 25.00% | $30,000.00 | $2,500.00 |
| 5,000 sq ft | 50.00% | $60,000.00 | $5,000.00 |
| 8,000 sq ft | 80.00% | $96,000.00 | $8,000.00 |
Area share is tenant area ÷ rentable area. Modeled annual CAM is annual expenses × area share. Modeled monthly CAM is the annual figure ÷ 12. Every column comes from the same two inputs held constant across the table.
The area-share ratio does all the work here: every suite size in this table sits at exactly $12 of modeled annual CAM per square foot, because $120,000 ÷ 10,000 sq ft is a constant rate. That is what makes the relationship proportional — a 2,500 sq ft suite owes exactly ten times what a 250 sq ft suite owes, because it is exactly ten times the area, nothing else.
What this table cannot tell you is whether your own lease actually uses a straight area ratio. Some leases fix a tenant's share by exhibit rather than by current measured area, some carve out anchor tenants or vacant space from the denominator, and some cap increases or exclude categories entirely — any of which moves the real number away from this arithmetic.
Open the calculator and enter your building's actual expenses, total area, and your suite's area to see your own modeled share, plus a variance check against what you were actually billed. Nothing you enter is uploaded.
This page is arithmetic, not legal, tax, financial, or real-estate advice. It does not tell you what your lease requires or whether a bill is correct.
Common questions
How do I calculate how much CAM I owe based on my square footage?
Your modeled CAM share is your tenant square footage divided by the building's total rentable square footage, multiplied by the building's annual CAM expenses. On a 10,000 sq ft building with $120,000 in annual CAM expenses, a 1,000 sq ft suite — 10.00% of the building — models to $12,000 a year, or $1,000 a month. A 2,500 sq ft suite in the same building is 25.00% of the building and models to $30,000 a year ($2,500 a month).
Does a bigger suite always mean a proportionally bigger CAM bill?
Yes, in this arithmetic model — area share is a straight ratio of your square footage to the building's total, so doubling your area exactly doubles your modeled share. Going from a 1,000 sq ft suite to a 2,000 sq ft suite in the same 10,000 sq ft building doubles the area share from 10.00% to 20.00% and the modeled annual CAM from $12,000 to $24,000. What can break that proportionality in a real lease is a fixed pro-rata share stated in an exhibit, exclusions or caps that apply differently by tenant, or a base-year stop that only some tenants have — none of which this model assumes for you.