To calculate triple net lease payments, add your base rent to your pro rata share of the building’s three operating expenses: real estate taxes, building insurance, and common area maintenance. The mechanics are straightforward. Add the three annual expense totals, divide by the building’s total rentable square footage to get a per-square-foot rate, multiply that rate by the size of your space, divide by twelve, and add the result to your monthly base rent. The variables feeding that formula are where tenants get tripped up, because listings quote NNN charges separately from base rent and the estimates shift every year.
A listing that reads “$20.00/SF NNN (Estimated NNN = $3.25/SF)” means base rent is $20.00 per square foot per year and the three nets are estimated at another $3.25. Your actual annual occupancy cost in that example is $23.25 per square foot, not $20.00. Reading the base rate as your total cost is one of the most common budgeting mistakes in commercial leasing.
The Five Numbers You Need First
Before running any math, gather five figures: the building’s annual real estate tax bill, the annual insurance premium, the projected annual common area maintenance costs, the total rentable square footage of the building, and the rentable square footage of your unit. Property tax records are public, so you can verify the tax figure through the county assessor rather than relying on what the landlord provides.
The insurance component usually covers more than basic hazard protection. A landlord’s policy typically bundles property and casualty coverage with commercial general liability, and some leases pass through the cost of rental interruption insurance as well. Ask for the policy declarations page so you can see exactly what premium you’re helping to pay.
Square footage matters more than most tenants realize. Usable square footage is the space you actually occupy. Rentable square footage adds your proportional share of hallways, lobbies, and restrooms. Leases almost always charge based on rentable square footage, so if your usable space is 4,500 square feet but your rentable allocation is 5,000, all your costs are calculated on 5,000. Verify the measurement in the lease against an independent floor plan.
Building the Per-Square-Foot Operating Rate
Add the three annual expense categories to get the total building-wide operating cost. Say real estate taxes run $50,000, insurance costs $10,000, and common area maintenance is projected at $40,000. Total: $100,000 for the year. That’s the pool every tenant in the building shares.
Divide by the building’s total rentable square footage. A $100,000 expense load across a 20,000-square-foot building works out to $5.00 per square foot. That per-square-foot rate is what you’ll use to size your own share and to compare this building against other properties. Older mechanical systems or extensive common areas push the rate up.
Your Pro Rata Share and Monthly Payment
Your pro rata share is the percentage of building expenses assigned to you. Divide your rentable square footage by the building’s total rentable square footage. A tenant in 5,000 square feet of a 20,000-square-foot building has a 25% pro rata share. That percentage follows you through every expense calculation for the life of the lease.
Multiply the per-square-foot operating rate by your unit’s square footage to get your annual NNN obligation. At $5.00 per square foot across 5,000 square feet, your annual share is $25,000. Divide by twelve, and monthly NNN comes to roughly $2,083. Add that to your monthly base rent. If base rent is $15.00 per square foot ($75,000 annually, or $6,250 per month), your total monthly payment is approximately $8,333.
The full formula in one place:
- Total annual operating expenses = taxes + insurance + CAM
- Per-square-foot rate = total expenses ÷ building’s total rentable square footage
- Your annual NNN cost = per-square-foot rate × your rentable square footage
- Your monthly NNN payment = annual NNN cost ÷ 12
- Total monthly payment = monthly base rent + monthly NNN payment
What Sits Inside CAM
Common area maintenance is the broadest and most unpredictable of the three nets, so it deserves a closer look before you accept the number a landlord plugs into the estimate. Typical CAM includes landscaping, parking lot maintenance and resurfacing, janitorial services for lobbies and hallways, snow and ice removal, elevator maintenance, HVAC servicing for common areas, exterior lighting, and security. Some buildings fold utilities for shared spaces into CAM as well.
Property management fees also sit inside CAM. They typically run between 4% and 12% of collected rent. On a property collecting $500,000 in annual rent, that’s $20,000 to $60,000 flowing into the CAM pool. If the management fee looks high relative to the building’s size and complexity, ask for a breakdown before you accept it into the calculation.
Gross-Up Clauses When the Building Isn’t Full
Occupancy changes the math. If the building isn’t fully leased, the tenants who are present would normally shoulder variable operating costs that were sized for a full building. A gross-up clause lets the landlord calculate variable expenses as if the building were fully occupied, or at a specified occupancy threshold, so the per-square-foot rate reflects normal operating conditions rather than an unusually thin denominator.
Gross-up applies only to costs that actually move with occupancy: janitorial, electricity and water, trash removal, and management fees. Property taxes and insurance don’t change with vacancy, so they aren’t grossed up. A building with $80,000 in variable operating costs at 60% occupancy might have run $133,000 in those same costs at full occupancy. With a gross-up clause, your pro rata share is calculated against the $133,000 figure. Without one, you pay a lower absolute amount but a larger effective percentage of actual costs, and that percentage climbs every time another tenant leaves. Check which category your lease is in before you rely on the estimate.
Why the Calculated Number Isn’t Final
Monthly NNN payments during the year are estimates. Property taxes get reassessed, insurance premiums shift at renewal, and maintenance depends on weather, equipment failures, and contractor pricing. Reconciliation is the annual process where the landlord compares what you paid against what the building actually cost to operate.
Once final invoices and tax bills are in, the landlord tallies actual expenses against estimated amounts collected. Overpayments come back as a credit against future rent or, less commonly, a direct refund. Shortfalls get billed to you, often due within 30 days of the reconciliation statement. Build a small cushion into your operating budget for exactly that scenario, and expect the statement to itemize every expense category with supporting documentation. A single lump number with no backup is worth pushing back on before you pay.
Projecting Payments Over the Lease Term
The number you calculate today is a year-one number. Two mechanisms will move it. First, most commercial leases include a base rent escalation clause: a fixed percentage each year (often around 3%), a fixed dollar increase per square foot (such as $1.00/SF annually), or an adjustment tied to the Consumer Price Index. Second, the NNN estimate itself resets every year based on the reconciliation and updated projections.
A cap on annual CAM increases limits how fast the NNN side can grow. A 5% cap means that even if actual CAM costs jump 12%, your obligation only rises 5% over the prior year. Watch whether the cap is cumulative or noncumulative. With a noncumulative cap, each year stands alone: costs up 7% then 3% means you pay 5% then 3%. With a cumulative cap, the landlord carries forward the excess from a capped year and applies it later, so the same sequence produces 5% then 5% as the prior year’s overage rolls into the next year. Over a ten-year term, cumulative caps quietly erase most of the protection.
When you model total occupancy cost over five or ten years, run base rent escalation and NNN increases separately. Fixed-rate escalations are predictable. NNN increases are not, because they depend on tax reassessments, insurance markets, and maintenance needs nobody can forecast precisely. Running a worst-case scenario at the maximum rate your cap allows (or at historical averages if you have no cap) gives you a realistic ceiling. The tenants who get into trouble are the ones who budgeted for year-one numbers and assumed they’d hold.