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How commercial insurance premiums are calculated
Premium is an exposure base times a rate, adjusted for limits, deductible, experience and schedule rating, plus flat charges. A worked example shows the math.
A commercial premium is built from an exposure base, such as payroll, sales, square footage or vehicles, multiplied by a rate for your class of business. The insurer then adjusts that amount for the limits and deductible you choose, your loss experience and its own judgment of the account, and adds flat charges, taxes and fees. For many standard lines the rate starts as an advisory loss cost from ISO or NCCI that the insurer multiplies by its own loss cost multiplier. Specialty and surplus lines are priced by underwriter judgment and carrier models rather than manual rates.
The core formula
Every rating plan, however detailed, boils down to this:
Premium = (exposure units) x (rate per unit) x (modification factors) + flat charges
The exposure base is chosen because it tracks the chance and size of a loss. The rate is set by classification, and for most lines by territory too.
| Coverage | Common exposure base | Rate expressed per |
|---|---|---|
| Workers' compensation | Payroll by class code | $100 of payroll |
| General liability | Gross sales, payroll, area or units, depending on the class | $1,000 of sales or payroll, or per 1,000 square feet |
| Commercial property | Insured value of building and contents | $100 of value |
| Commercial auto | Number and type of vehicles, with driver and use rating | Per vehicle |
| Professional liability, cyber | Revenue, headcount and services, set by carrier models | Carrier-specific |
| Umbrella | The underlying liability exposures and the limit | Per $1 million layer, set by underwriting |
An exposure base you misstate flows straight into the premium. That is why payroll and sales figures carry so much weight on an application and in a premium audit.
Where the rate comes from
Insurers price from loss data. In many states for workers' compensation, and in many for ISO lines, an advisory organization (NCCI, ISO, or a state bureau) files a prospective loss cost: the part of the rate that pays for claims. Missouri's Department of Insurance describes the pure cost of claims as medical care, lost wages and prescriptions, with general business expenses such as salaries, rent and utilities excluded. Each insurer then applies its own loss cost multiplier (LCM) to cover its operating expenses, taxes, commissions and profit. Missouri's illustration uses a carpentry loss cost of 5.62: at a 1.250 multiplier the rate is 7.025 per $100 of payroll, and at 1.500 it is 8.43, which is about 20% more for the same class (Missouri Department of Insurance, About Loss Cost Multipliers).
Say an insurer expects expenses and profit to take 20% of each premium dollar. Its multiplier is 1 divided by (1 minus 0.20), or 1.25. The arithmetic shows why a carrier with lower expenses or a higher appetite for a class can quote less for identical coverage.
Whether the rate must be filed and approved before use depends on the state and the line: some use prior approval, others file-and-use, and many exempt larger commercial risks. See commercial rate filing: prior approval, file-and-use and deregulation and admitted vs non-admitted insurers: what the difference means. Non-admitted insurers (excess and surplus lines insurance explained) are generally not bound by filed rates and price risk by underwriting judgment.
The insurer's pricing also reflects what happens to the rest of the premium dollar: losses and loss adjustment expense, commissions and operating costs, taxes, reinsurance and profit. The ratio of all of those to premium is the combined ratio. A line running above 100 loses money on underwriting and tends to see rate increases, which is the logic behind hard and soft insurance markets: what the cycle means for buyers.
The modifications that move the price
After the base premium is set, the plan applies adjustments in a defined order.
- Classification and territory. The class code sets the base rate; the location can change it. Workers' comp rates also vary by state, as covered in workers' comp class codes and why they matter.
- Limits. Liability rates are generally built at a "basic" limit, and higher limits are priced with increased limit factors. Doubling the limit rarely doubles the premium. See commercial policy limits: per occurrence, aggregate and sublimits.
- Deductibles. A higher deductible earns a credit, because the insurer no longer pays the small, frequent losses. See deductibles vs self-insured retentions: who pays first.
- Experience rating. For workers' comp, the experience mod compares your actual losses with those expected for businesses in your classes; a 0.85 mod cuts the premium by 15% and a 1.25 mod adds 25%. NCCI's experience rating plan applies in 39 jurisdictions, with other states using their own bureaus, per NCCI's ABCs of Experience Rating. See The experience modification rate (EMR) explained. Liability lines may use a similar experience rating for larger accounts.
- Schedule rating. Underwriters can apply a debit or credit for characteristics the base rate does not capture: management quality, safety program, premises condition, protective devices. Most states cap how far schedule modifications can go. See schedule rating.
- Premium discount. Larger workers' comp policies earn a graduated discount on standard premium, with tiers set in each state's rating plan.
Flat charges, taxes and fees
After the percentage adjustments come the items that do not scale with exposure:
- Expense constant in workers' comp, a flat per-policy charge.
- Minimum premium, a floor below which the rated premium is raised.
- Terrorism premium. Treasury's June 2026 Terrorism Risk Insurance Program report found the embedded terrorism premium averaged 2.3% to 2.8% of policy premium over 2023 through 2025 (U.S. Treasury). See Terrorism insurance and TRIA: how the federal backstop works.
- State assessments, surplus lines taxes and stamping fees, which vary by state (surplus lines taxes, stamping fees and diligent search).
- Policy and inspection fees, charged by some carriers and agencies.
A worked hypothetical
Say a 10-employee landscaping company in a state that uses NCCI-style rating. The rates below are assumed for illustration. They are not market rates.
| Step | Calculation | Result |
|---|---|---|
| Workers' comp, landscaping | $350,000 payroll / 100 x $6.00 | $21,000 |
| Workers' comp, clerical | $60,000 payroll / 100 x $0.30 | $180 |
| Subtotal | $21,000 + $180 | $21,180 |
| Experience mod 0.90 | $21,180 x 0.90 | $19,062 |
| Expense constant | assumed $200 | $19,262 |
| General liability | $900,000 sales / 1,000 x $9.00 | $8,100 |
| Property (equipment, yard) | $150,000 value / 100 x $0.60 | $900 |
| Total before taxes, fees and terrorism | $28,262 |
In this example the $6.00 rate could come from an advisory loss cost of 4.80 times a 1.25 LCM, and the 0.90 mod means the company's losses have run better than expected for its classes.
Now the audit. If actual landscaping payroll at the end of the year is $420,000 instead of $350,000, the workers' comp premium rises by 700 units x $6.00 x 0.90, or $3,780. If sales reach $1.1 million, general liability rises by 200 units x $9.00, or $1,800. Neither number reflects a rate increase; both flow from exposure. Read premium audits: how they work and how to prepare for the process and what to keep on file.
Why the premium changes when the business does not
Three things move a premium at renewal even when your exposures stay flat: the rate (the insurer's filed or judgment rate, driven by the market), the modifications (your mod, schedule credits, loss history) and the exposure (payroll growth, new vehicles, higher property values from construction inflation). why your business insurance premium went up sorts these for an actual renewal, and how reinsurance pricing flows into business insurance premiums covers the part that originates with reinsurers.
How to read your own premium
- On the quote or declarations, find the exposure base for each coverage and check it against your records.
- Confirm each class code against what employees actually do. workers' comp class codes and why they matter explains how misclassification happens.
- Ask which modifications are applied: mod, schedule credit or debit, deductible credit, and the premium discount.
- Ask whether the premium is estimated, subject to audit and with a minimum.
- For comp, the workers' comp premium calculator and Experience mod (EMR) calculator let you reproduce the math.
Premium math is only half of buying; how much does business insurance cost, and what drives the price? covers what drives the total and how to compare business insurance quotes line by line covers comparison.
Key takeaways
- Premium is exposure times rate, adjusted for limits, deductible, experience and schedule rating, with flat charges and taxes added.
- The rate usually starts with an advisory loss cost, which each insurer multiplies by its own loss cost multiplier for expenses, taxes and profit.
- Misstated payroll, sales or class codes distort the premium, and audits usually correct them after the fact.
- Surplus lines and specialty coverages such as cyber and professional liability are priced by underwriting judgment and carrier models, not published manuals.
- The same coverage can differ in price between carriers because of different multipliers, appetite and expense structure, so compare terms and totals, not rates alone.