Workers comp premium calculation starts with payroll: each classification’s payroll is divided by 100 and multiplied by that class code’s rate, and the results are added to give a base premium. If your business is large enough to qualify, an experience modification factor then raises or lowers that figure based on your own claims history. The final premium is not known until a payroll audit after the policy year ends.
Once you can follow those three steps, you can check your own quote, spot a wrong class code and avoid most audit surprises.
Step 1: Payroll and class codes
The California Department of Insurance explains that workers’ compensation premium calculations are based on how employers are classified. Each classification has a rate expressed in dollars and cents, the payroll for each classification is estimated and multiplied per each $100 of payroll, and the total is the base premium (California Department of Insurance, Workers’ Compensation).
The Texas Department of Insurance describes the same method: each employee’s payroll is assigned to the appropriate classification, and the total payroll for each classification is multiplied by the company’s rate for it (TDI, Texas workers’ compensation rate guide).
Why the class code matters so much
A class code groups businesses with similar work and similar injury costs. NCCI explains that under manual rating, employers are grouped by business operation or classification, and the rates are averages reflecting normal conditions in each class (NCCI, ABCs of Experience Rating). Oregon notes that it has about 600 different classifications, each with its own rate (Oregon DCBS Small Business Ombudsman, How much does it cost?).
Rates for an office clerical class and a roofing class can differ many times over, so payroll placed in the wrong class changes the premium more than any other single input.
The formula, with Oregon’s example
Oregon’s Small Business Ombudsman gives the formula plainly: 12-month projected payroll multiplied by the classification rate, divided by 100, equals estimated premium. Its example is a business with a $5 rate and $100,000 of projected payroll, which gives $5,000. Oregon notes that this estimate does not include taxes, premium credits or experience modifications.
Where the rate comes from
In many states a rating organization files loss costs, and each insurer applies its own multiplier. TDI’s rate guide shows a Texas example: a loss cost of 1.946 for code 5551 (roofing) as of July 1, 2026, and an insurer loss cost multiplier of 1.50, giving a rate of 2.919 per $100 of payroll. That is why two insurers can quote different premiums on identical payroll.
Step 2: The experience modification factor
NCCI explains that experience rating compares an employer’s payroll and loss experience with the average employer in the same classification. The result is an experience rating modification, or mod, which can increase, decrease or leave premium unchanged. An employer with better-than-average loss experience generally receives a credit mod, and one with worse-than-average experience a debit mod. The California Department of Insurance describes it the same way: the mod compares the employer’s claims history to all other employers and is applied to the base premium.
Who gets a mod
NCCI states that experience rating is mandatory for employers that meet a state’s premium eligibility threshold, and the thresholds are set state by state. Small employers below the threshold pay manual premium without a mod. NCCI also notes that its plan does not apply in the four monopolistic states, North Dakota, Ohio, Washington and Wyoming, which administer their own plans.
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Which years count
NCCI says the experience period is usually the latest available three years of data, and that the current policy is not used. Its example: a policy renewing January 1, 2026 generally uses losses from policies effective 1/1/2022 to 1/1/2025.
Why frequency hurts more than one big claim
NCCI’s plan gives more weight to how often injuries happen than to how much each costs. Each claim is split at a state-approved split point into primary loss, which counts more, and excess loss, which counts less. Very large claims are also capped by a state per-claim accident limitation. NCCI further notes that most states apply an Experience Rating Adjustment that includes only 30% of a medical-only claim in the calculation. The practical lesson: reporting every injury promptly and getting people back to work safely matters more to your mod than trying to absorb small claims yourself.
Step 3: Other adjustments
TDI notes that a schedule rating debit or credit may be applied based on individual characteristics of the business, and that optional rating plans such as deductibles or retrospective rating may reduce premiums. Taxes, assessments and minimum premiums also appear on many bills. Which of these apply is set by your state and insurer.
A worked illustration
The table below uses made-up rates for illustration only. It is not a quote and the rates are not real for any state.
| Line item | Example | Calculation |
|---|---|---|
| Clerical payroll | $80,000 at an illustrative $0.20 rate | $80,000 / 100 x 0.20 = $160 |
| Field installer payroll | $150,000 at an illustrative $4.00 rate | $150,000 / 100 x 4.00 = $6,000 |
| Base (manual) premium | Sum of classes | $6,160 |
| Experience mod | Illustrative 0.90 credit | $6,160 x 0.90 = $5,544 |
| Schedule credits, discounts, taxes | Vary by state and insurer | Shown separately on the bill |
Now imagine the auditor finds that $40,000 of the “clerical” payroll was actually people working in the field. That $40,000 moves to the higher-rated class, and the premium rises accordingly.
The premium audit
The premium you pay at the start is an estimate. The California Department of Insurance explains that final premium cannot be calculated until the policy term is over; if payroll went up from the estimate, the employer owes additional premium. It notes insurers generally have the right to audit payroll records during the policy and for up to three years after, that refusing an audit can make an employer liable for a premium of three times the estimated policy premium, and that deliberately under-reporting payroll is insurance fraud.
Common reasons an audit bill is higher than expected:
- Payroll grew during the year and the estimate was never updated.
- Employees’ duties put them in a higher-rated class than the policy shows.
- Uninsured subcontractors’ costs were counted as your payroll.
- Overtime, bonuses or owner pay were treated differently from what you assumed.
No primary source publishes a ranking of which cause is most common; all four are worth checking every year.
Disputing a classification
In California, the Department of Insurance says an insurer must tell the employer in writing within 30 days of a classification change that increases premium. The employer can then file a written complaint with the insurer and, if unresolved, appeal to the Department; for classification decisions by the state rating bureau, the employer may first file a written inquiry with it. Other states have their own procedures, usually through the state insurance department or rating bureau.
How to keep the premium accurate
- Read the class codes on your declarations page and the description of each.
- Keep payroll records split by job duty, not just by employee.
- Update your insurer when payroll or operations change mid-year.
- Collect certificates of insurance from subcontractors.
- Ask for your experience rating worksheet if you have a mod, and check every claim on it.
Whether you need coverage at all is a state question, covered in who needs workers’ comp. Workers’ comp is also excluded from a business owner’s policy and from general liability, so it is always priced on its own. If you have a mod, NCCI publishes it on an experience rating worksheet that lists every claim used, and that worksheet is the first document to request when your renewal premium jumps.