Workers' comp insurance premiums: how your rate is calculated

Workers’ compensation insurance covers your employees’ medical costs and lost wages if they’re injured on the job. As a small business owner, the premium is a real operating cost — and unlike most insurance, the premium calculation is largely transparent. Understanding it lets you verify that you’re being rated correctly and spot the levers that affect cost.

The premium formula

The standard workers’ comp premium formula is:

Premium = (Payroll / 100) × Class Rate × Experience Mod × Schedule Mod

Each variable is explained below.

Class codes

Every type of work has a classification code set by the National Council on Compensation Insurance (NCCI) or the relevant state rating bureau. A software developer working in an office carries a very different class code than a roofer or a warehouse picker. Each class code has a base rate expressed in dollars per $100 of payroll. Rates vary significantly by state and risk level: office clerical workers might carry a rate of $0.20–$0.35/$100 of payroll, while roofing contractors in high-risk states can run $15–$30/$100 of payroll.

Common small business class codes and approximate rates (national average, varies by state):

Misclassification is one of the most common workers’ comp billing errors. If your employees are doing office work but are coded as a higher-risk class, you’re overpaying.

Payroll base

The payroll used in the premium calculation is your estimated annual payroll at the start of the policy year. At audit time (the end of the year), the insurer checks your actual payroll against the estimate and bills or credits the difference. Getting the payroll estimate close to actual avoids large audit adjustments.

Note: executive officers are often included in payroll but subject to minimum and maximum payroll amounts set by state law, regardless of their actual compensation.

Experience modification factor (Experience Mod / EMR)

Once a business has at least three years of workers’ comp history and reaches a certain payroll threshold, the insurer calculates an Experience Modification Rate (EMR or Mod). The EMR compares your actual claims to the expected claims for a business of your size and class. An EMR of 1.00 is average. An EMR below 1.00 means fewer claims than average — you get a premium credit. Above 1.00 means more claims — you pay a surcharge.

A single serious claim can push your EMR above 1.0 for up to three years (EMRs use a 3-year rolling window, excluding the most recent year). At $1 million in annual payroll and an EMR of 1.40 versus 1.00, the additional cost is: ($1,000,000 / $100) × $3.00 × 0.40 = $12,000/year extra.

Reducing your premium

The most effective levers:

1. Verify class codes. Request the policy’s class code listing and confirm each employee is coded correctly.
2. Implement a return-to-work program. Getting injured employees back to modified-duty work faster reduces the total claim cost, which flows through to your EMR.
3. Safety program documentation. Many states and insurers give a schedule credit for documented safety training programs.
4. Shop the market. Workers’ comp rates are state-filed but carriers can apply schedule modifications (±25% in most states) based on their underwriting judgment.
5. Professional employer organization (PEO). Small businesses often get better rates through a PEO because the risk is pooled across many employers.

How the workers’ comp calculator works

The workers’ comp calculator on this site takes your payroll, job classifications, and (optional) experience mod to estimate your annual premium. It also shows how changes to payroll mix, EMR, or class code affect the total — useful for planning when hiring or restructuring roles.