In California, the experience modification is calculated by the Workers' Compensation Insurance Rating Bureau (WCIRB), not by your insurance carrier. Because it multiplies your entire premium, the X-Mod is often the single largest lever an employer controls over what workers' comp actually costs.
How the X-Mod is calculated
The WCIRB compares your actual losses (what your claims cost) against your expected losses (what a typical employer with your payroll and class codes would be expected to incur). The formula weighs claim frequency more heavily than a single large claim, because frequent small claims are a stronger predictor of future losses.
- Actual losses — the reported cost of your claims over the experience period (typically three years, excluding the most recent policy year).
- Expected losses — a benchmark derived from your payroll by class code and the WCIRB's expected loss rates.
- Primary vs. excess — the first portion of each claim (primary losses) counts more heavily than the amount above the split point, which is why many small claims hurt more than one large one.
Who is required to have an X-Mod
Not every employer is experience-rated. In California, the WCIRB assigns an experience modification once an employer's payroll produces enough expected losses to meet the current eligibility threshold. Larger employers, and most businesses in higher-rate class codes such as construction, trucking, and manufacturing, are experience-rated. Smaller employers below the threshold pay manual rates without a mod.
6 ways to lower your experience mod
- Report and manage claims fast. Early medical care and prompt reporting reduce the ultimate cost of a claim, which is what feeds the mod.
- Run a return-to-work program. Bringing injured workers back on modified duty limits indemnity (lost-time) costs, the most heavily weighted part of a claim. See our return-to-work guide.
- Reduce claim frequency with safety training. Because the formula punishes frequency, preventing several small claims moves the mod more than avoiding one large one.
- Verify your class codes. Misclassified payroll inflates expected losses and can distort your mod. Audit your codes annually.
- Review open claim reserves. Carriers set reserves that count against your mod. Reserves left too high on stale claims can be challenged and reduced.
- Check the WCIRB worksheet for errors. Request your experience rating worksheet and confirm payroll, claim values, and class codes are correct before it hits your premium.
Why the X-Mod matters beyond price
Many general contractors, government agencies, and large facility owners require a mod at or below 1.00 to bid on work. A high mod can therefore cost you jobs, not just premium dollars. Treating the X-Mod as a year-round operating metric, rather than something you look at once at renewal, is what separates employers who control their comp costs from those who absorb them.