Workers Comp Experience Mod Calculator
The experience modification factor adjusts a workers compensation premium for an employer's own loss record. Losses are split into a primary part (small, frequency-driven, fully weighted) and an excess part (large, partly weighted), then combined with a ballast value and a weighting value from the rating bureau. The mod is the ratio of actual to expected on this weighted basis: above 1.00 means worse than the class average, below 1.00 means better. This calculator applies the bureau formula to your figures, with the ballast and weighting as user-editable inputs because those values come from bureau tables.
Experience rating formula
Numerator = Ap + B + W * Ae + (1 - W) * Ee
Denominator = Ep + B + W * Ee + (1 - W) * Ee
Experience mod = numerator / denominator
Premium effect = (mod - 1) * 100
Ap and Ae are actual primary and excess losses; Ep and Ee are the expected counterparts; B is the ballast value and W the weighting value, both from the rating bureau tables.
Experience rating context
- A mod of 1.00 is the class average; above is a surcharge, below is a credit.
- Primary losses are fully weighted because claim frequency is more predictive than severity.
- Ballast and weighting values come from bureau tables and scale with expected losses.
- Rating bureaus such as NCCI or independent state bureaus publish the plan rules.
- The mod is recalculated annually from a rolling multi-year experience period.
Experience mod: frequently asked questions
What is the experience modification factor?
The experience modification factor, or experience mod (EMR), adjusts a workers compensation premium up or down based on an employer's own loss history versus the expected losses for its class. A mod above 1.00 raises premium (worse than average); a mod below 1.00 lowers it (better than average); 1.00 is the class average.
What is the experience rating formula?
The experience rating formula combines primary and excess losses, a ballast value and a weighting value. Mod = (actual primary + ballast + W times actual excess + (1 - W) times expected excess) divided by (expected primary + ballast + W times expected excess + (1 - W) times expected excess). This calculator applies that formula to your figures.
Where do the ballast and weighting values come from?
They come from the rating bureau's tables (for example NCCI or an independent state bureau) and depend on the expected loss size. Because those tables are bureau-specific, the ballast and weighting are user-editable inputs here, so nothing is assumed.
What are primary and excess losses?
Each claim is split at a state-specific limit. The portion at or below the split point is the primary loss, weighted fully because it reflects frequency. The portion above is the excess loss, partly weighted because large single claims are less predictive. The split keeps the mod responsive to frequency, not severity.
How is the experience mod used?
Premium equals manual premium times the experience mod (before other factors). A lower mod reduces cost directly, which is why loss control and accurate payroll classification matter. The mod is recalculated annually by the rating bureau from a rolling experience period.
Official sources
- U.S. Occupational Safety and Health Administration: OSHA workplace injury recordkeeping.
- Workers' Compensation Insurance Rating Bureau of California: WCIRB experience rating plan.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.