Experience Modifier (X-Mod) Explained
The experience modification, usually called the X-Mod or mod, is the number that ties your workers’ compensation premium to your own injury record. This guide explains how California calculates it, what 1.00 means, how to read the worksheet and which steps bring a mod down over time.
What an experience modification is
The California Department of Insurance defines the experience modification as a rating factor, expressed as a percentage, that adjusts the workers’ compensation premium of employers who qualify for it. It compares your loss history with that of other employers in the same industry and of similar size. In the CDI’s words, a mod under 100 percent generally reflects better-than-average experience, and one over 100 percent reflects worse-than-average experience.
The mod is applied to the base premium, the figure produced by multiplying your payroll in each class by the class rate per $100 of payroll, along with any schedule or judgment rating the insurer uses. Insurers and brokers usually write it as a decimal, so a mod of 0.85 trims the base by 15 percent and a mod of 1.25 adds 25 percent. If you are new to how a workers’ compensation policy is priced, start with California Workers’ Comp Basics for Employers.
Who calculates it and which employers are rated
Your insurer does not set your mod. The Workers’ Compensation Insurance Rating Bureau of California (WCIRB) calculates it from the payroll and loss data that insurers must report to it each year, using a formula filed with and adopted by the Insurance Commissioner as the California Workers’ Compensation Experience Rating Plan. Any employer that meets the eligibility test must be experience rated; it is not optional for the employer or the insurer.
Eligibility is a test of size. The WCIRB multiplies your payroll in each class during the experience period, per $100, by that class’s expected loss rate, and if the total reaches the eligibility threshold, you are rated. Only completed policy periods count. The threshold moves each year with wages: the WCIRB’s filing for mods effective on or after September 1, 2026 asked to raise it from $10,800 to $11,700, so confirm the figure in force for your rating date.
Because the test runs through expected loss rates, the payroll it takes differs widely by trade. Using the rates proposed in that filing, a business with only clerical office payroll (class 8810, at 0.093 per $100) would need roughly $12.6 million of payroll across the experience period to qualify, while a carpentry contractor in class 5403 (at 4.051 per $100) would qualify at about $289,000. That is why many small contractors carry a mod years before an office business of the same headcount does.
The experience period and the claim values used
The Experience Rating Plan sets a three-year experience period that begins four years and nine months before your rating effective date and ends one year and nine months before it, as the Department of Insurance has quoted the rule in its published decisions. The gap gives insurers time to audit payroll, value claims and send the data in. The WCIRB’s 2026 filing notes that mods effective from September 2026 through August 2027 generally draw on policies that began between September 1, 2022 and August 31, 2025.
Each policy is reported at set stages. Insurers file a first report valued 18 months after the policy began, and later reports at 30 and 42 months, so the same policy year can show up in three mods at three different maturities. The CDI explains that the losses counted include both amounts already paid and the loss reserves the adjuster has set aside for what is still expected, which is why an open claim with a large reserve can weigh on a mod before most of that money is spent.
Expected losses, actual losses, primary and excess
Expected losses are what the plan predicts for a business like yours: your payroll in each class across the experience period, divided by 100, times the class’s expected loss rate. Actual losses are the values of your own claims. Before any comparison, the plan removes the first $250 of each claim and limits each claim to a maximum value, $175,000 in the 2026 filing, so a minor first-aid claim reported properly costs the mod little or nothing.
Each claim is then split. The primary portion is the part above the first $250 and up to your primary threshold, and anything above it is excess. The threshold grows with the employer: the 2026 tables run from $4,500 for the smallest rated employers to $75,000 for the largest. On the expected side, a class figure called the D-ratio sets how much of your expected losses count as primary; the remainder is expected excess.
California’s formula adds your actual primary losses to your expected excess losses and divides the total by your total expected losses. Your own excess losses do not enter at all. The result is that how often injuries happen matters more than how expensive any single one turns out, because each claim contributes its primary slice in full.
What 1.00 means: a worked example
A mod of 1.00 means your actual primary losses equal the primary losses expected for your size and classes, and the base premium is left as it is. Take a carpentry contractor in class 5403 with $600,000 of payroll across the experience period, using the rates in the WCIRB’s 2026 filing. Expected losses are 6,000 × 4.051, or $24,306. That size falls in the band with a $9,500 primary threshold, where the class’s D-ratio is 0.172, so expected primary losses are about $4,181 and expected excess losses about $20,125.
- No claims: ($0 + $20,125) ÷ $24,306 gives a mod of about 0.83.
- One $60,000 claim: only $9,250 counts as primary, so the mod is about 1.21.
- Three $4,000 claims: each contributes $3,750, a total of $11,250, so the mod is about 1.29.
Twelve thousand dollars of small claims cost this employer more than one $60,000 claim. To see the effect on premium, the Insurance Commissioner adopted an average advisory pure premium rate of $1.65 per $100 of payroll for policies starting on or after September 1, 2026. On $400,000 of annual payroll at that average, the base is $6,600; a 0.83 mod brings it to about $5,478, while 1.21 lifts it to about $7,986 and 1.29 to about $8,514. Treat these figures as arithmetic, not a quote, because insurers file their own class rates, and see How to Save on Workers’ Comp for the other levers on the base.
How to read your mod worksheet
The WCIRB issues an experience rating worksheet with each mod, and it reads from top to bottom like the formula. Start with the rating effective date and the policy periods listed, and confirm they belong to your business and fall inside the experience period. Then check the payroll shown for each class code against your audited payroll, since payroll that never made it into the data shrinks your expected losses and pushes the mod up.
Next come the expected loss rate, the expected losses and the D-ratio for each class, followed by the totals for expected primary and expected excess losses. The claims section lists each claim by policy period with its reported value, whether it is open or closed, and the primary amount used. Look for claims that are not yours, duplicates, injuries reported in the wrong year and open files that have since closed for less.
Questions about the worksheet go first to the WCIRB, which offers a policyholder ombudsman for classification and experience rating issues. Title 10 of the California Code of Regulations, sections 2509.40 to 2509.78, sets out the process for disputing a mod, including an appeal to the Department of Insurance. Insurance Code section 11751.9 also requires a mod to be recalculated when the closing values of its claims fall below 60 percent of the values first used, provided the result is a lower mod.
How to bring a mod down
Report every injury promptly and through the insurer. The CDI has warned employers against arrangements that keep injury reports away from the insurer, and the first $250 of each claim is already left out of the mod. The DIR’s employer rules, including the one-working-day claim form deadline, keep a claim from growing while nobody is handling it.
Bring injured employees back on modified or alternative duties as soon as the treating doctor allows. Temporary disability keeps paying two-thirds of lost wages while the employee is off work, so a quicker return holds down the value the claim carries into the mod. Ask the adjuster about reserves on open claims well ahead of each unit statistical report, and close files that are finished. Our guide on what to do after a business insurance claim covers that follow-up.
Finally, keep classification and payroll accurate, because expected losses come straight from them, and keep your Cal/OSHA Injury and Illness Prevention Program working, since fewer injuries is the only lasting way to lower a mod. Each clean year replaces a year with claims as the experience period rolls forward. How to Save on Business Insurance covers the steps that carry across your other policies.
A few useful answers
Common questions
Who calculates the experience modification for California employers?
The Workers’ Compensation Insurance Rating Bureau of California, not your insurer. It uses the payroll and claim data insurers report to it and the Experience Rating Plan the Insurance Commissioner has adopted, and every insurer applies the same mod.
Why can several small workers’ comp claims raise an X-Mod more than one large claim?
California’s formula counts only the primary part of each claim, up to the employer’s primary threshold, and leaves actual excess losses out. Three modest claims each contribute their primary slice in full, while one large claim contributes a single capped slice.
Does a workers’ comp claim under $250 affect a California experience mod?
No. The plan removes the first $250 of each claim before calculating the mod, so a claim of $250 or less has no effect on it. Report it anyway, because the reporting rules apply regardless of size.
How long does one workplace injury stay in an experience modification?
The experience period covers three years of policies and ends one year and nine months before the rating date, so a claim typically appears in three consecutive mods, then drops out as newer years replace it.
What can an employer do if its experience modification looks wrong?
Compare the worksheet’s payroll and claims with your own records, ask your insurer to correct any reporting error, and contact the WCIRB’s policyholder ombudsman. Formal disputes follow Title 10 sections 2509.40 to 2509.78, with an appeal to the Department of Insurance.