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How car insurance rates are calculated
How insurers build a car insurance price: a filed base rate, multiplied by rating factors for driver, car, place and coverage, then adjusted by discounts.
Your car insurance price starts as a base insurance rate the insurer files with your state, built from what it expects to pay in claims and expenses. That base is then multiplied up or down by rating factors that describe you, your car, where it is kept and the coverage you choose, and finally reduced by any discounts. Two drivers with the same car can pay very different amounts because each factor compounds the others.
Step one: the insurer's base rate
Every insurer starts with a statewide estimate of the cost of covering an average car for a year. Actuaries project future claim payments (how often claims happen and how much each one costs), add the cost of handling claims, add operating expenses such as commissions, salaries, premium taxes and advertising, and leave a margin for profit and contingencies. The result, divided across the cars the insurer expects to cover, is the base rate for each coverage: liability, collision, comprehensive, uninsured motorist and so on.
That base rate is not secret. Insurers submit it to the state insurance department in a rate filing, along with the data that supports it. Depending on the state, the regulator must approve the filing before it is used, or can review it after it takes effect; our guide to how rate approval works explains the difference. According to the Insurance Research Council (November 2024), U.S. personal auto insurers submitted roughly 10,200 rate filings a year between 2010 and 2023.
When an insurer's claims run hotter than expected, its loss ratio rises and the next filing usually asks for a higher base rate. That is why your premium can go up even if nothing about you changed; see loss ratios and rate filings for how that math works.
Step two: rating factors multiply the base
The base rate is then adjusted for your individual risk. In most rating plans each factor is a multiplier (a "relativity"): 1.00 is average, 1.25 means 25% more, 0.85 means 15% less. The table shows the factors most insurers use and what each one is trying to predict.
| Factor | What it measures | Typical effect |
|---|---|---|
| Driving record | At-fault crashes and violations on your motor vehicle report | Each incident adds a surcharge for several years |
| Claims history | Past claims, from your insurer or a CLUE report | Recent claims raise the price |
| Driving experience and age | Years licensed; age where allowed | Newest drivers pay the most |
| Annual mileage and use | Commuting, business use, miles per year | More miles, more exposure |
| Garaging location | Claim costs in your rating territory | Dense, high-theft, high-litigation areas cost more |
| The vehicle | Repair cost, theft rate, injury losses for that model | Captured in the vehicle rating symbol |
| Coverage choices | Limits and deductibles | Higher limits cost more; higher deductibles cost less |
| Insurance score | Credit-based score, where state law allows it | Lower scores pay more |
| Discounts | Multi-policy, telematics, good student, safety features | Each reduces the total |
Location is handled through territory rating, explained in our guide on why your ZIP code changes your price. The car is handled through a vehicle rating symbol that groups models with similar loss experience.
A worked example
Here is a simplified, hypothetical calculation for one coverage. Real rating plans have dozens of factors and differ by insurer, but the arithmetic works the same way.
Say an insurer's filed base rate for collision coverage in your state is $500 a year.
- Territory factor for your area: 1.20 (claims cost more than the state average). $500 x 1.20 = $600.
- Vehicle symbol factor for your car: 1.10. $600 x 1.10 = $660.
- Driver factor for a driver licensed 20 years with a clean record: 0.90. $660 x 0.90 = $594.
- Deductible factor for a $1,000 deductible instead of $500: 0.85. $594 x 0.85 = $504.90.
- Multi-policy discount of 10%: $504.90 x 0.90 = $454.41.
Your collision premium is about $454. Now change one input: add one at-fault crash surcharged at 1.30 and the same steps produce about $591. Because the factors multiply, a surcharge applies to everything above it, which is why a single crash can raise several coverages at once.
Underwriting comes before rating
Before any of that math, the insurer decides whether to accept you and on what terms. This is underwriting. It can place you in a rating tier or a separate company within the same insurance group, each with its own base rates, and it can decline applicants who fall outside the company's guidelines. Drivers with recent serious violations or lapses in coverage may be steered to a nonstandard carrier.
State law decides which factors are allowed
States regulate which characteristics can be used, so the list above is not universal. Some examples, each verified against the state's own law:
- California: under Proposition 103, rates must be determined by three mandatory factors in decreasing order of importance: driving safety record, miles driven annually and years of driving experience. Other factors can be used only if the Insurance Commissioner adopts them by regulation, and using any unapproved criterion counts as unfair discrimination (Cal. Ins. Code 1861.02). See California Proposition 103 explained.
- Michigan: since the 2019 no-fault reform, insurers may not base auto rates on sex, marital status, home ownership, education, occupation, the postal zone where you live, or credit score (MCL 500.2111).
- Hawaii: insurers may not base any rating plan on age, sex, length of driving experience, credit bureau rating or marital status (HRS 431:10C-207).
- Massachusetts: rates may not be based on credit information, and risks may not be grouped by sex or marital status, or by age except for a required discount for drivers 65 and older (M.G.L. c. 175E, s. 4).
For the details by factor, read our guides on credit-based insurance scores and car insurance and how age, gender and marital status affect car insurance. Your own state's rules are in our state guides.
What you can change, and what you cannot
Some inputs are fixed in the short term. Others you control:
- Coverage and deductibles: raising a collision deductible lowers that coverage's price, at the cost of more out of pocket per claim.
- Discounts: ask which ones you qualify for and which are missing from your declarations page.
- Mileage and use: report a lower commute or a switch to remote work.
- Telematics: a usage-based program can move your driver factor based on how you actually drive.
- The insurer: each company files its own base rates and factor tables, so the same driver can get very different quotes. Compare like-for-like coverage across insurers.
Key takeaways
- A premium is a filed base rate multiplied by rating factors, then reduced by discounts.
- Base rates rise or fall with the insurer's statewide claims experience, so prices can change at renewal even with a clean record.
- Factors compound: one surcharge raises every coverage it touches.
- State law limits which factors insurers may use; California, Michigan, Hawaii and Massachusetts restrict several common ones.
- Because every insurer weighs factors differently, comparing quotes is the most reliable way to find your price.