Perhaps you've had a spotless driving record for the past 15 years, but just last month you caused an accident. Now you have to make a claim on your auto insurance in order to pay for damages for which you're responsible. You suspect your rates will go up at renewal time, but what's a standard increase after just one accident?

It's a difficult question to answer because auto insurance companies vary on how much and whether they will raise rates after one accident. Insurance companies need to file rates with each state insurance department, and included in that is how they're going to determine the rates after a claim is made.

Many auto insurance companies follow the Insurance Services Office's (ISO) standard of increasing a premium by 20 to 40 percent of the insurer's base rate (not your current rate) after your first at-fault accident. A base rate is the average amount of claims paid plus the insurance company's claims-processing fee. According to the ISO, for multicar policies the surcharge is 20 percent of the base rate, and for single-car policies it is 40 percent.

For example, say you insure two cars at a premium of $300 each and a base rate of $400. After an accident, you may get a surcharge of $80 (20 percent of $400) on both, so your total surcharge would be $160, an increase of about 27 percent on the policy.

Under ISO standards, there is no surcharge on "property damage only" accidents where the damage is under $1,000. Also, if the accident is caused by new driver (driving less than two years) who already receives a surcharge for being inexperienced, there is no additional surcharge for his first accident. There are also state exceptions to surcharges.

But consequences could be different for you. Some insurance companies will not figure your increase based on their base rate, but rather on what you were paying before the accident. Keep in mind that your location, your age, and your driving record, as well as the "loss experience" (meaning claims made) of drivers similar to you, affect the percentage increase in your premium payment.

Most companies try to remove the rating variables that might skew your premium increase related to "loss experience." For example, if you live in a state in which the majority of your insurance company's customers live in urban areas, yet you live in a rural area, you don't want your premium increase to reflect the claims of urban motorists. Likewise, if your insurance company happens to insure more youthful drivers than middle-aged drivers, and you're middle-aged, you don't want your rate increase to be affected by those higher-risk, higher-charged, younger drivers.

By factoring out drivers who aren't in your age group and drivers who don't live in your area, insurance companies say they can derive an equitable increase in your premium.

Rate increases not exactly a science

When an insurance company decides to raise your premiums because you make a claim, it doesn't follow any hard and fast rules; many factors are involved. That's why you might find yourself getting double- or triple-whammied by your individual circumstances. For example, if you make a claim and have a birthday before renewal time, your birthday might bump you into a higher risk category along with the claim, shooting your rate through the roof.

Or, if you've made a claim and bought a more expensive car before renewal time, you'll likely see a significant increase — perhaps as much as 100 percent — in what you pay for car insurance.

Remember, too, that circumstances can work in your favor at times. If you turn 40 and enter a lower-risk category, or if you buy a car that's less expensive to insure, your savings might help offset any increase due to an accident.

The unforgiven

Then again, some insurance companies give their customers a one-time "get out of jail free" pass. When you make a claim on your first at-fault accident, you might not see any increase in your auto premium at renewal time. This practice — sometimes known as forgiveness — is not industry-wide, so if your insurance company holds your rate steady, consider yourself lucky.

Companies that forgive first-time accidents often require that you fit a certain profile in order to escape a rate increase. At USAA, for example, in 38 states and Washington D.C. (California, Massachusetts and North Carolina excluded), you must have had no chargeable at-fault accidents and been insured with USAA for five years before your accident occurred in order to be forgiven.

State Farm Insurance Co. increases your premium for any "chargeable" accident — meaning any accident in which the company pays more than $750 in liability or collision claims. State Farm policyholders should expect to see the increase at renewal time on their liability, collision, and PIP or medical payments coverages. But State Farm also has a program called "forgive the first accident" for its policyholders who have been with the company, accident-free, for at least nine years. If you fit this description and have a chargeable accident, you won't see any increase in your premium.

When you're shopping for insurance, it's a good idea to ask a company or independent agent whether the insurers offer first-time accident forgiveness. It might save you 20 percent or more on your car insurance over the long haul.