How to Calculate the Pro Rata Share of Insurance

"Pro-rata" refers to parceling something out proportionally. For example, you might prorate monthly rent payments if you only rent an apartment for part of the month or have your biweekly salary prorated if you start or leave your job in the middle of a pay period. In insurance, pro-rata can refer to adjusting insurance payments based on an incomplete period or dividing liability between insurers and the insurance buyer based on degrees of coverage.

One example of where you'll see the term pro-rata in insurance is in pro-rata cancellation terms in insurance policies. These provide that if an insurance policy is canceled during its normal term, such as a year, the only premium due to the insurance company is the portion of the premium based on what percentage of the term the policy was in effect.

For example, if you have a policy that is good for a year and it is canceled halfway through the year, only half of the premium would be due. Depending on how much of the premium was prepaid, this could lead to money owed from the insured party to the insurance company or vice versa. You can prorate with a calculator for insurance to determine the amount of premium that will apply under different cancellation scenarios. A calculator tool for figuring this out is sometimes called a pro-rata wheel or simply insurance wheel, referring to a physical device that was used for these calculations before online calculators became widespread.

Some insurance policies have stricter cancellation terms that effectively penalize the insured party for canceling the policy early. Read the terms of a policy you have purchased or are considering to understand what happens if you or the insurer cancel early.

In some cases, you may deliberately buy a shorter-than-usual insurance policy, such as if you only need to insure a house, car or liability related to a professional engagement for less than a year. In such circumstances, you will likely work with the insurance company to spell out the terms for how the normal premium will be prorated, often based simply on the number of days that you need the insurance coverage.

It's a good idea to make sure the length of time you need the coverage and how the proration works is spelled out in any contract you sign so there is less likely to be a dispute later on. Check the bills you receive and any other materials from the insurance carrier and agent to make sure they accurately reflect what you agreed to.

In addition to premiums being prorated, the terms of a policy might also specify how claim payments are prorated based on the amount of coverage. For example, if three-quarters of the value of an item such as a house is covered by insurance, the policy might specify that only three-quarters of the cost of any damage will be paid out. Such a term is sometimes called the pro-rata condition of average. Any additional costs must then be borne either by the policyholder or by additional coverage.

Note that if you have multiple policies with such terms, they may collectively pay more. For example, you might insure three-quarters of the value of an asset with one insurance carrier and the other 25 percent with another carrier. If a claim is filed with both and both pay out according to the pro-rata condition of average, 100 percent of the claim may still be covered. Check your insurance policies to understand if such conditions may apply.