UIM insurance, short for underinsured motorist insurance, is auto coverage that pays for your injuries and damages when the driver who hit you has insurance but not enough to cover your losses. If someone with a $25,000 liability policy causes an accident that leaves you with $80,000 in medical bills, their coverage runs out long before your bills do. UIM fills that gap using your own policy.
How UIM Differs From Uninsured Motorist Coverage
UIM and UM (uninsured motorist) coverage often appear together on a policy, sometimes written as “UM/UIM,” but they respond to different situations. Uninsured motorist coverage kicks in when the at-fault driver carries no insurance at all, or in hit-and-run scenarios where no driver can be identified. Underinsured motorist coverage applies when the other driver does have a policy, but its limits are too low to pay for the harm they caused.
The distinction matters because a surprising number of drivers carry only the legal minimum liability coverage, which in many states is $25,000 per person for bodily injury. A single surgery, a few days in the hospital, or ongoing physical therapy can blow past that amount quickly. UIM exists to protect you in exactly that scenario: the other driver technically has insurance, but it’s not nearly enough.
When a UIM Claim Gets Triggered
You can’t simply file a UIM claim whenever you think the other driver’s coverage seems low. In most cases, the at-fault driver’s liability limits must actually be exhausted first. That means their insurer pays out the full policy limit, and you still have remaining losses that weren’t covered. Only then does your own UIM coverage step in to cover the difference.
You also typically need to file the claim within a set window after the accident. Some states give you as little as two years, others up to three, from the date of the accident or from the date you learn the other driver’s coverage is insufficient. Your own insurer will require written notice that you intend to pursue a UIM claim, so reporting the situation promptly is important.
How Payouts Are Calculated
Not all UIM policies pay the same way. The two main methods are the “offset” approach and the “add-on” approach, and which one applies depends on your state’s laws and your policy language.
With the offset method (sometimes called “limits-triggered” or “topping-off”), your UIM coverage brings the total available insurance up to your own policy limit, then subtracts whatever the at-fault driver’s insurer already paid. Say you carry $100,000 in UIM coverage and the at-fault driver has a $50,000 policy. Under the offset method, your insurer would pay up to $50,000 in UIM benefits (your $100,000 limit minus the $50,000 already paid by the other driver’s policy). The goal is to put you in the same position you’d be in if the other driver had carried the same limits you carry.
With the add-on method (sometimes called “damages-triggered” or “excess”), your UIM limit stacks on top of whatever the at-fault driver’s insurer pays. Using the same example, you’d have access to up to $150,000 total: the other driver’s $50,000 plus your full $100,000 UIM limit. Add-on coverage is more generous, though policies using this method sometimes cost more.
Understanding which method your policy uses makes a real difference in how much protection you actually have. Check your declarations page or call your insurer to find out.
What UIM Typically Covers
UIM coverage primarily pays for bodily injury: medical bills, lost wages, pain and suffering, and rehabilitation costs that exceed what the at-fault driver’s insurance can handle. Some states and policies also offer underinsured motorist property damage coverage, which helps pay for vehicle repairs or replacement, though this is less common.
UIM generally covers you whether you’re driving your own car, riding as a passenger in someone else’s vehicle, or even walking or cycling when struck by an underinsured driver. It also typically extends to members of your household listed on the policy. The specifics vary by insurer and state, but the coverage tends to follow you rather than being tied strictly to one vehicle.
Is UIM Coverage Required?
Roughly a third of states require drivers to carry UIM coverage. In those states, the coverage is built into your policy automatically, though the required minimums are often modest. In other states, your insurer is required to offer UIM coverage, but you can decline it in writing. A handful of states bundle UM and UIM into a single mandatory coverage.
Even where it’s optional, UIM is one of the more valuable add-ons you can buy. The cost is relatively low compared to other coverage types, often adding $50 to $150 per year depending on your limits and location. Considering that a single serious accident can generate six-figure medical bills, the protection is disproportionately large relative to the premium.
Stacking UIM Across Multiple Vehicles
If you insure more than one vehicle, some states allow you to “stack” your UIM limits, which multiplies your coverage by the number of vehicles on your policy. For example, if you carry $50,000 in UIM coverage and insure three cars on the same policy, stacking would give you access to $150,000 in UIM benefits after a single accident.
This is called vertical stacking because the limits combine within one policy. Horizontal stacking, where limits combine across separate policies with the same carrier, is also possible in certain states. Not every state permits stacking, and insurers in states that do will typically charge a higher premium for stacked coverage. If your state allows it, stacking is one of the most cost-effective ways to increase your injury protection without buying an entirely separate policy.
Choosing the Right UIM Limits
When selecting UIM limits, think about what a serious accident could actually cost you. A broken leg with surgery can run $50,000 to $100,000 in medical bills alone. A traumatic brain injury or spinal cord injury can push costs into the hundreds of thousands. Your UIM limits represent the ceiling of what your own insurer will pay after the at-fault driver’s coverage is exhausted.
A common approach is to match your UIM limits to your own liability limits. If you carry $100,000/$300,000 in bodily injury liability (meaning $100,000 per person and $300,000 per accident), selecting the same amounts for UIM ensures you’re as well-protected when someone else is at fault as others are protected when you’re at fault. Many insurers won’t let you buy UIM limits higher than your liability limits anyway.
If you have significant assets or income to protect, higher limits make sense. The incremental cost of moving from $50,000 to $100,000 in UIM coverage is often only a few dollars per month, making it one of the cheaper ways to close a large financial gap.