Key Takeaways
- UM and UIM are distinct coverages that address different scenarios, though insurers often sell them together.
- Uninsured motorist coverage activates when the at-fault driver carries no liability insurance whatsoever.
- Underinsured motorist coverage activates when the at-fault driver's policy limits are too low to cover your losses.
- Both coverages typically apply to bodily injury; property damage versions exist but vary by state.
- Many states require one or both of these coverages, though minimums differ significantly across state lines.
- Your own deductible and policy limits govern how much your insurer will ultimately pay under either coverage.
Option A
Uninsured Motorist Coverage (UM)
Protection when the at-fault driver has no insurance at all.
Best for: Drivers who want financial protection against hit-and-run accidents or collisions with drivers carrying zero liability insurance.
Option B
Underinsured Motorist Coverage (UIM)
Protection when the at-fault driver's insurance isn't enough.
Best for: Drivers seeking to bridge the gap when an at-fault driver's liability limits fall short of covering actual damages.
If you want protection against drivers who carry no insurance
Uninsured Motorist Coverage (UM)
UM coverage is specifically designed for accidents where the at-fault driver has no liability policy in force, including most hit-and-run situations.
If you want protection when the at-fault driver's limits are simply too low
Underinsured Motorist Coverage (UIM)
UIM steps in after the at-fault driver's policy pays out its maximum, covering the remaining gap up to your own UIM limit.
If you want the broadest protection against other drivers' coverage failures
Both UM and UIM together
Carrying both ensures you are covered whether the at-fault driver is completely uninsured or simply underinsured — two separate and common scenarios.
If you live in a state with high rates of uninsured drivers
Uninsured Motorist Coverage (UM)
In states where a significant share of drivers lack insurance, robust UM limits provide meaningful financial protection after a not-at-fault accident.
Why These Two Coverages Are Routinely Confused
Uninsured motorist (UM) and underinsured motorist (UIM) coverage are typically listed together on a policy's declarations page, often abbreviated as UM/UIM. That pairing leads many drivers to assume they're essentially the same protection — they aren't. Each addresses a distinct problem, and understanding the line between them directly affects whether you have the right limits in place.
For a broader grounding in policy terminology, see our glossary of auto insurance terms that covers concepts like subrogation and declarations pages. And if you want to understand how UM/UIM fits within the wider coverage picture, our guide to liability, collision, and comprehensive coverage is a useful starting point.
| Criterion | Uninsured Motorist (UM) | Underinsured Motorist (UIM) |
|---|---|---|
| When it activates | At-fault driver has no insurance | At-fault driver's limits too low |
| Hit-and-run coverage | Typically yes | Typically no |
| Bodily injury protection | Yes, in all requiring states | Yes, where available |
| Property damage protection | Available in many states | Available in some states |
| Pays after at-fault insurer | No — at-fault has no insurer | Yes — pays the remaining gap |
| State mandate | Required in most states | Required in many, optional in some |
| Limit structure | Per-person / per-accident | Per-person / per-accident |
How Uninsured Motorist Coverage Actually Works
UM coverage pays for your injuries — and in some states your vehicle damage — when you're hit by a driver who carries no liability insurance. It also typically applies in hit-and-run situations where the at-fault driver cannot be identified. According to the Insurance Research Council, roughly one in eight U.S. drivers was uninsured as of a recent industry estimate, though the rate varies considerably by state.
There are two forms of UM coverage worth knowing:
- UM Bodily Injury (UMBI): Covers medical expenses, lost wages, and pain and suffering for you and passengers in your vehicle.
- UM Property Damage (UMPD): Covers damage to your vehicle caused by an uninsured driver. Not all states offer or require this; where available, it may carry its own deductible.
Your UM limits work similarly to a standard liability policy — they're expressed as per-person and per-accident maximums (for example, 50/100 means $50,000 per person, $100,000 per accident). Once those limits are reached, your insurer pays no more under this coverage type.
1 in 8
U.S. drivers estimated to be uninsured
According to Insurance Research Council estimates, approximately 12.6% of U.S. motorists were uninsured as of their most recently published study.
$15,000
Minimum bodily injury liability in some states
Several U.S. states set per-person bodily injury liability minimums as low as $15,000 — an amount that can be exhausted quickly by a single hospitalization.
Varies by state
Whether UM/UIM is legally required
State insurance regulators set their own UM and UIM requirements; some mandate both, others only one, and a few allow policyholders to waive coverage in writing.
How Underinsured Motorist Coverage Fills a Different Gap
UIM coverage addresses a scenario that UM does not: the at-fault driver has insurance, but their limits are too low to cover your actual damages. This is more common than many drivers expect. A driver carrying the state minimum — sometimes as low as $15,000 per person in bodily injury liability — can cause injuries that generate tens of thousands of dollars in medical bills.
Here's how UIM typically works in practice: the at-fault driver's liability policy pays out its maximum first. Your UIM coverage then pays the difference between that payout and your actual covered losses, up to your own UIM limit. Note that most policies require your UIM limit to exceed the at-fault driver's liability limit before your UIM coverage can be triggered — this is a detail worth reading carefully in your policy documents.
As with UM, UIM generally comes in bodily injury and property damage versions, with availability depending on state law. To understand how your deductible interacts with these limits, see our explanation of how deductibles work in auto insurance.
Many drivers discover gaps in their coverage only after filing a claim. Our article on coverage gaps drivers discover too late walks through common scenarios where policies fall short.
UIM Triggers Require Careful Limit Matching
Many UIM policies will not pay a benefit if your UIM limit equals or is lower than the at-fault driver's liability limit. For example, if the at-fault driver carries $50,000 in liability and your UIM limit is also $50,000, some policies treat this as no underinsurance gap and decline to pay. Setting your UIM limits meaningfully above typical state minimums is one way to ensure the coverage can actually activate.
State Requirements and What Your Policy May Require You to Read
State law governs whether UM, UIM, or both are required, optional, or offered with a signed waiver option. Some states mandate both; others require only UM; a handful make both entirely optional. Because requirements differ significantly, it's essential to verify what your state mandates — and what your specific insurer's policy language says — rather than assuming bundled UM/UIM means identical protections.
A few additional policy details to look for:
- Stacking vs. non-stacking: Some states allow you to "stack" UM/UIM limits across multiple vehicles on the same policy, effectively multiplying available coverage. Others prohibit stacking entirely.
- Offset clauses: Some policies reduce your UIM benefit by any amount you receive from the at-fault driver's insurer, while others use an excess approach. This distinction can meaningfully change what you collect.
- Consent-to-settle provisions: Settling with the at-fault driver's insurer without your own insurer's consent could affect your UIM claim. Read this clause carefully.
To see how UM/UIM compares alongside other coverage decisions, our guide on state minimum vs. full coverage explains the real trade-offs of carrying only what's legally required. You can also review your auto insurance declarations page to confirm exactly what limits you currently carry.
This article is for general informational purposes only and does not constitute personalized insurance, financial, or legal advice. Coverage terms, requirements, and availability vary by state and insurer. Consult a licensed insurance agent or adviser to evaluate your specific situation.
