Stacked vs Unstacked Uninsured-Motorist Coverage

A worked example

You have a 3-vehicle household policy with 50/100 UM-BI on each vehicle. An uninsured driver hits you with $200,000 in medical bills.

• Unstacked: UM pays the per-vehicle limit of 50/100 = $100,000. You eat the remaining $100,000 personally.
• Stacked (if your state and policy allow): 3 vehicles × 50/100 = 150/300 effective. UM pays $200,000 (within the $300,000 per-accident effective limit). You eat $0.

States that permit stacking

Stacking is explicitly permitted by statute in Florida, Pennsylvania, Louisiana, Mississippi, Texas (limited), Oregon, Washington, Wisconsin, and a few others. The exact list and the stacking formula vary by state. Pennsylvania is one of the clearest permission states; stacking is standard unless you sign a specific waiver.

States that prohibit or restrict

Stacking is prohibited or materially restricted in California, New York, Michigan, Minnesota, Massachusetts, and most no-fault states. In California, UM-BI limits are per-vehicle only; there is no stacking available at any premium.

Typical cost premium

Stacking adds roughly 10 to 25 percent to the UM-BI line. On a 3-vehicle policy with $100/year UM-BI per vehicle, stacking might add $30 to $75 per year. Compared to the potential recovery (tripling your effective per-accident UM limit), the math is usually favorable if your state permits and you have household assets to protect.

When to carry stacked UM

• Multiple vehicles on the policy (stacking requires at least 2).
• Live in a high-uninsured-driver state (FL, MS, NM, LA, TN have 20%+ uninsured rates).
• Household net worth that would be meaningfully exposed by exhausting unstacked limits.
• Carry an umbrella: umbrella usually requires UM-BI at or above a floor, and stacked helps you meet it.