Standard License Plate Bolt Sizes in the United States

Most U.S. license plates use 1/4-inch diameter bolts, typically in either 5/8-inch or 3/4-inch length. The 5/8-inch length is most common for mounting directly to a bumper or frame. If you are drilling through a plastic bumper cover or need extra length to reach through a bracket, 3/4-inch bolts work as well.

The bolt thread is almost always UNC (Unified Coarse), which is the standard thread pitch for 1/4-inch fasteners in North America. When you buy bolts at a hardware store, a 1/4-inch UNC bolt is the default — you do not need to ask for a special thread type.

The head style matters less than the diameter and length. Hex head bolts (six-sided) are easiest to tighten with a wrench, but Phillips head or square head bolts work just as well. What matters is that the bolt fits through the oval hole in your license plate frame and the mounting bracket underneath.

Key Takeaways

  • Standard U.S. license plates mount with 1/4-inch diameter bolts, usually 5/8-inch long, with UNC threads.
  • The bolt head style (hex, Phillips, or square) does not affect function — choose whichever you can tighten easily.
  • If your bolts spin without tightening, the threads are stripped; replace the bolt and use a nylon-insert lock nut to prevent future spinning.
  • Some states and Canadian provinces use different bolt sizes; check your local vehicle code or the mounting holes on your plate before buying.

Why Bolt Size Matters for License Plates

The oval holes stamped into a license plate frame are sized to fit a 1/4-inch bolt snugly. If you use a smaller bolt (like 3/16-inch), it will rattle and may fall out. If you use a larger bolt (like 5/16-inch), it will not fit through the hole at all.

The length is equally important. A bolt that is too short will not reach the threaded hole or nut on the other side of the bumper, leaving the plate loose. A bolt that is too long will stick out past the nut and can catch on debris or create a safety hazard. The 5/8-inch length is designed to pass through a standard license plate frame and reach a threaded hole in a typical bumper or bracket with room to spare.

What You Need to Buy at the Hardware Store

Walk into any hardware store and ask for 1/4-inch UNC bolts, 5/8-inch long. You will find them in the fastener aisle, usually sold in small boxes of 4, 8, or 10. The cost is typically 50 cents to $1.50 per bolt. Buy at least four — two for the front plate and two for the rear, plus one or two spares in case one gets lost or damaged.

You will also need 1/4-inch UNC nuts and 1/4-inch washers if your bumper does not have pre-drilled threaded holes. The washers sit under the bolt head and nut to spread the clamping force and prevent the bolt from pulling through the plastic frame. If your bumper already has threaded holes, you may not need nuts — the bolt will screw directly into the threaded hole.

If the bolts keep spinning and will not tighten, buy 1/4-inch nylon-insert lock nuts (also called nylock nuts). The nylon ring inside the nut grips the bolt threads and prevents it from loosening or spinning on its own. This is the most common fix for a license plate that rattles or spins.

Regional and International Variations

Most U.S. states use the 1/4-inch standard, but a few states and Canadian provinces have different requirements. Some older vehicles or specialty plates may use different bolt sizes. Before you buy, check the mounting holes on your actual license plate frame — measure the hole diameter with a ruler or calipers if you are unsure.

If you are mounting a plate in Canada, verify the bolt size with your provincial vehicle code or the documentation that came with the plate. Some Canadian provinces use metric bolts (6mm or 8mm) instead of imperial sizes. If you are unsure, bring the plate frame to the hardware store and test-fit a bolt before you buy a full box.

How to Install License Plate Bolts Correctly

Start by positioning the license plate frame against the bumper or mounting bracket. Mark the bolt holes with a marker or tape so you know exactly where to drill if the holes are not already there. If the bumper already has holes, skip to the next step.

Insert the bolt through the oval hole in the license plate frame, then through the mounting bracket or bumper. If you are using a nut, slide a washer onto the bolt first, then thread the nut onto the bolt by hand. Tighten with a wrench — snug enough that the plate does not move, but not so tight that you crack the plastic frame. A quarter-turn past hand-tight is usually enough.

If the bolt spins without tightening, the threaded hole is stripped or the nut is not catching. Remove the bolt, replace it with a new one, and use a nylon-insert lock nut. If the hole itself is stripped, you may need to drill out the hole and use the next size up (5/16-inch), or move the plate slightly and drill a new hole.

Troubleshooting Loose or Missing Bolts

License plate bolts loosen over time because of road vibration. Check your bolts every few months and tighten them if they have come loose. A loose bolt can fall out and leave your plate hanging or dragging.

If a bolt keeps loosening no matter how tight you make it, the threads are probably stripped. Replace it with a new 1/4-inch bolt and use a nylon-insert lock nut. If the bolt hole itself is damaged or enlarged, you can drill a new hole a few inches away and mount the plate there instead. Make sure the new hole is still within the bumper or bracket — do not drill into a structural part of the vehicle.

If you lose a bolt on the road, you can temporarily hold the plate with a single bolt and nut while you buy a replacement. Do not drive long distances with only one bolt — the plate can shift and become unreadable or fall off entirely.

Frequently Asked Questions

Can I use stainless steel bolts instead of regular steel?

Yes. Stainless steel bolts resist rust better, especially in areas with road salt or high humidity. They cost a bit more but last longer. Make sure you buy stainless steel nuts and washers too — mixing metals can cause corrosion.

What if my license plate frame has different sized holes?

Some aftermarket frames have oversized or oddly-shaped holes. Measure the largest diameter of the hole and buy bolts that fit snugly. If the holes are much larger than 1/4-inch, you may need larger washers to prevent the bolt head from pulling through the plastic.

Do I need a lock washer under the bolt head?

A lock washer (the kind with teeth or ridges) can help prevent loosening, but it is not required if you use a nylon-insert lock nut. If you use a regular nut, a lock washer under the bolt head and another under the nut will reduce vibration loosening.

Can I use a bolt that is longer than 3/4-inch?

You can, but it is not necessary and the extra length can catch on debris or create a snag hazard. Stick with 5/8-inch or 3/4-inch — both are designed for this job and are widely available.

What if the bolt holes in my bumper are not threaded?

Use a bolt, washer, and nut. The bolt passes through the bumper hole, and the nut on the back side holds it in place. This is the most common setup for plastic bumpers. Tighten the nut with a wrench while holding the bolt head steady with another wrench so the bolt does not spin.