Standard License Plate Screw Sizes in the United States

License plate screws in the United States are almost always 5/16 inch diameter with a 18 threads-per-inch pitch, which means they are metric-compatible with M8 bolts. The head is typically a Phillips or slotted style, and the length runs between 3/4 inch and 1 1/4 inches depending on whether you are fastening through a single bracket or through the bumper itself. Most states do not mandate a specific screw size, so manufacturers have settled on this single standard because it works with nearly every bracket and bumper design sold in North America.

The reason this size became standard is practical: 5/16 inch is large enough to hold a plate securely without stripping, small enough to fit through standard bracket holes, and common enough that any hardware store carries replacements. If you lose a screw or need to replace a rusted one, you can walk into any auto parts store or home improvement retailer and ask for a 5/16-inch stainless steel bolt, and it will fit.

Key Takeaways

  • Standard U.S. license plate screws are 5/16 inch in diameter with 18 threads per inch, compatible with M8 metric bolts.
  • Length varies from 3/4 inch to 1 1/4 inches depending on whether you are fastening through a bracket alone or through a bumper.
  • Stainless steel screws resist rust better than zinc-plated ones and are worth the small extra cost in coastal or wet climates.
  • Most hardware stores and auto parts retailers stock 5/16-inch bolts as standard inventory, making replacements straightforward to find.

Why Material Matters More Than Size

The material of the screw matters more than getting the exact length right. Stainless steel resists corrosion and will not rust even after years of exposure to road salt, rain, and humidity. Zinc-plated steel screws are cheaper but will eventually corrode, especially in coastal regions or areas that salt roads in winter. Once rust forms, the screw becomes difficult to remove without breaking, and you may end up drilling it out.

If you are replacing screws, buy stainless steel 5/16-inch bolts with Phillips heads. The extra cost is usually less than a dollar per screw, and you will not have to replace them again in five years. Avoid plated screws unless your vehicle is garaged year-round in a dry climate.

How to Measure a Screw You Already Have

If you have a screw in hand and need to match it, measure the diameter with a ruler or caliper at the widest part of the threaded shaft — not the head. Then count the threads in one inch of length. For a standard license plate screw, you should get 5/16 inch (or about 8 millimeters) and 18 threads per inch.

If the screw is corroded or the threads are damaged, do not try to reuse it. Buy a new one in stainless steel. A damaged screw will not hold the plate securely and may fall out while you are driving.

Bracket and Bumper Variations

Most modern vehicles use a plastic or metal bracket that bolts to the bumper, and the license plate bolts to that bracket. In this setup, you typically need 3/4-inch screws because they only pass through the bracket and the plate. Older vehicles or those with custom bumpers sometimes require 1-inch or 1 1/4-inch screws to pass through the bumper material itself.

If you are unsure which length you need, remove one existing screw and measure it, or take a photo to an auto parts store and ask. Most staff can identify the correct length by looking at your vehicle or the bracket. Do not guess — a screw that is too short will not hold, and one that is too long may poke through and damage the bumper or bracket.

Where to Buy Replacement Screws

Auto parts chains like AutoZone, O'Reilly, and NAPA stock 5/16-inch stainless steel bolts in bulk packs of four to eight. Home improvement stores like Home Depot and Lowe's carry them in the fastener aisle. Online retailers including Amazon and Bolt Depot offer larger quantities if you want to keep spares. Prices typically range from 50 cents to $2 per screw depending on material and where you shop.

If you need the screw when ready, a local auto parts store is fastest — most have them in stock and can sell you one or two without requiring a bulk purchase. If you are stocking up or want the best price, online ordering is usually cheaper per unit.

What Happens If You Use the Wrong Size

A screw that is too small in diameter will not grip the bracket or plate and will spin loose within days. A screw that is too large will not fit through the holes at all. A screw that is too short will not reach the threaded hole in the bracket, leaving the plate loose. A screw that is too long will poke through the back of the bracket or bumper and may damage paint or plastic.

If you install the wrong size and the plate falls off while driving, you are breaking the law in most states — license plates must be visible and securely fastened. You may also be liable for any damage the plate causes if it strikes another vehicle. Always verify the size before installing.

Frequently Asked Questions

Can I use a bolt instead of a screw?

Yes. A 5/16-inch bolt with a nut works just as well as a screw, and many people prefer it because you can tighten it from both sides. Use a washer under the bolt head and nut to prevent them from sinking into plastic brackets. Stainless steel bolts and nuts resist rust the same way stainless screws do.

What if the hole in my bracket is stripped?

If the threaded hole no longer grips the screw, you can drill the hole larger and use the next size up — typically 3/8 inch — or install a threaded insert (a small metal sleeve that restores the original thread size). Threaded inserts are sold at hardware stores and take about five minutes to install with a drill. This is cheaper and faster than replacing the entire bracket.

Do I need to use threadlocker on license plate screws?

Threadlocker (a liquid adhesive that prevents bolts from loosening) is not necessary for license plate screws because the vibration is minimal and the screw is not under high stress. If you want extra security, a small drop of removable threadlocker (blue, not red) will not hurt, but it makes future removal harder. Stainless steel screws that are tight enough will stay in place without it.

Why do my license plate screws keep rusting?

You are likely using zinc-plated or mild steel screws, which corrode quickly in wet climates or near salt. Switch to stainless steel and the rust will stop. If the old screws are already rusted, remove them and replace them with stainless ones. Stainless steel costs slightly more but lasts the life of the vehicle.

Can I use a different head style, like a hex head?

Yes. The head style does not matter as long as the diameter and thread pitch are correct. Hex heads are easier to tighten with a wrench and less likely to strip than Phillips heads. Many people prefer them for this reason. Use stainless steel regardless of head style.