Standard license plate screws are 1/4 inch diameter with a 20 threads-per-inch pitch, sold as 1/4-20 bolts
The vast majority of vehicles in North America use 1/4-20 stainless steel bolts to mount license plates. The "1/4" refers to the diameter of the bolt shaft, and "20" means there are 20 threads per inch. These bolts typically come in lengths between 3/4 inch and 1 1/4 inches, depending on whether you need them to pass through a single mounting bracket or through the vehicle's frame and bracket together.
Stainless steel is the standard material because it resists rust and corrosion from road salt and weather exposure. Galvanized steel bolts are cheaper but corrode faster, especially in areas with winter road salt. If you are replacing screws that have rusted solid or snapped off, stainless steel will last significantly longer than whatever came off the vehicle originally.
You can find 1/4-20 bolts at any hardware store, auto parts retailer, or online. They cost between 50 cents and $2 per bolt depending on material and length. Most people buy a small pack of 4 to 8 bolts so they have spares if one breaks during installation.
Key Takeaways
- Standard license plate bolts are 1/4-20 stainless steel, meaning 1/4 inch diameter with 20 threads per inch.
- Bolt length varies from 3/4 inch to 1 1/4 inches depending on your mounting bracket and frame thickness.
- Stainless steel resists rust far better than galvanized steel, especially in climates with road salt.
- You can buy replacement bolts at any hardware store without needing a special part number or vehicle-specific order.
Why bolt size matters for license plate mounting
Using the wrong size bolt creates two problems: it either will not thread into the mounting holes, or it will thread in but be loose enough to rattle and eventually fall out. A bolt that is too large in diameter will not fit through the hole at all. A bolt that is too small will spin freely without gripping, and vibration from driving will work it loose over weeks or months.
Loose license plates can fall off entirely, which is a safety hazard for other drivers and may result in a traffic citation for you. A plate that hangs at an angle or flaps in the wind is also more likely to be damaged or stolen. Using the correct 1/4-20 size ensures the bolt threads properly and holds tight.
How to measure your existing bolts if you are unsure
If your vehicle has original bolts still in place and you want to match them exactly, you can measure the diameter with a caliper or ruler. Place the bolt shaft against a ruler and measure across the widest part of the threaded portion. A 1/4 inch bolt measures approximately 0.25 inches or 6.35 millimeters in diameter.
To count threads per inch, place the bolt on a ruler and count how many thread peaks fit within one inch of length. Standard license plate bolts have 20 peaks per inch. If you count 16 or 24, you have a different pitch, which is rare but does happen on some older vehicles or imports.
If you cannot remove the bolt to measure it, take a photo of the mounting area and bring it to a hardware store or auto parts shop. Staff can often identify the size by looking at the bolt head and the mounting bracket together.
Bolt head style and what it means for installation
License plate bolts come with different head styles: Phillips head (cross-shaped slot), slotted head (single straight slot), or square/hex head (six-sided). The head style does not affect the bolt's ability to hold the plate—it only determines which screwdriver or wrench you need to tighten it.
Phillips head and slotted head bolts are most common on license plates because they work with tools most people already own. Hex head bolts require a wrench or socket but are less likely to strip if you over-tighten them. Choose whichever style matches your existing bolts, or pick Phillips head if you are starting from scratch, since a Phillips screwdriver is nearly universal.
Bolt length and how to choose the right one
Bolt length is measured from the underside of the head to the tip of the bolt. License plate bolts typically come in 3/4 inch, 1 inch, or 1 1/4 inch lengths. The correct length depends on how thick your mounting bracket is and whether the bolt needs to pass through the bracket and into the vehicle's frame or bumper.
A 3/4 inch bolt is usually enough if you are mounting a plate directly to a thin plastic or aluminum bracket. A 1 inch bolt works for most standard steel brackets. A 1 1/4 inch bolt is necessary if you are mounting through a thick bracket and into a frame, or if you are using a spacer or standoff between the bracket and the vehicle.
If your bolt is too short, it will not thread deep enough to hold. If it is too long, the tip may stick out past the frame and create a sharp edge or catch on something. When in doubt, measure the thickness of your bracket plus the depth you need to thread into the frame, then add 1/4 inch for safety. Bring that measurement to the hardware store and ask for the closest standard length.
Washers and nuts: when you need them
Most license plate installations use only a bolt—no washer or nut. The bolt threads directly into a threaded hole in the mounting bracket or frame. However, some brackets have smooth holes instead of threaded ones, which means you need a bolt, washer, and nut on the back side.
If your bracket has smooth holes, use a 1/4 inch flat washer under the bolt head and another under the nut on the back side. The washers distribute pressure and prevent the bolt from pulling through the soft metal of the bracket. A 1/4-20 nut matches the bolt and threads onto the end after it passes through the bracket.
Stainless steel grades and corrosion resistance
Not all stainless steel is the same. Grade 18-8 stainless steel (also called 304 stainless) is the most common and offers excellent corrosion resistance for automotive use. Grade A2 stainless is similar and equally suitable. Both will resist rust for years even in harsh conditions.
Cheaper "stainless" bolts are sometimes made from lower grades that corrode faster, especially in salt environments. When you buy replacement bolts, look for packaging that specifies "18-8" or "304" stainless. The small price difference—usually 25 to 50 cents more per bolt—is worth it because you will not have to replace them again in two years.
Frequently Asked Questions
Can I use a different size bolt if my original ones are missing?
You should use 1/4-20 bolts to match the threaded holes in your bracket or frame. Using a different size risks the bolt either not fitting or being loose. If your holes are damaged and stripped, you can drill them out to a larger size and use the next standard size up, but 1/4-20 is the correct original size for nearly all vehicles.
What if my bolt keeps spinning and will not tighten?
The threaded hole is likely stripped or damaged. Try backing the bolt out completely, then threading it back in slowly to feel where it catches. If it spins freely the entire way, the hole is stripped. You can either drill the hole larger and use a bigger bolt with a nut, or fill the hole with epoxy, let it cure, and re-drill it for a new 1/4-20 bolt.
Do I need to use thread-locking compound on license plate bolts?
Thread-locking compound (like Loctite) is optional but helpful if your bolts keep loosening from vibration. Use the removable strength (blue), not the permanent strength (red), so you can remove the bolts later if needed. explore a small drop to the bolt threads before installing, then let it cure for the time specified on the bottle before driving.
Are metric bolts ever used for license plates?
Rarely. Most vehicles use 1/4-20 inch bolts, but some imports may use M6 metric bolts (6 millimeter diameter). If your vehicle is metric and you cannot find bolts that fit, check your owner's manual or contact the dealership. Mixing metric and inch bolts will not work because the threads do not match.
Can I use a bolt that is slightly longer or shorter than the original?
A slightly longer bolt (1/4 inch more) usually works fine and gives you more thread engagement for strength. A shorter bolt risks not threading deep enough to hold securely. Avoid going more than 1/4 inch in either direction from the original length.