Bolt Size Chart: Find the Right Fastener for Every Project

Nothing kills a weekend project faster than realizing you’ve stripped a thread because you guessed on the hardware. We’ve all been there—holding a bolt that looks close enough, forcing it into a tapped hole, and feeling that sickening "pop" of ruined metal. That’s why you need a reliable bolt size chart taped to your workbench. It’s the difference between a secure assembly and a structural failure waiting to happen. You can download our printable version as a PDF to keep on your garage wall for those moments when you’re elbow-deep in a repair and can't afford a trip back to the hardware store.

The chart above acts as your primary reference for identifying fasteners. But it won't help if you don't understand the fundamental divide between systems. Hardware in the US generally falls into two camps: Imperial (SAE) and Metric. Mixing these up is a common mistake that leads to cross-threading, which essentially welds a bolt into a hole—making removal a nightmare.

Standard vs. Metric: What’s the Difference?

The core distinction lies in how we measure thread pitch. Imperial bolts use threads per inch (TPI). A 1/4-inch bolt with a coarse thread has 20 threads per inch, meaning the distance between each peak is exactly 0.05 inches. Metric bolts, conversely, measure the distance between threads in millimeters. An M6 bolt with a 1.0mm pitch has a peak-to-peak distance of exactly 1mm. These systems aren't interchangeable, even if they look identical at a glance.

Consider the 1/4-inch bolt versus the 6mm bolt. A 1/4-inch bolt is roughly 6.35mm in diameter. Because the 0.35mm difference is small, you might be tempted to force a 6mm nut onto a 1/4-inch bolt. Don't. The thread geometry—the angle of the cut—is different enough that you will chew through the metal, leaving you with a fastener that holds nothing. Always verify your hardware before you start turning a wrench. If it doesn't spin on by hand for the first few turns, stop immediately. You’re likely trying to force a metric peg into an imperial hole.

Check the bolt thread pitch chart often. It saves you from buying the wrong box of hardware. Trust me, it’s worth the five seconds of checking.

Decoding the Specs: A Practical Workflow

Reading this chart requires a simple three-step verification process. First, identify your system—Imperial or Metric. Second, measure the major diameter of your bolt. Finally, cross-reference that diameter with the thread specification to confirm you’ve got the right match. If you’re staring at a bolt that measures 0.375 inches, you are looking at a 3/8" fastener. Depending on your application, you’ll then choose between the 16 UNC coarse thread or the 24 UNF fine thread. Use the coarse version for general construction where speed matters, but grab the fine version if you're dealing with vibration-heavy engine components.

You can print this out and keep it near your hardware bins. It cuts down on guessing.

Real-World Hardware Mismatches

Not all fasteners are created equal, and the data reveals exactly where people trip up. Look at the M8 metric bolt in the chart—it requires a 13mm wrench. A common mistake is reaching for a 1/2" wrench, which is roughly 12.7mm. While it might grab, you’re almost guaranteed to round off the edges of the bolt head over time. You’re trading a secure fit for a damaged tool surface.

Consider the #6 (0.138") Imperial bolt. It’s tiny, yet it demands a 1/4" wrench. If you try to use a 6mm wrench—which is just slightly smaller—you’ll be fighting the tool every time. Conversely, the M20 bolt is a beast, requiring a 30mm wrench. If you don't have that specific size, don't try to use an adjustable wrench. The clout needed for an M20 bolt will likely slip and strip the hex head entirely. Use the right tool. Every single time.

Navigating the Gray Areas

What happens when your bolt doesn't match a row on the chart? You might have a specialty fastener, like a carriage bolt or a socket cap screw. These often have different head dimensions than the standard hex head bolts listed here. If your bolt head is significantly smaller or larger than the listed wrench size, stop. You are likely working with a specialized grade or a different head style that requires a specific socket type.

Another common snag is the "#" designation for small Imperial bolts. Sizes like #6, #8, and #10 aren't measured in fractions of an inch, but in machine screw gauges. A #10 bolt is 0.190 inches, which sits right between the 3/16" and 1/4" marks. Don't eyeball it. If you try to force a standard 1/4" nut onto a #10 bolt, it will fall right off. If you try the reverse, you’ll ruin the threads on the bolt. Use a thread gauge or a set of calipers to confirm the diameter before you start.

• Verify the pitch: If a bolt resists turning after half a rotation, back it out. You’ve likely mixed a 1.25mm pitch M8 bolt with a 1.5mm pitch M10 hole.
• Check the fit: The wrench size is just a starting point. Always test the socket fit on a spare nut before applying torque.
• Avoid the "close enough" trap: A 0.5mm difference in wrench size feels negligible until you’re trying to remove a rusted bolt under a vehicle.

Save the PDF version of this table to your phone. It keeps the data in your pocket when you’re standing in the hardware aisle.

Mastering Fastener Strength and Selection

Hardware isn't just about fitting the right hole; it’s about holding the load. You’ll see markings on the heads of bolts—usually lines or numbers. These aren't decorative. They indicate the grade, or tensile strength, of the steel. A Grade 5 bolt, marked with three radial lines, handles moderate stress. If you swap that for a Grade 8 bolt—which has six lines—you’re stepping up to a hardened alloy capable of managing much higher tension. Using a Grade 5 bolt in a high-stress spot like a suspension component is a recipe for a snapped fastener. Match the grade to the job.

Washers are your best friend for protecting the surface finish of your project. They distribute the clamping force over a wider area. Without one, the underside of a bolt head can dig into soft aluminum or wood, creating a burr that invites rust. Always use a flat washer under the nut or bolt head unless the design specifically calls for a lock washer to prevent vibration-induced loosening.

Keep a small, labeled container of common hardware on your workbench. Having a dozen extra M6 nuts or 1/4-inch washers saves you a twenty-minute drive to the store when you drop one behind the workbench. It’s cheap insurance against frustration.

Common Bolt Size Questions

What do I do with a mystery bolt from my junk drawer?

If you find a random bolt, don't guess. Use a thread gauge to check the pitch and a set of calipers to measure the diameter. If you don't have those, take it to the hardware store and use their physical test boards. You’ll know in seconds if it’s a standard fine thread or a metric coarse thread. Never force a bolt into a hole if it feels tight; you'll destroy the internal threads of your project.

Can I swap an M8 bolt for a 5/16-inch bolt?

No. Even though an M8 bolt measures 8mm and a 5/16-inch bolt is roughly 7.93mm, the thread counts are wholly different. The M8 usually has a 1.25mm pitch, while a 5/16-inch bolt typically has 18 threads per inch. They won't thread together correctly. Mixing them results in a loose, unsafe connection that will fail under load. Stick to the system the hole was tapped for.

How do I measure bolt length correctly?

Length measurement depends entirely on the head style. For a standard hex bolt, you measure from the underside of the head to the tip of the threads. For countersunk or flat-head screws, you measure the total length including the head, because the head sits flush inside the material. Get this wrong, and your bolt will either bottom out or stick out too far.

You can download the PDF version of our bolt size chart to keep near your workspace. Having this reference handy keeps your projects moving and your hardware pile organized. You can print this chart out and tape it directly to your tool cabinet for quick access whenever you’re staring down a pile of mystery parts. Save this guide as a bookmark—you’ll thank yourself the next time you’re in the middle of a teardown.