Drill Bit Size Chart: Your Complete Guide to Hole Accuracy

Stop guessing at your bits. If you’ve ever snapped a bolt head off. Because your clearance hole was too tight, or watched a screw spin uselessly in a hole that was too large, you know the frustration of poor fitment. A reliable drill bit size chart is the difference between a professional finish and a total scrap-pile disaster. Using the wrong bit ruins expensive materials and wastes hours of your time. Keep this reference saved on your phone or printed for your shop wall—accuracy forces you to measure twice and drill once.

Understanding the Three Systems

Most shops rely on three distinct sizing systems, and mixing them up is a classic rookie mistake. Fractional bits are standard, moving in 1/64-inch increments. These are your bread and butter for general woodworking and basic metalwork. If your bit isn't marked, use a set of calipers to check the decimal equivalent against the table above. Don't trust your eyes.

Then you have the number and letter gauges. These were designed for specific machine shop tolerances—like standardized tap drill sizes. It’s counter-intuitive, but in the number system, a higher number means a smaller diameter. A #80 bit is a microscopic 0.0135 inches, while a #1 is a hefty 0.228 inches. Letter bits pick up where the numbers leave off, running from A (0.234 inches) through Z (0.413 inches). Using the wrong system leads to stripped threads. Always verify the decimal size before you pull the trigger.

Decoding the Table for Real-World Work

When you look at the chart, think of it as a map of the machine shop floor. Each row links four different ways to define a diameter. If you're working on a project that calls for a #80 drill bit, the table shows you're dealing with a tiny 0.0135-inch hole. This is the size of a standard sewing needle—perfect for delicate electronics work or clearing a clogged nozzle, but useless for structural fasteners. If you try to use a 1/8" bit here, you'll be off by nearly 0.11 inches, which is the difference between a secure fit and a destroyed workpiece.

Always align your hardware requirements with the decimal column first. For instance, if your bolt calls for a clearance hole of 0.25 inches, the chart confirms a 1/4" bit is your direct match. However, if you're tapping a hole for a thread, you need the tap drill size. Using a #10 bit at 0.1935 inches for a project requiring a 0.25-inch hole will result in a tap that snaps off inside your material. That’s a ruined part every single time.

Common Pitfalls and Field Observations

In the shop, I often see technicians grab a bit by eye. Never do this. A 9/64" bit (0.1406 inches) looks dangerously close to a 5/32" bit (0.1562 inches) if you aren't paying attention. That 0.0156-inch difference is enough to make a screw feel sloppy or prevent a press-fit pin from holding. Always check the shank markings or use a digital caliper to verify your bit against the decimal values listed here.

• The "Between Sizes" Problem: If your project requires a size not on the chart, go with the next smallest bit and ream it out. It's far easier to enlarge a hole than it's to fill one back in.
• Metric Confusion: Don't assume a metric bit is a perfect swap for a fractional one. A 3mm bit is roughly 0.118 inches, which is smaller than the 1/8" (0.125") standard. Using the wrong system leads to loose fasteners—a classic amateur mistake.
• Wear and Tear: Bits dull over time. A worn bit often cuts slightly smaller than its rated size due to heat expansion and metal displacement.

Consulting official engineering standards for drill tolerances is the best way to ensure your hole quality remains consistent. When you're working with hardened steel or aluminum, remember that the material type forces you to compensate for bit deflection. Print this chart out and tape it to your drill press. Keep it clean. It’s your best insurance against turning a good project into a pile of scrap.

Getting the Most from Your Measurements

If those shank markings have rubbed off, don't guess. Use a set of digital calipers to measure the shank diameter—not the tip, as the cutting edges often wear down first. You can download the PDF version of the complete drill bit size chart to verify your findings against industry standard decimal equivalents. Keep this reference handy on your phone for quick hardware store trips.

Pro Tips for Better Holes

• Pilot Holes: Always drill a pilot hole about 70% of the minor diameter of your screw. This prevents the wood from splitting under pressure.
• Clearance Holes: For machine screws, pick a drill size that's 1/64" larger than the bolt shank. This gives you just enough wiggle room to start the nut without stripping the threads.
• Bit Maintenance: If your bits look dull, toss them. A dull bit creates heat, which hardens the material and ruins your hole tolerance.

Common Questions

Can I use a slightly smaller bit for a tight fit?
Only if you’re reaming the hole later. If you don't, you’ll likely snap the bolt or crack your workpiece due to excess friction.

Does bit wear change the hole diameter?
Yes. As the cutting edges round off, the bit creates more heat and vibration. This often results in an undersized, rough hole that won't accept your fastener properly.

Should I trust my eyes when picking a bit?
Never. A 1/16" difference is nearly invisible to the naked eye but catastrophic for a precision fit.

Where should I store this chart?
You can print this chart out and tape it directly to your drill press. Having the data right in front of you prevents the "just one more hole" mistake that ruins a project.

Stop wasting good material on bad holes. Save this guide, keep your bits sharp, and let the numbers do the work for you.

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