Drill Bit Decimal Equivalent Chart: Your Quick Reference Guide

If you've ever spent ten minutes hunting for the right bit only to realize your "1/4-inch" hole is actually 0.260 inches wide, you know the frustration of sloppy tolerances. A drill bit decimal equivalent chart is the only way to kill that guesswork before it ruins your workpiece. When you're tapping threads or seating press-fit pins, being off by even 0.005 inches—the thickness of a piece of paper—is enough to strip a hole or snap a tap. Keep a printed copy of this data near your mill or drill press. It’s the difference between a professional assembly and a pile of scrap metal.

Decoding the Sizing Systems

Most hobbyists stick to fractional bits, but that limits you to 1/64-inch increments. That’s rarely enough for tight-tolerance engineering standards. It demands master the three common systems to get the fit you actually want:

• Fractional: These are your standard 1/16, 1/8, and 1/4 inch bits. They're fine for rough work but lack the granularity needed for precision.
• Letter Sizes (A-Z): These fill the gaps between fractions, ranging from 0.234 inches (A) to 0.413 inches (Z). They allow for incremental sizing that fractional sets simply ignore.
• Wire Gauge (1-80): This is the secret weapon for threaded fasteners. A #7 bit is 0.201 inches, perfect for a 1/4-20 tap.

Using the wrong system is a trap. Don't fall for it.

Reading the Chart Like a Pro

Columns in this chart are your map. The Type column tells you which standard you're dealing with, while the Decimal Equivalent provides the raw math required for your caliper readings. Don't eyeball it. If your digital display reads 0.015, you're looking at a #80 bit, not a 1/64-inch bit. That 0.0006-inch difference—the gap between the #80 at 0.0135 and the 1/64 at 0.015625—can be the difference between a pin that slides in and one that binds tight.

Data Points That Save Your Workpiece

Look at the transition between #68 and 1/32. The #68 bit sits at 0.0310 inches, while the 1/32-inch bit is 0.03125. That is a tight margin. If you're reaming a hole for a press-fit dowel, using the wrong one will leave your part loose or impossible to assemble. Similarly, notice the #56 bit at 0.0465 inches compared to the 3/64-inch bit at 0.046875. You're dealing with a variance of less than 0.0004 inches. Use the wrong one, and you'll ruin the interference fit of your assembly.

Common Traps and Field Observations

• The Rounding Error: Never round up or down when using this chart. If your spec calls for 0.050, use the #55 bit exactly. Rounding to a nearby fraction often puts you outside your tolerance zone.
• Bit Wear: In the shop, I often see guys use bits until they're dull. A worn bit doesn't just cut slow; it cuts small. A 0.03125 bit that's been run through hardened steel for a month might measure closer to 0.0305. Always check your bits with a micrometer before starting a critical hole.
• The "Between Sizes" Dilemma: If your required decimal doesn't appear on the list, you have two choices. Either step up to the next size and use a reamer to clean it up, or choose the closest bit and accept a slight deviation in your engineering fit. Never force a bit that's too small. It will chatter and likely snap in the hole.

Print this out and tape it to your drill press guard. It’s better to be slow and accurate than fast and wrong.

Quick Tips for Your Drill Index

Organize your bits by size, not by the order you bought them. Use a dedicated metal index case—it keeps your bits from banging around and dulling their edges. If a bit is missing, mark the slot with a piece of tape so you don't waste time looking for it later. Always wipe your bits down with a light coat of machine oil before putting them away to prevent rust from eating the cutting geometry.

Frequently Asked Questions

• Why do some sets skip sizes? Manufacturers often omit rarely used sizes to keep costs down. If you need a specific hole, you have to buy that individual bit.
• Is it okay to round up? No. Rounding up creates a loose fit that can cause your screw to strip or your pin to wobble.
• How do I keep track of tiny bits? Use a numbered index block. It’s the only way to avoid mixing up a #60 and a #61.

You can download the PDF version of this guide to keep at your bench. Print this chart out and laminate it—grease and metal chips will destroy a paper copy in a week. Keep this reference handy whenever you're prepping for a project. It’s the easiest way to make sure your holes are exactly the size you intended.

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