If you've ever stared at a drawer full of bits and guessed between a 1/4 inch and a slightly smaller wire gauge, you know the frustration. That "close enough" mentality is exactly how you strip a thread or snap a tap off inside a $500 workpiece. Precision machining forces you to stop guessing and start measuring. A reliable drill bit size decimal equivalent chart is the difference between a clean fit and a ruined project. It cuts through the noise of three overlapping sizing systems so you can grab the right tool the first time.
Here’s the reference guide it is requisite to keep on your shop wall:
Decoding Drill Bit Systems
Most shops juggle three distinct systems. Fractional bits are standard in 1/64th increments, but they don't cover the fine gaps required for threading. That's where wire gauge (number) and letter bits come in. A #30 bit is 0.1285 inches, while a 1/8 inch bit is 0.1250 inches. That 0.0035-inch difference—roughly the thickness of a sheet of paper—is enough to make a screw feel sloppy or a tap bind in the hole. You can read more about these standardized sizing systems here.
Decimal values are the universal language of the machine shop. Every bit, regardless of its label, has a specific diameter. When one must drill a hole for a 1/4-20 tap, you aren't looking for a "fractional" bit; you're looking for a #7 bit, which is exactly 0.2010 inches. If you reach for a 13/64 bit (0.2031 inches) instead, you lose about 10% of your thread engagement. That’s a massive hit to structural integrity. Always check the decimal.
Interpreting the Data
The chart is organized by size type—Number and Fractional—to help you cross-reference quickly. When you look at the Decimal Equivalent column, remember that these are absolute dimensions in inches. You aren't just looking at labels; you're looking at the physical limit of the tool. If your project calls for a 0.0465-inch hole, the #56 bit is your target. Using a 3/64 bit instead puts you at 0.0469 inches. That 0.0004-inch difference might seem trivial, but it’s the difference between a press-fit pin staying put and falling out under vibration.
Data Points That Matter
Specific rows in the table act as benchmarks for common shop tasks. Take the #80 bit at 0.0135 inches—that’s the smallest size on this list. If you're clearing a clogged carburetor jet or working on precision electronics, it is requisite to verify your stock against this value before you break the bit. On the other end, the #8 bit at 0.1990 inches is a standard clearance size for many #10 machine screws. If you accidentally grab a 13/64 bit (0.1910 inches), your screw will bind, and you'll end up tapping the clearance hole by accident.
Watch the jump between the #30 bit (0.1285 inches) and the 1/8 inch fractional bit (0.1250 inches). I’ve seen many apprentices grab the 1/8 inch bit because it’s "close enough" to the #30. That 0.0035-inch gap is enough to ruin a sliding fit. Always use a micrometer to verify the shank if you aren't sure, or just keep a calibrated digital caliper handy to confirm the measurement against the chart.
Common Pitfalls in the Shop
Don't rely on memory. The most common mistake I see is assuming that wire gauge sizes follow a linear progression. They don't. Look at the shift from #74 (0.0225 inches) to #73 (0.0240 inches). The increment isn't consistent across the entire range. If you try to interpolate the size of a bit that isn't on the list, you’re guessing. Guessing leads to scrap metal. If your required diameter isn't on the chart, calculate the decimal equivalent manually—or better yet, print this out and tape it to your drill press column for instant access.
One final tip: check your bit wear. A bit that's been sharpened three times isn't the same diameter as a fresh one. The chart gives you the nominal size, but real-world tool deflection and wear mean you should always test your hole size in a piece of scrap material first. If the fit is tight, don't force it—grab the next size up from the list.
Workshop Best Practices
Keep a physical copy of this drill bit size decimal equivalent chart right next to your drill press. I keep mine inside a clear plastic sleeve to protect it from stray oil and metal chips. If you need a clean copy, you can download the PDF version and print this chart out for your shop wall. Having it within arm's reach stops you from grabbing the wrong bit when you're in a rush.
Always distinguish between pilot holes and clearance holes. A pilot hole is meant to guide a larger bit or a tap, so it needs to be smaller than the final fastener diameter to provide enough material for threads to bite into. A clearance hole should be slightly larger than the screw shank—usually by 0.005 to 0.010 inches—so the fastener passes through without binding. Verify the decimal size before you pull the trigger. Once the hole is drilled, there's no going back.
Common Questions
- How do I measure an old, unmarked bit? Use a set of digital calipers. Measure the shank, not the flutes, as the flutes often wear down over time.
- Does the chart cover metric sizes? This guide focuses on standard imperial wire, letter, and fractional sizes. If you work with metric hardware, you'll need a separate conversion table for millimeters.
- Why does my bit feel loose in the hole? You likely used a bit that's too large by just a few thousandths. Check the decimal equivalent on the chart against your caliper reading to find the right step down.
Save this guide and keep this reference handy whenever you're setting up a job. Precision is a habit—not an accident. Don't let a simple labeling error wreck your project.
Also Check: Transpose Chords Chart: The Easy Way to Change Keys