When you need to drive a screw but don’t know what size drill bit to grab, this is how it usualy goes: You look around at your drawer of random drill bits until you find one that feels about right size. That’s great if you’re hanging a picture frame, not so good if you are building a piece that will support some weight or fit together preciseley.
The drill index sizes chart converts hole size into four different systems, and knowing which one to work with make drilling a repeatable (rather than a guessing) game.
How to Choose the Right Drill Bit
The numbering system trips you up because it’s counterintuitive: The higher the number, the smaller the bit. We normaly expect a bigger number to mean a bigger size. This makes sense if you are working with wire gauges where #1 is big and #80 is small. For small taps like these, that kind of precision matter: A fraction of an inch off is enough to throw off the pitch of the threads. And when you’re tapping metal components that will see high torque, you want your thread to be precise.
Letter bits fill the middle ground where fractional inches get clumsy. You can ask for a Letter G bit which is unambiguous, rather than try to explain how a three-sixteenths-inch hole differs from a five-thirty-seconds-inch hole over the phone. The letter sizes range from roughly a quarter inch to slightly more than seven sixteenths. That covers most common furnitures hardware and machine bolt clearance holes.
You don’t need to do any mental math with fractions. It works for both heavy structural drilling and fine detail work, filling the gap between the two. For general carpentry, however, fractional bits is still the norm, not just because they match your woodworking measurements, but because they makes sense. If you’ve ever grabbed a tape measure, you know when someone says “a quarter-inch” they’re talking about a one-quarter-inch bit. That’s generally as big as a standard three-jaw chuck will reliably hold in place, which is why most beginning bit collection top out at half an inch. Beyond that, it gets into special tooling, so learning those bits through half an inch should of get you through most homeowner projects on the weekend.
In contrast, Metric sizes goes up in a more straightforward linear fashion, increasing only by half-millimeters. So if you use standard metric fasteners or European machines, it’s easy to scale up. The transition from a 5mm to a 6mm bit is both easy to recognize and simple to compute. There is no conversion between decimals and fractions, so there is no mental friction when you’re under pressure and just has to reach for the next one.
While bit size is important, so is the material from which it’s made. For soft aluminum and wood, high-speed steel will do fine; it won’t last long in hardened alloys or stainless steel. Titanium nitride and cobalt coatings can absorbs to make your bits last longer because they can take more abuse before going dull. Instead of a different set for each task, select materials that best match what you’re working on.
An overlooked practical consideration is picking a suitable shank type for your drill. Hex shanks are designed for use with quick-change chucks and hold fast without slipping when used at high torque. Round (standard) shanks will work in all handheld drills. Reduced shanks allows you to use larger bits in small chucks, which is ideal for confined areas.
So what we have here isn’t just a set of numbers representing dimensions. It’s a decision making roadmap. If you know how each of these systems work, you don’t view the drawer as a puzzle to be solved by finding whatever fits close enough, you reach for the tool that will do the job. That peace of mind is priceless, and it’s well-worth the price of a good set of bits.
