Making sense of drill bit sizes and the markings stamped on each bit takes the guesswork out of choosing the right one. Once you understand the four common systems, any bit is easy to read.
The Four Sizing Systems
In the workshop you will meet four ways of naming a bit: fractional inch, metric, number gauge and letter gauge. Fractional bits step up in fractions such as one eighth or one quarter inch. Metric bits are measured in millimetres. Number and letter gauges fill in the fine sizes between fractions, which is why machinists rely on them for precise work.
- Fractional: Sized in inch fractions like 1/4 or 3/8 inch.
- Metric: Sized in millimetres, common worldwide.
- Number gauge: Runs from large No.1 down to tiny No.80.
- Letter gauge: Runs A to Z, filling gaps above the number sizes.
How Number and Letter Sizes Work
These two gauges can feel backwards at first. In the number system, a smaller number means a larger bit, so a No.1 is bigger than a No.60. The letter system then picks up above the number range, running from A as the smallest through to Z as the largest. Together they give fine steps that fractions alone cannot.
Reading the Markings
Most bits are laser-etched or stamped near the shank with their size, and quality bits often add the material, such as HSS or cobalt. On very small bits the marking can be hard to read, so a set with a marked index or a drill gauge is invaluable for confirming a loose bit. When the etching wears off, a gauge plate tells you the diameter instantly.
- Check the shank: Size and often the material are marked there.
- Use a gauge: A drill gauge plate identifies unmarked bits.
- Keep the index: Storing bits in a labelled case avoids mix-ups.
Converting Between Systems
Jobs often mix systems, and a metric fastener may call for a bit only listed in inches or a number gauge. A conversion chart lets you find the nearest equivalent quickly, and a tap drill chart tells you the exact bit for threading a given size. Keeping these references handy prevents costly wrong-size holes.
Choosing With Confidence
Once you can read the markings and move between systems, selecting the right bit becomes second nature. Pair that knowledge with quality American-made bits held to tight diameter tolerances, and every hole comes out the size you intended.