You can actually custom order a drill bit to any size, but most drill bits – the actual cutting part on the end of the drill – are manufactured to standard sizes.
In the United States, fractional inch sizes are still used; in most other parts of the world, metric bit sizes are normally used.
Because of the two common different measuring systems – fractional and metric – it can be a little confusing buying the right drill bit size. To confuse things even more, there are two other ways of measuring – letter sizes and wire-gauge sizes.
If you are purchasing drill bit sizes in the United States, the size will probably be measured in fractions. The standard “twist” drill bits, which most people use begin at 1/64 of an inch and continue in 1/64 increments up to 1 inch. One disadvantage of this method of bit sizing is that the size increment between drill bits is extremely large for the smaller sizes – a difference of 100% between the first two sizes.
Metric drill bit sizes were introduced by the British Standard in 1959. The British Standard BS 328 identifies bit sizes ranging from the smallest, 0.2mm, to the largest which measures 25mm.
Letter sizes for drill bits are perhaps the easiest to understand. These bits are used to make precise small hole sizes and are simply labeled from the smallest – A – to the largest – Z. Even the largest of these is a mere 0.4 inches in diameter, so you would use letter sizes for detailed and close up work.
Wire gauge sizes, as the name suggests are standard measurements used for drilling holes for particular diameters of wire, especially wires conducting electricity. Most of these sizes are extremely small – a size AWG 36, for example is just 0.005 inches in diameter.
Drill bits with a “twist” on the end are also sold in standard lengths as well as sizes – the length is in proportion to the diameter of the bit.
Unless you are an expert and need a particular size, it is probably best just to purchase a variety of drill bits of varying sizes, ensuring you always have one that’s the right size for the job.