Drill size chart - inch and metric | Downloadable PDF - Mechanical Engineering (2024)

• September 9, 2018
• EngineeringClicks

Every drill size chart you’ll ever need is in this article! No more scratching your head wondering which gauge relates to which size or how to convert inches to mm or vice versa. This is your one-stop-shop for all drill bit size conversions and tables.

But, there’s more to this article than just charts.

You will also get some background on how the various drill sizes came about, how they are calculated and some of their applications. If you’ve ever wondered why there are so many different gauge designations and drill bit sizes, then read on.

How to Measure Drill Bits

To measure the diameter of a drill bit, it’s best to measure the cutting edge rather than the shank (the shank is the part that is held by the drill chuck). To get an accurate measurement, use Vernier callipers, but be aware that depending on the quality of the drill bit, there could be a tolerance error of between 5 and 15 thousandths of a unit.

Metric Drill Bit Sizes

Metric drill sizes are used primarily in Europe and use metric units to measure the drill bit diameter. There are a variety of standards used to define drill sizes, the most common one in the UK is the British Standard BS 328 set which includes twist drill bit sizes between 0.2 mm and 25 mm.

BS 328 drill sizes are defined as shown in the drill size chart below:

Please note that BS 328 sizing only applies to twist drill bits. Other bit types, such as Forstner bits, will contain different size cutting surfaces.

Another example of a metric set of drill sizes is the Renard series, which uses a simple factor to determine the diameter:

• The R5 series uses a factor of 1.58 and contains the following sizes: M2.5, M4, M6, M10, M16 and M24
• The R10 series uses a factor of 1.26 and contains the following sizes: M3, M5, M8, M12, M20 and M30

Drill Size Chart for Number and Letter Gauges

Number and letter gauges do not follow a formula or standard increments. They are loosely based on the Stubs Steel Wire Gauge, with gauge sizes that are similar (but not entirely identical) to that system.

Although they originated in the 19^thcentury, these gauges are still commonly used in the U.S. They are also seen in the UK to a lesser degree, having been largely replaced by metric sizes. Most other countries, especially in Europe, have abandoned them entirely in favour of metric systems.

The drill size chart below contains imperial and metric drill bit diameter measurements for all number and letter gauges.

Fractional-inch Drill Size Chart

Fractional drill bits are definedby the ANSI B94.11M-1979 standard.

Most commonly used in the US, but can still be found occasionally in the UK, these imperial measurement drill bits come in 1/64 inch increments. The fractions used are always in their simplest form, i.e. reduced as far as possible, so 2/64 becomes 1/32 for instance.

The drill size chart below gives fractional sizes and their metric conversions up to 1 inch.

Centre Drill Bit Sizes

These specialist drill bits are used for drilling centre holes at two different angles, 60 degrees and 90 degrees. They are commonly used to drill lathe centres and spotting drills, i.e.marking holes to guide twist drilling.

The drill size chart below contains all commonly used centre drill bit sizes.

Drill Bit Lengths

The length of drill bits vary depending on their application.

• Jobber-length drill bits are the most common, with a length somewhere between 9 to 14 times the diameter.
• Screw-machine-length drills arethe shortest available standard sized bits. They are used primarily in machinist workshops for screw machinesand are not usually available in hardware stores.
• Aircraft-length drills are extended length drill bits that are manufactured in fixed length such as 4, 6, 8, 12, 18, 24-inch bits, rather than proportional lengths.