Metric drill bit sizes measure in millimetres. In United States people still use fractional drills. Useful drill charts work as fast converters between decimal and metric values and help to move between them
The chart shows standard drills in various systems: fractional, metric, wire gauge and letter. It shows diameters in inches and millimetres. Fractional values use inches, metric use millimetres.
Metric and Inch Drill Sizes and How to Convert
For all drills use metric dimensioning, although BS 328 counts only for twist drills. For instance, Forstner bits commonly have cutters in 10, 15, 20, 25 and 30 mm. In stores sell standard metric drill sets of 1 to 10 mm in 1 mm steps; sets until 12 or 15 mm also availabel.
To convert inches to millimetres, multiply by 25.4. Otherwise, divide millimetres by 25.4. Like this half-inch drill matches 12.7 mm.
Only few fractional sizes exactly match metric. Near fractions not always work, so for M-thread tapping best choose metric.
Metric drills are sold in every size by 0.1 mm steps. That matches around 0.004 inches. 0.1 mm difference suffices for many cases.
For general holes, 1/64-inch drills well serve, if a bit of tension or play is acceptable. But for precise tasks as tapping or alignment with existing hole, tenth-millimetre drills are the right choice.
Number drill sizes come from wire standards. Bigger numbers mean smaller diameters. ASME B94.11M standard defines sizes of 1 to 97.
Letter sizes go from A to Z. 60-piece number set of 1 to 60 covers drills for tapping or clearance of almost every fastener, whether inch, whether metric, until 1/4 inch or 6 mm.
Standard imperial set has 29 bits instead of 25 metric and covers a bit smaller range of 1.6 to 12.7 mm, so its step is finer. Metric charts usually add information about tap-drills and clearance holes. Available printable PDF-charts with sets for 19-piece and 25-piece kits.
Such converters from standard sizes commonly use in CNC-production and lower cost of special tools. Metric drill reaches inside two thousandths of any imperial size, what means precise thread-engagements without rarebits.
