The fractional drill bit sizes base on the American imperial system and follow ANSI standards. They grow in steps of 1/64 inch, usually from 1/64 to 1 inch, although larger sizes with Morse taper shanks also exist. The standard ANSI B94.11M-1979 defines them from 1/64 inch and upward to 1 inch in steps of 1/64 inch.
Fractional drill bits measure in parts of the inch, like 1/16 or 1/8 inch, and they are the most used for everyday work.
Fractional Drill Bit Sizes
Fractional sizes commonly use in the United States, usually from 1/64 to 1 3/4 inch in steps of 1/64 inch. They start at 1/64 inch and go upward to around 1 1/2 inch in 1/64, then in 1/32 inch steps. The smallest fractional drill bits are 1/64 inch.
It is not a perfect system, but it works for more than 150 years. Much old equipment requires repair and care, so this system will last yet another century.
Cheap sets of drill bits most carry steps of 1/16 to 1/4 inch, along with 5/16, 3/8 and 1/2 inch, usually 17 bits. Because imperial drill bits depend only on inches, just one single unit, no different than metric.
Charts of drill bit sizes show several measurement systems. This covers fractional, metric, number wire gauge and letter sizes. Excellent chart of drill bit sizes with fractions have columns for fraction in inches, decimal in inches and metric in millimeters.
Start by the fractional column for fast identification later read the decimal and millimeter values. Free printable charts with all 318 standard sizes, including fractional, metric, number and leter series with decimal matches, tap sizes and clearance holes.
Only few fractional drill bits genuinely match metric sizes. For instance, a 1/2-inch drill bit match 12.7 millimeters. Both 31/64 and 1/2 inch match 12.5 millimeters, so they both work well, but simple drill bit sets commonly lack 31/64.
For a tapped hole in metal for a 1/4-20 screw, a number 7 drill bit in 0.201 inch is recommended. The nearest fractional size is a 13/64 inch bit. Standard metric or fractional drill bit works well, but for better fit, change one size up or down according to a conversion chart.
Instead of a 1/4 inch or 0.250 bit, move some thousandths up or down for better result. Buying a set with 116 bits, you get access to more in-between sizes.
A hobby drill gauge with 29 holes to check drill bits gives fast and accurate way to find sizes of 1/16 to 1/2 inch. It helps well to identify fractional drill bit sizes in the spot.
