Drill Bit Size Converter
Convert fractional inch drill bit sizes to millimetres. Covers 1/16" through 1/2".
How to use this tool
- Enter drill bit size in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your diameter and the full breakdown beneath it.
Convert fractional inch drill bit sizes to metric (mm) equivalents for any drilling project.
Formula
Diameter (mm) = Fraction (inches) × 25.4
For example: 1/4 in × 25.4 = 6.35 mm
The converter uses a pre-built lookup table of standard fractional sizes (1/16" through 1/2") rounded to two decimal places.
How it works
Each fractional inch drill bit size is mapped to its millimetre equivalent by multiplying the decimal value of the fraction by 25.4 (the exact number of millimetres in one inch). The lookup table stores pre-rounded values matching standard manufactured bit sizes, so results correspond directly to what you will find on a drill index.
A common mistake is confusing fractional inch sizes with number or letter gauge drill bits — those are entirely separate sizing systems and cannot be converted with a simple fraction-to-mm formula; they require their own lookup tables.
Worked example
Convert a 1/4-inch drill bit to millimetres
- Identify the drill bit size: 1/4 inch.
- Convert the fraction to a decimal: 1 ÷ 4 = 0.25 inches.
- Multiply by 25.4 mm/in: 0.25 × 25.4 = 6.35 mm.
A 1/4-inch drill bit has a diameter of 6.35 mm.
Common mistakes to avoid
- Selecting the next fractional size up rather than the closest metric equivalent when using metric fasteners -- a 6 mm screw requires a 6.0 mm drill bit; the nearest fractional inch size is 15/64" (5.95 mm), which is slightly undersized.
- Using a drill bit chart for wood when drilling metal -- metal drilling often requires pilot holes and step-down sizes not standard in fractional inch wood bit sets.
- Rounding 25.4 mm/inch to 25 mm for mental math -- a 1/4" bit is 6.35 mm, not 6.25 mm; the 0.4 mm error compounds across hole patterns in precision woodworking or metalwork.
Key terms
- Fractional inch drill bit
- A drill bit whose diameter is expressed as a common fraction of an inch (e.g., 1/8", 3/16"). The most widely used sizing system in North America.
- Millimetre (mm)
- The standard metric unit for small lengths. There are exactly 25.4 mm in one inch.
- Number/Letter gauge bits
- Two alternative imperial sizing series (numbers #1–#80 and letters A–Z) that overlap the fractional range but follow different diameter progressions and are not interchangeable with fractional designations.
- Drill index
- A set of drill bits stored in a labelled holder; fractional indexes typically cover 1/16" to 1/2" in 1/64" increments.
Frequently asked questions
- What is a 1/4 inch drill bit in mm?
- A 1/4" drill bit is 6.35 mm in diameter. To convert any fraction to mm, multiply the decimal value by 25.4.