Drill Bit Size Converter (fractional inch ⇄ mm)

Understanding the drill bit size converter — concept and principle

This converter turns a fractional or decimal inch drill size, such as 3/8, 1 1/4 or 0.25, into millimetres and suggests the nearest common metric drill bit. It helps when plans, screws or anchors are specified in inches but your bit set is metric, or the other way round.

How it is calculated

• The input is read as a decimal number of inches: 3/8 = 0.375, 1 1/4 = 1.25.
• Millimetres = inches × 25.4, because the inch is defined as exactly 25.4 mm.
• The nearest metric bit is chosen from a list of common sizes: 1 to 12 mm in 0.5 mm steps, then 13, 14, 15, 16, 18, 20, 22 and 25 mm. The difference (metric − exact) is shown in brackets.

The principle

Since the international yard and pound agreement of 1959, one inch has been exactly 25.4 mm, so the conversion itself has no rounding error. The practical question is matching sets: fractional drill sets usually step by 1/64 inch (0.396875 mm), while metric sets step by 0.5 mm or 0.1 mm, so the two grids rarely line up. Picking the nearest metric bit is fine for many wood and general holes, but the direction of the difference matters. A bit slightly larger gives clearance; a slightly smaller one gives a tighter fit.

Worked example

Default input: 3/8 inch.

1. 3 ÷ 8 = 0.375 inch.
2. 0.375 × 25.4 = 9.525 mm.
3. Nearby metric bits are 9.5 mm (0.025 mm smaller) and 10 mm (0.475 mm larger). The closer one is 9.5 mm.

The calculator shows 9.525 mm, nearest metric bit 9.5 mm (−0.025 mm), matching these steps.

How to read the result

A negative difference means the metric bit is smaller than the inch size; a positive one means it is larger. Differences of a few hundredths of a millimetre are usually irrelevant in wood and plastic. For a dowel, bearing or press fit, even 0.1 mm can matter, and for a bolt clearance hole you should round up rather than to the nearest size.

Common mistakes

• Using the nearest size for tap drill holes. Tap drill size depends on thread pitch and must come from a tap drill chart.
• Typing 3/8 as 3.8. The calculator expects fractions or decimal inches, not millimetres.
• Forgetting that real holes often end up slightly larger than the bit because of wobble and material.
• Confusing fractional sizes with number (#1–#80) or letter (A–Z) drill gauges, which are separate inch series.

Frequently asked questions

What is 3/8 inch in millimetres?

9.525 mm; a 9.5 mm bit is the closest metric size.

What is 1/4 inch in mm?

6.35 mm. The nearest common metric bits are 6 mm and 6.5 mm.

Should I round up or down?

Round up for clearance holes, choose slightly smaller for pilot holes in wood, and use a chart for tapped threads.

Sources: NIST Special Publication 811, Appendix B (inch = 25.4 mm exactly) · ASME B94.11M, Twist Drills