Drill bit size chart: fractions to millimetres

A nominal 1/4-inch drill-bit size converts to exactly 6.35 mm, using the NIST definition of 1 inch as exactly 25.4 mm, rechecked 3 October 2026. Use the chart below to convert fractional sizes. For a screw pilot hole, check the screw and the drill guide: a fraction-to-mm conversion alone cannot choose the bit.

What are common fractional drill-bit sizes in millimetres?

These are exact conversions, not promises that a named metric bit is an identical substitute. The downloadable CSV covers every 1/64-inch step from 1/64 to 1/2 inch.

How do you convert an imperial drill size yourself?

Divide the fraction, then multiply by 25.4 mm per inch. For example, 3/32 inch × 25.4 = 2.38125 mm. The inch definition is exact, so these results require no rounding; a bit maker may print a rounded metric label. NIST explains the exact factor.

Is the fraction printed next to a metric bit an exact match?

Check the actual diameter in the maker's specification. Trend labels a 2.4 mm drill in its QR/DBG/SET guide as 3/32 inch, while the exact 3/32-inch conversion is 2.38125 mm. The 0.01875 mm difference is a difference between a nominal label and a mathematical conversion. It does not show the manufacturing tolerance of either bit.

What pilot bit should you use for a No. 6 screw?

The screw number alone does not determine one pilot diameter. Trend lists drill diameters and screw sizes separately for its SNAP/DBG/A and QR/DBG/SET self-centring guide sets; those lists do not explicitly identify which bit pairs with a No. 6 screw. Do not infer a No. 6 pilot size from their order.

If you have a guide, use its supplied bit or replacement specification. Otherwise identify the screw's shank and thread, the material and whether the joint needs a clearance hole. Test on matching scrap material when the fixing maker's instructions leave a choice. A masonry plug or metal tapping hole needs its own specified diameter.

How should you choose a drill bit for the job?

First identify the required hole diameter from the fixing, plug, hardware or drawing. Use the chart to interpret a fractional specification, then check the bit's marked metric diameter and the material it is designed to drill. Our drill-bit buying guide covers bit types; the cordless combi drill guide covers the tool that drives them.

How was this chart made?

We multiplied reduced fractions from 1/64 through 32/64 inch by NIST's exact 25.4 mm/inch factor. The CSV download includes the fraction, exact decimal inch, exact millimetres and source URL for each of the 32 rows. To reproduce a row, divide the fraction's numerator by its denominator and multiply by 25.4. Source and data were rechecked on 3 October 2026. The chart does not assign a pilot bit to a screw size.

What are the limits of this reference?

The chart converts nominal fractional sizes. It does not measure a particular bit, specify tolerances, or tell you which metric size to substitute when a hole's fit matters. The Trend example illustrates a nominal fraction label, not a pilot-hole recommendation. Product specifications can change; check the current manual or product page before drilling a finished part.

How can you cite this reference?

Handy Tools Home, “Drill bit size chart and pilot-hole reference”, verified 3 October 2026. For the unit definition, cite NIST directly. For Trend's nominal 3/32-inch label, cite Trend QR/DBG/SET. The CSV is available for reuse with source attribution.

Frequently asked questions

How many millimetres is a 1/4-inch drill bit?

The nominal 1/4-inch size converts to exactly 6.35 mm using NIST's exact 25.4 mm per inch definition. A physical bit may have a manufacturing tolerance.

How many millimetres is a 3/32-inch drill bit?

The nominal 3/32-inch size converts to exactly 2.38125 mm. A manufacturer may label a 2.4 mm bit as 3/32 inch; individual bits may have manufacturing tolerances.

Is a No. 6 screw pilot hole always 2 mm?

No. The screw number alone cannot establish one pilot diameter. Match the bit to the guide, screw and material using the fixing maker's instructions; do not infer a bit pairing from separate product lists.

Does the chart show which metric bit to buy?

It shows exact unit conversions, not equivalent stocked bit sizes or manufacturing tolerances. Check the required hole fit and the bit maker's marked diameter.

Can I use a wood-screw pilot size for a wall plug?

No. Use the wall plug or fixing maker's specified drill diameter, material and hole depth.