Convert common drill bit sizes between fractional inch, decimal inch and millimeters.
Tools • drill sizes
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Drill bits come in fractional-inch, metric (mm), wire-number and letter sizes, and workshops mix all four. This chart converts the common drill sizes between fractional inch, decimal inch and millimeters.
It is essential for woodworking, metalworking and DIY when matching a bit to a hole, screw or tap — for example a 1/4" bit is 0.25" = 6.35 mm, with 6.5 mm being the nearest metric bit.
US drill bits come in four overlapping systems that together provide fine coverage of diameters: fractional inch sizes in sixty-fourths, number sizes from 80 up to 1 (smaller number means larger bit), letter sizes from A to Z, and metric. The systems interleave, so a complete index set moves in small increments across the whole range. The practical use is tap drilling, where the correct hole size is the thread's major diameter minus its pitch, and the required bit is frequently a number or letter size rather than a convenient fraction.
Fractional: sixty-fourths of an inchNumber sizes: 80 (smallest) to 1 (largest) — counter-intuitively orderedLetter sizes: A (smallest) to Z (largest), spanning above the number sizesTap drill diameter ≈ major diameter − thread pitchwhere:
Assumptions: Tap drill charts assume about 75% thread engagement, which balances strength against tapping torque. Going to 100% engagement adds little strength and greatly increases the risk of snapping the tap.
Convert three bits from different systems to millimetres and order them.
Result#7 = 5.105 mm · ¼ in = 6.35 mm · F = 6.528 mm
The number-7 tap drill for a ¼-20 thread is the clearest illustration of why the extra systems exist: no convenient fraction sits at 5.105 mm, and using the nearest one (13/64 at 5.16 mm) would leave slightly shallow threads. Fine increments are the whole purpose.
American drill sizing runs on four separate scales that interleave rather than align. Fractional bits step in sixty-fourths of an inch; wire-gauge numbers run from 1 to 80 with the numbers rising as the bits get smaller; letter sizes A through Z pick up above the number series; and metric bits step in tenths of a millimetre. A full index therefore contains sizes only fractions of a thousandth of an inch apart, which is exactly the point — the gaps in one system are covered by another.
This matters most when drilling for a thread rather than for clearance. A tapped hole needs a specific percentage of thread engagement, so the drill is chosen from whichever scale lands nearest the required diameter: a 1/4-20 UNC tap calls for a number 7 bit at 0.201 inch, not for any fraction, while a metric M6 × 1.0 tap simply takes a 5.0 mm bit. Substituting the closest fractional size instead is the usual cause of a tap that binds or a thread that strips.
Substitution across systems is safe in one direction and risky in the other. Going slightly larger on a clearance hole costs nothing; going slightly larger on a tapped or dowel hole removes material that cannot be put back. Where a chart shows a metric and an imperial bit as near-equivalents, the sensible habit is to check which of the two is undersized relative to the target before reaching for it.