Drill Bit Size Converter Calculator

Work out drill bit size converter instantly with clear inputs, formula shown and shareable results.

About Drill Bit Size Converter Calculator

Drill sizes come in three incompatible systems: metric millimetres, imperial fractions in sixty-fourths, and number gauges. A 6.5 mm bit is 0.2559 inches, closest to 16/64 or a quarter inch at 6.35 mm - a 0.15 mm difference that matters for a press fit but not for a clearance hole.

Formulas

Metric to imperial

inches = mm / 25.4

Fractional size

sixty-fourths = inches x 64

Worked example

With the default values below, Drill Bit Size Converter Calculator returns Size In Millimetres of 6.500. Change any input above and every figure here is recalculated in your browser.

Inputs used

Size to convertminimum 0: 6.5
That figure is inone of Millimetres, Inches (decimal), Sixty-fourths of an inch, Number gauge (approximate): Millimetres

Results

Size In Millimetres: 6.500 mm
Size In Inches: 0.2559
Nearest Fractional Size: 16/64 in
Clearance Drill For A Matching Bolt: 7.75 mm

Step by step

1Values used
2Metric to imperial
3Fractional size
4Size In Millimetres= 6.500 mm
5Size In Inches= 0.2559
6Nearest Fractional Size= 16/64 in
7Clearance Drill For A Matching Bolt= 7.75 mm

Assumptions and limits

The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why is a quarter inch not 6.5 mm?

It is 6.35 mm. The 0.15 mm gap is enough to turn an interference fit into a sloppy one, so mixing systems needs care.

What is a tap drill size?

The hole diameter for cutting a thread - roughly the bolt diameter minus the thread pitch. The clearance figure here is for a hole the bolt passes through.