Conversion · exact first, nominal second
Convert the diameter.
Then find a real bit.
Inches and millimetres convert exactly at 25.4 mm per inch. Drill designations are discrete, so the nearest named bit is shown separately with a signed diameter difference.
Exact unit conversion 5.5 millimetres
Decimal inches
0.216535 in
5.500 mm Nearest fractional 7/32 7/32 in · 5.556 mm Δ +0.056 mm
Nearest number #3 0.2130 in · 5.410 mm Δ -0.090 mm
Nearest letter A 0.2340 in · 5.944 mm Δ +0.444 mm
Nearest metric 5.5 mm 0.2165 in · 5.5 mm Δ +0.000 mm
Do not round away the application
A close conversion does not prove interchangeability. Tap drill, reamer allowance, press fit, clearance, coating, material, and machine capability can all make a small diameter difference consequential.