number drill · nominal diameter

#8
drill bit size.

A #8 bit measures 0.1990 in across (5.055 mm).

The inch decimal is the published nominal designation; millimetres are a conversion.

Diameter conversion cross-checked

Calibrated relative diameter · 1 in reference

#8 · number

0.1990 in

5.055 mm

Designation#8number

Inch decimal0.1990nominal

Metric conversion5.055millimetres

Where this size sits from 0 to 1 inch

#8 · 5.055 mm on a zero to 25.4 millimetre scale. Neighbor marks: 5.1 mm, #7.

0 25.4 mm

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Measured neighbors

What sits beside #8.

“Nearest” means nominal diameter only. It does not make a neighboring size acceptable for a tolerance, tapping, reaming, or fastener application.

Other naming systemsABSOLUTE DIAMETER DELTA

5.1 mmmetric 0.2008 inΔ +0.045 mm 5 mmmetric 0.1969 inΔ -0.055 mm 13/64fractional 0.2031 inΔ +0.105 mm 5.2 mmmetric 0.2047 inΔ +0.145 mm

Same seriesNEAREST NOMINAL ROWS

#7number 5.105 mmΔ +0.051 mm #9number 4.978 mmΔ -0.076 mm

Source record

ASME.

The standard text is paywalled. Number and letter series are cross-checked against public manufacturer charts; exact standard edition/page verification remains a release gate.

Open source record

#8drill bit size.

What sits beside #8.

ASME.

#8
drill bit size.