number drill · nominal diameter

#21
drill bit size.

A #21 bit measures 0.1590 in across (4.039 mm).

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

Diameter conversion cross-checked

Calibrated relative diameter · 1 in reference

#21 · number

0.1590 in

4.039 mm

Designation#21number

Inch decimal0.1590nominal

Metric conversion4.039millimetres

Where this size sits from 0 to 1 inch

#21 · 4.039 mm on a zero to 25.4 millimetre scale. Neighbor marks: 4 mm, #22.

0 25.4 mm

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Where this bit is specified

What #21 is for.

#21 is the cutting-tap drill for the thread below— usually the reason this size gets looked up. Engagement is calculated from the basic 60° profile; open a thread for its published row, evidence, and limits.

Cutting-tap rows naming #21 CUT TAPS ONLY · CALCULATED ENGAGEMENT

#10-32 UNF UNF · manufacturer-published row 76.4% calculated thread engagement

Measured neighbors

What sits beside #21.

“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

4 mmmetric 0.1575 inΔ -0.039 mm 4.1 mmmetric 0.1614 inΔ +0.061 mm 5/32fractional 0.1563 inΔ -0.070 mm 3.9 mmmetric 0.1535 inΔ -0.139 mm 4.2 mmmetric 0.1654 inΔ +0.161 mm

Same seriesNEAREST NOMINAL ROWS

#22number 3.988 mmΔ -0.051 mm #20number 4.089 mmΔ +0.051 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

#21drill bit size.

What #21 is for.

What sits beside #21.

ASME.

#21
drill bit size.