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Drill & Tap Chart

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Swipe tables sideways on a phone. Fractional options are approximate—check the size comparison and notes.

Inch Threads — UNC & UNF

*Approximate substitute—not the listed tap drill. Check the size comparison before using.

Thread / Tap Listed Tap Drill Approx. Fractional Alternative* Size Comparison
4-40 #43 3/32* 0.0048 in. larger; reduced thread depth
6-32 #36 7/64* 0.0029 in. larger
8-32 #29 9/64* 0.0046 in. larger; reduced thread depth
10-24 #25 5/32* 0.0068 in. larger
10-32 #21 5/32* 0.0028 in. smaller; harder tapping
12-24 #16 11/64* 0.0051 in. smaller; harder tapping
1/4-20 #7 13/64* 0.0021 in. larger
1/4-28 #3 7/32* 0.0058 in. larger
5/16-18 F 17/64* 0.0086 in. larger
5/16-24 I 9/32* 0.0092 in. larger; reduced thread depth
3/8-16 5/16 5/16 Same size
3/8-24 Q 11/32* 0.0117 in. larger; reduced thread depth
7/16-14 U 3/8* 0.0070 in. larger
7/16-20 25/64 25/64 Same size
1/2-13 27/64 27/64 Same size
1/2-20 29/64 29/64 Same size
9/16-12 31/64 31/64 Same size
9/16-18 33/64 33/64 Same size
5/8-11 17/32 17/32 Same size
5/8-18 37/64 37/64 Same size
3/4-10 21/32 21/32 Same size
3/4-16 11/16 11/16 Same size
7/8-9 49/64 49/64 Same size
7/8-14 13/16 13/16 Same size
1-8 7/8 7/8 Same size
1-12 59/64 59/64 Same size
1-14 15/16 15/16 Same size

Metric Threads

*Approximate substitute—not the listed tap drill. Check the size comparison before using.

Thread / Tap Listed Drill (mm) Approx. Fractional Alternative* Size Comparison
M3 × 0.5 2.5 3/32 — not suggested 0.0047 in. smaller; use listed tap drill
M4 × 0.7 3.3 1/8 — not suggested 0.0049 in. smaller; use listed tap drill
M5 × 0.8 4.2 11/64* 0.0065 in. larger
M6 × 1.0 5.0 13/64* 0.0063 in. larger
M8 × 1.25 6.8 17/64* 0.0021 in. smaller
M8 × 1.0 7.0 9/32* 0.0057 in. larger
M10 × 1.5 8.5 11/32* 0.0091 in. larger
M10 × 1.25 8.8 11/32* 0.0027 in. smaller
M12 × 1.75 10.2 13/32* 0.0047 in. larger
M12 × 1.25 10.8 27/64* 0.0033 in. smaller
M14 × 2.0 12.0 15/32* 0.0037 in. smaller; harder tapping
M14 × 1.5 12.5 1/2* 0.0079 in. larger
M16 × 2.0 14.0 35/64* 0.0043 in. smaller; harder tapping
M16 × 1.5 14.5 37/64* 0.0073 in. larger
M18 × 2.5 15.5 39/64* 0.0009 in. smaller
M18 × 1.5 16.5 21/32* 0.0066 in. larger
M20 × 2.5 17.5 11/16* 0.0015 in. smaller
M20 × 1.5 18.5 47/64* 0.0060 in. larger
M22 × 2.5 19.5 49/64* 0.0021 in. smaller
M22 × 1.5 20.5 13/16* 0.0054 in. larger
M24 × 3.0 21.0 53/64* 0.0014 in. larger
M24 × 2.0 22.0 55/64* 0.0068 in. smaller; harder tapping

NPT Pipe Threads

*Approximate substitute—not the listed tap drill. Check the size comparison before using.

Pipe Thread / Tap Listed Tap Drill Approx. Fractional Alternative* Size Comparison
1/16-27 NPT C 15/64* 0.0076 in. smaller; confirm pipe-tap specification
1/8-27 NPT Q 21/64* 0.0039 in. smaller; confirm pipe-tap specification
1/4-18 NPT 7/16 7/16 Same size
3/8-18 NPT 9/16 9/16 Same size
1/2-14 NPT 45/64 45/64 Same size
3/4-14 NPT 29/32 29/32 Same size
1-11-1/2 NPT 1-9/64 1-9/64 Same size

Chart Notes

Find your tap drill, then compare it with a common fractional-inch drill from your shop’s drill index.

Have a regular fractional drill set? The chart compares nearby sizes in 1/64-inch steps, as found in a full fractional drill index. Smaller kits may skip some sizes. These are approximate alternatives, not exact equivalents. A smaller hole makes tapping harder; a larger hole leaves less thread. Confirm that the size is within your tap manufacturer’s recommended range for the material and required thread fit. Use the listed tap drill for critical work.

Tap drill ≠ bolt clearance hole. These sizes are for cutting internal threads. Forming taps and thread-repair inserts require different drill sizes.

Inch Threads

The number after the dash is threads per inch (TPI). # sizes are numbered drills; single letters are letter drills. All fractional drill sizes are in inches. Where the nearest fraction would make tapping excessively hard, a nearby larger fraction is shown instead. “Not suggested” means there is no close fractional option in this index; get the listed drill.

Metric Threads

The number after × is pitch in millimeters. Example: M8 × 1.25 is an 8 mm thread with 1.25 mm pitch. The fractional column helps when you do not have metric drills; check the size comparison before choosing one.

NPT Pipe Threads

NPT is tapered pipe thread. Nominal pipe size is not the drill diameter. For pipe threads, confirm your specific tap’s drill and reaming instructions before substituting, and verify finished threads with the proper gauge. The fractional column is a diameter comparison only.

Quick Tapping Tips

  • Match both the thread diameter and pitch before drilling.
  • Clamp the work securely and keep the tap square to the hole.
  • Use cutting fluid appropriate for the material and tap.
  • For blind holes, allow room for the tap’s lead and chips below the required full-thread depth.
  • Use the tap manufacturer’s instructions for chip control and tapping technique.
Need the right drill or tap?
Call Crowbar’s at 319-200-2769 or stop in at 4151 3rd Ave, Suite 101, Marion, Iowa.

Reference: Viking Drill & Tool (inch threads), Guhring (metric cutting-tap guidance), and AutoDrill (NPT). General shop reference; the tool manufacturer’s specifications take priority.