Quick Tap Drill Size Chart (M3–M30): Common Sizes and the Formula

The tap drill (pre-drill) diameter is one of the most critical tapping parameters, directly affecting thread accuracy, tool life and stability. Different pitches and materials need the right drill size, based on metric coarse and fine standards. This guide gives M-thread tap drill charts, HSS vs carbide differences, the selection formula and machining tips to raise tapping success and quality.

1. What a Tap Drill Is

The tap drill size (pre-drill) is the hole drilled before tapping. It affects thread form completeness, tapping torque and tool wear. Too small → hard tapping, broken taps; too large → shallow thread, reduced strength.

Scope of this page: a quick chart of common sizes (coarse, fine and M20 and up) and the formula. For the complete M1–M100 reference with minor-diameter limits and adjustments by material, see the Metric Tap Drill Size Chart.

For roll tap (form tap) drill sizes specifically, see the roll tap / form tap drill sizes chart.

2. Metric Coarse Thread Tap Drill (reference)

Both columns follow the tap drill chart in Mitsubishi Materials’ technical data; HSS and carbide mean the material of the drill used for the pre-drilled hole, and this table gives no carbide-drill size from M24 up. For M12 ×1.75 the HSS size is 10.3 mm, while some manufacturers list 10.2 mm (see the Metric Tap Drill Size Chart Guide). Both sit inside the 6H minor-diameter band of 10.106–10.441 mm, with 10 − 1.75 = 10.25 right between them; the difference is only the rounding direction. The carbide size of 10.5 mm sits slightly above that limit; see Section 5.

3. Metric Fine Thread Tap Drill (reference)

Fine threads have a smaller pitch, so the drill is closer to the major diameter; higher thread density and strength suit precision fastening.

Drill sizes follow the EMUGE tap drill chart (DIN 13 / ISO 965-1); D1 is the min/max of the 6H internal minor diameter, so any drilled hole inside that band is fine. Some charts give 5.3 and 7.3 mm for M6 ×0.75 and M8 ×0.75; those also sit inside the band, the difference being only the rounding direction. The minor-diameter limits for M12 ×1.25 are not listed in this table.

4. Large Threads (M20 and Above)

Large threads have a high tapping load; use machine tapping with coolant and lubrication.

Source and D1 as in the previous section; the minor-diameter limits for M20 ×2.0 are not listed in this table.

5. HSS Drill vs Carbide Drill: Tap Drill Size

HSS and carbide here refer to the material of the drill used for the pre-drilled hole, not the tap material. Section 2 gives both columns for coarse threads; the table below gives them for fine threads and M20, also from Mitsubishi Materials’ technical data. The fine-thread sizes in Sections 3 and 4 follow EMUGE, and a few rows round differently from the HSS column below (e.g. M6 ×0.75: 5.2 vs 5.30); all sit inside the minor-diameter band. To compare HSS and carbide, read both columns of the same table rather than subtracting across tables.

Note: wherever this article gives a carbide size, the carbide drill is 0.05–0.3 mm larger than the HSS drill. Six rows sit slightly above the 6H minor-diameter maximum: coarse M12 ×1.75 at 10.5 mm (maximum 10.441), and the ×1.5 fine threads of M12, M14, M16, M18 and M20, each 0.024 mm over (e.g. M12 ×1.5 at 10.7 mm, maximum 10.676). We have found no reliable explanation for why, so we do not guess at one here.

Mitsubishi’s technical data also notes that the drilled size varies with machining conditions, so measure the actual hole and change the drill diameter if it does not suit. After drilling to this chart with a carbide drill, measure the hole as in Section 7; if you are tapping a 6H thread and the hole comes out above the minor-diameter maximum, step down one drill size. When you switch drill material, look the size up again rather than reusing the other drill’s number.

6. Practical Selection Tips

Basic formula: tap drill ≈ major diameter − pitch. E.g. M10 ×1.5 → 10 − 1.5 = 8.5 mm. Fine-tune by condition: soft material slightly smaller, hard material larger, deep holes larger. For estimating tapping torque see the Tap Torque Guide; for bolt clearance holes see the Bolt Hole Size Chart.

7. Quality Control

Measure the drilled hole before tapping and check thread accuracy afterward (a go/no-go gauge helps); if it fails, adjust the drill size or cutting conditions.

For the full reading guide on this topic, see Thread Machining: A Complete Reading Guide.

Published: 2026-03-25 | Last updated: 2026-10-17