Tap and Drill Size Chart (SAE & Metric) | HandySkook
Tap and Drill Size Chart
The correct tap drill size for common SAE (inch) and metric machine-screw and bolt threads — so your tapped holes come out right.
SAE / inch tap drill sizes (coarse, UNC)
| Thread | Tap drill | Decimal (in) |
|---|---|---|
| #6-32 | #36 | 0.1065 |
| #8-32 | #29 | 0.1360 |
| #10-24 | #25 | 0.1495 |
| #10-32 | #21 | 0.1590 |
| 1/4-20 | #7 | 0.2010 |
| 5/16-18 | F | 0.2570 |
| 3/8-16 | 5/16 | 0.3125 |
| 1/2-13 | 27/64 | 0.4219 |
Metric tap drill sizes (coarse)
| Thread | Tap drill (mm) |
|---|---|
| M3 × 0.5 | 2.5 mm |
| M4 × 0.7 | 3.3 mm |
| M5 × 0.8 | 4.2 mm |
| M6 × 1.0 | 5.0 mm |
| M8 × 1.25 | 6.8 mm |
| M10 × 1.5 | 8.5 mm |
| M12 × 1.75 | 10.2 mm |
Rule of thumb for metric: tap drill ≈ major diameter − pitch (e.g. M6×1.0 → 6 − 1 = 5 mm). These give roughly 75% thread engagement, the usual target.
Frequently asked questions
How do I find the tap drill size?
For metric coarse threads, subtract the pitch from the major diameter (M6×1.0 needs a 5 mm bit). For inch threads, use a tap & drill chart — the bit is sized for about 75% thread engagement.
What happens if the tap drill is too big?
The threads will be shallow and strip easily. Too small and the tap can bind or break. Match the chart for clean, strong threads.
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