Reference
Tap drill chart
Tap drill sizes for 75% thread — UNC and UNF inch threads with the standard drill, plus metric coarse — and how to size the hole yourself.
A tap drill is the hole drilled or punched before a hole is threaded (tapped). Its diameter sets how much of the thread actually gets cut: too large and the threads are shallow and weak, too small and the tap binds and snaps. The standard target for most work is 75% thread engagement, which gives almost the full strength of a fully formed thread at a fraction of the tapping torque. The tables below give the tap drill for common inch and metric threads at 75%.
UNC — Unified coarse threads (75%)
Theoretical 75% tap drill diameter next to the nearest standard drill and its decimal size.
| Thread | TPI | 75% drill (in) | Standard drill (in) |
|---|---|---|---|
| 2-56 | 56 | 0.0686 | #50 (0.0700) |
| 4-40 | 40 | 0.0876 | #43 (0.0890) |
| 6-32 | 32 | 0.1076 | #36 (0.1065) |
| 8-32 | 32 | 0.1336 | #29 (0.1360) |
| 10-24 | 24 | 0.1495 | #25 (0.1495) |
| 12-24 | 24 | 0.1754 | #16 (0.1770) |
| 1/4-20 | 20 | 0.2010 | #7 (0.2010) |
| 5/16-18 | 18 | 0.2584 | F (0.2570) |
| 3/8-16 | 16 | 0.3141 | 5/16 (0.3125) |
| 1/2-13 | 13 | 0.4251 | 27/64 (0.4219) |
UNF — Unified fine threads (75%)
| Thread | TPI | 75% drill (in) | Standard drill (in) |
|---|---|---|---|
| 4-48 | 48 | 0.0917 | #42 (0.0935) |
| 6-40 | 40 | 0.1137 | #33 (0.1130) |
| 8-36 | 36 | 0.1369 | #29 (0.1360) |
| 10-32 | 32 | 0.1596 | #21 (0.1590) |
| 1/4-28 | 28 | 0.2152 | #3 (0.2130) |
| 5/16-24 | 24 | 0.2719 | I (0.2720) |
| 3/8-24 | 24 | 0.3344 | Q (0.3320) |
| 1/2-20 | 20 | 0.4513 | 29/64 (0.4531) |
Metric coarse threads (75%)
For metric coarse threads the tap drill is the major diameter minus the pitch, taken to the nearest standard drill.
| Thread | Pitch (mm) | Tap drill (mm) |
|---|---|---|
| M2 × 0.4 | 0.40 | 1.60 |
| M2.5 × 0.45 | 0.45 | 2.05 |
| M3 × 0.5 | 0.50 | 2.50 |
| M4 × 0.7 | 0.70 | 3.30 |
| M5 × 0.8 | 0.80 | 4.20 |
| M6 × 1.0 | 1.00 | 5.00 |
| M8 × 1.25 | 1.25 | 6.80 |
| M10 × 1.5 | 1.50 | 8.50 |
| M12 × 1.75 | 1.75 | 10.20 |
Source: inch threads per the Unified thread standard (ASME B1.1); metric threads per ISO metric thread standards (ISO 261 / ISO 965). Tap drill at 75% thread ≈ major diameter − 0.974 / TPI for inch, and major diameter − pitch (rounded to the nearest standard drill) for metric coarse. The listed standard drill is the nearest commercial size, so the actual thread percentage varies a point or two from 75%. Verify against your drawing and thread class before cutting.
How to read the chart
Find your thread, read the 75% drill diameter, and pick the standard drill on the same row. If your drawing specifies a different thread percentage or a class-3 fit, size from the formula instead: inch tap drill = major diameter − 0.974/TPI at 75%, scaled up or down for other percentages. For metric, subtract the pitch. When in doubt, drill to the standard size in the table — it is the most reliable choice for production tapping.
Common mistakes
- Chasing 100% thread. It adds almost no strength and roughly doubles tapping torque and tap breakage. 75% is the right default.
- Tapping thin sheet directly. A straight tap in thin material gives too few threads to hold — extrude-and-tap or use a clinch nut instead.
- Confusing coarse and fine. 10-24 and 10-32 use different drills (#25 vs #21). Read the full thread callout, not just the size number.
- Reaming the tapped hole open. An oversized tap drill leaves shallow threads that strip. Match the drill to the table.
Where this fits our shop
Drilling and tapping are in-house at Nevatronix Sheet Metal. For thin sheet, our CNC punch cell forms tapped extrusions that add real thread depth, and hardware insertion installs self-clinching nuts where a straight tap will not hold — see the PEM hardware hole size chart for those hole sizes. Put the thread callout on the print and we confirm the best approach on the quote.
Frequently asked questions
- What is the tap drill size for a 1/4-20 thread?
- For a standard 75% thread, 1/4-20 UNC uses a #7 drill (0.201 in / 5.11 mm). The math: tap drill ≈ major diameter − 0.974 / threads-per-inch = 0.250 − 0.974/20 = 0.201 in, which the #7 drill matches almost exactly. 75% thread is the normal target for most work; it gives nearly full thread strength at a fraction of the tapping torque of a 100% thread.
- How do I calculate a tap drill size?
- For inch threads at 75% engagement, tap drill diameter ≈ major diameter − (0.974 / threads-per-inch). For metric coarse threads the simple rule is tap drill diameter = major diameter − pitch (for example M6 × 1.0 → 6 − 1 = 5.0 mm), which lands close to 75% thread. Round to the nearest standard drill, then confirm against a tap-drill table for the exact recommended size.
- What does 75% thread mean and why not 100%?
- Percent thread describes how much of the theoretical thread depth is actually cut. Going from 75% to 100% thread adds only about 5% more thread strength but can roughly double the tapping torque — sharply increasing tap breakage and heat. 75% is the industry-standard compromise: nearly all the strength, far easier and more reliable to cut. Use it unless a drawing specifically calls for more.
- How do you tap threads in thin sheet metal?
- Thin sheet is the hard case: a straight tapped hole only has as many threads as the sheet is thick, so a 1.5 mm panel gives barely one full thread — not enough to hold. The fixes are to extrude-and-tap (punch a formed collar that adds thread depth), use a self-clinching nut, or add a weld nut. Our CNC punch cell forms tapped extrusions, and hardware insertion installs clinch nuts — both add real thread engagement to thin material.
- Do you tap and thread parts in-house?
- Yes. Drilling and tapping are in-house operations, alongside punched and formed hole features and self-clinching hardware insertion. Put the thread callout and class on the print; for thin material we will recommend an extruded-and-tapped hole or a clinch nut so the threads actually hold. Send the part for a quote and we confirm the approach.
Threaded features on your part? Send the print.
Tapped holes, extruded-and-tapped collars, or clinch nuts — we pick the approach that actually holds in your material and quote it. Most parts turn a quote within a business day.