Enter major diameter, threads per inch and a chosen engagement percentage for a cutting 60-degree thread approximation. For assumed 1/4-20 and 75%, the estimate is 0.2013 inch. The selected 75% is not a universal target; forming taps and specific materials need the maker’s drill table.
The approximation is for cutting 60-degree threads. A forming tap requires different hole sizes. Guhring advises 60–70% for many applications; the selected 75% example is not a universal strength requirement.
Follow the tap maker’s table and material guidance.
Tap drill diameter (inches) from major diameter
| Major diameter | 0.25 |
|---|---|
| Threads per inch | 20 |
| Target thread engagement | 75.0% |
| Tap drill diameter (inches) | 0.201 |
| Nearest 1/64 in. drill size (inches) | 0.203 |
Common error: Drilling to the major diameter. The hole must be smaller than the screw's outside diameter, since the threads are cut into the metal in between.
Tap Drill Size: formula
ASME B1.1: Unified Inch Screw Threads · Guhring: Tap drill selection
Tap Drill Size: changing Major diameter
| Major diameter | Tap drill diameter (inches) | Nearest 1/64 in. drill size (inches) |
|---|---|---|
| 0.2 | 0.151 | 0.156 |
| 0.225 | 0.176 | 0.172 |
| 0.25 (example) | 0.201 | 0.203 |
| 0.275 | 0.226 | 0.219 |
| 0.3 | 0.251 | 0.25 |
Tap Drill Size: compare input ranges
| Input | Tested from | To | Tap drill diameter (inches): low input | Tap drill diameter (inches): high input | Swing |
|---|---|---|---|---|---|
| Major diameter | 0.225 | 0.275 | 0.176 | 0.226 | 0.05 (25%) |
| Threads per inch | 18 | 22 | 0.196 | 0.206 | 0.00984 (4.9%) |
| Target thread engagement | 73.0% | 77.0% | 0.203 | 0.2 | 0.0026 (1.3%) |
Tap Drill Size: individual input comparisons
| Input | Value | Tap drill diameter (inches) | Nearest 1/64 in. drill size (inches) |
|---|---|---|---|
| Major diameter | 0.2 | 0.151 | 0.156 |
| 0.225 | 0.176 | 0.172 | |
| 0.25 | 0.201 | 0.203 | |
| 0.275 | 0.226 | 0.219 | |
| 0.3 | 0.251 | 0.25 | |
| Threads per inch | 16 | 0.189 | 0.188 |
| 18 | 0.196 | 0.203 | |
| 20 | 0.201 | 0.203 | |
| 22 | 0.206 | 0.203 | |
| 24 | 0.209 | 0.203 | |
| Target thread engagement | 71.0% | 0.204 | 0.203 |
| 73.0% | 0.203 | 0.203 | |
| 75.0% | 0.201 | 0.203 | |
| 77.0% | 0.2 | 0.203 | |
| 79.0% | 0.199 | 0.203 |
Tap drill diameter (inches): Major diameter against Threads per inch
| Major diameter / Threads per inch | 16 | 20 | 24 |
|---|---|---|---|
| 0.2 | 0.139 | 0.151 | 0.159 |
| 0.225 | 0.164 | 0.176 | 0.184 |
| 0.25 | 0.189 | 0.201 | 0.209 |
| 0.275 | 0.214 | 0.226 | 0.234 |
| 0.3 | 0.239 | 0.251 | 0.259 |
Tap Drill Size: values and definitions
| Quantity and definition | Value used |
|---|---|
| Major diameter — Basic major diameter of the screw thread, in inches. | 0.25 |
| Threads per inch — Threads per inch (TPI) of the thread being tapped. | 20 |
| Target thread engagement — Target percent of full thread engagement (75% is the common practical target). | 75.0% |
| Tap drill diameter (inches) | 0.201 |
| Nearest 1/64 in. drill size (inches) | 0.203 |
Major diameter: half and double
| Major diameter | Tap drill diameter (inches) |
|---|---|
| 0.125 | 0.076 |
| 0.25 | 0.201 |
| 0.5 | 0.451 |
The drill for a tapped hole decides how much thread you get. For a quarter inch, 20 threads per inch screw at 75 percent engagement, the drill is about 0.201 inches, close to a number 7 drill, and the nearest fraction is 13/64, or 0.203. A smaller hole gives more thread and needs more force to tap, and a larger one gives less thread and less strength.
Around 75 percent is a common balance, and softer metals often use less.
Look at the tap's own chart and the material. Hard metals and small taps break easily at high engagement, and 60 to 75 percent is the usual range.
Check the achieved engagement on the thread engagement calculator, machining speeds on the feeds and speeds page and surface speed on the surface speed calculator. The fastener pilot hole and length converter pages help with wood screws and units, and the torque converter handles fastener torque.
Is 75% engagement required?
No. It is the selected example. Guhring lists 60–70% for many applications; the appropriate hole depends on tap type, material, strength requirements and the maker’s table.
What thread engagement should I use?
Use the tooling and fastener specifications for thread form, pitch, material and required connection. An engagement percentage alone does not determine tap-drill diameter or strength. Forming taps and cutting taps can need different drill sizes.
What drill do I use for a 1/4-20 tap?
For the conventional 1/4-20 cutting-tap example, a number 7 drill is 0.201 inch. Check the tap maker’s chart for the material and engagement requirement; a forming-tap recommendation can differ from a cutting-tap recommendation.
Does more engagement always mean a stronger thread?
Greater thread engagement can increase cutting effort and tap-breakage risk without a proportional strength improvement. The useful choice depends on material and connection requirements. Follow the relevant drill chart and design specification rather than applying one percentage universally.