Enter the major diameter of the thread in inches, the threads per inch and the actual diameter of the hole you drilled, measured with a pin gauge or calipers. The percentage of thread engagement is the difference between the major diameter and the hole diameter times the threads per inch times 75 divided by 0.974. Use this after drilling to check the hole, or before tapping to confirm a drill choice gives the engagement you intended.
This approximation uses inches and TPI for a 60-degree cutting thread. Metric pitch requires consistent conversion; forming taps and tapered pipe threads need other guidance. Guhring recommends lower percentages for many applications, so do not select a hole from percentage alone.
Thread engagement from major diameter
| Major diameter | 0.25 |
|---|---|
| Threads per inch | 20 |
| Drilled hole diameter | 0.201 |
| Thread engagement | 75.5% |
| Diameter gap from major diameter (inches) | 0.049 |
Common error: Assuming 100 percent is best. The last few percent add little strength and greatly raise the tapping force and the risk of breaking the tap.
Thread Engagement: formula
ASME B1.1: Unified Inch Screw Threads · Guhring: Tap drill selection
Thread Engagement: changing Major diameter
| Major diameter | Thread engagement | Diameter gap from major diameter (inches) |
|---|---|---|
| 0.2 | -1.5% | -0.001 |
| 0.225 | 37.0% | 0.024 |
| 0.25 (example) | 75.5% | 0.049 |
| 0.275 | 114.0% | 0.074 |
| 0.3 | 152.5% | 0.099 |
Thread Engagement: compare input ranges
| Input | Tested from | To | Thread engagement: low input | Thread engagement: high input | Swing |
|---|---|---|---|---|---|
| Major diameter | 0.225 | 0.275 | 37.0% | 114.0% | 77.0% (102%) |
| Drilled hole diameter | 0.181 | 0.221 | 106.3% | 44.7% | 61.6% (82%) |
| Threads per inch | 18 | 22 | 67.9% | 83.0% | 15.1% (20%) |
Thread Engagement: individual input comparisons
| Input | Value | Thread engagement | Diameter gap from major diameter (inches) |
|---|---|---|---|
| Major diameter | 0.2 | -1.5% | -0.001 |
| 0.225 | 37.0% | 0.024 | |
| 0.25 | 75.5% | 0.049 | |
| 0.275 | 114.0% | 0.074 | |
| 0.3 | 152.5% | 0.099 | |
| Threads per inch | 16 | 60.4% | 0.049 |
| 18 | 67.9% | 0.049 | |
| 20 | 75.5% | 0.049 | |
| 22 | 83.0% | 0.049 | |
| 24 | 90.6% | 0.049 | |
| Drilled hole diameter | 0.161 | 137.1% | 0.089 |
| 0.181 | 106.3% | 0.069 | |
| 0.201 | 75.5% | 0.049 | |
| 0.221 | 44.7% | 0.029 | |
| 0.241 | 13.9% | 0.009 |
Thread engagement: Major diameter against Threads per inch
| Major diameter / Threads per inch | 16 | 20 | 24 |
|---|---|---|---|
| 0.2 | -1.2% | -1.5% | -1.8% |
| 0.225 | 29.6% | 37.0% | 44.4% |
| 0.25 | 60.4% | 75.5% | 90.6% |
| 0.275 | 91.2% | 114.0% | 136.8% |
| 0.3 | 122.0% | 152.5% | 183.0% |
Thread Engagement: 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 |
| Drilled hole diameter — Actual diameter of the drilled hole before tapping, in inches. | 0.201 |
| Thread engagement | 75.5% |
| Diameter gap from major diameter (inches) | 0.049 |
Thread engagement is how much of the full thread depth the tap cuts, and it follows from the drilled hole size. A quarter inch, 20 threads per inch tap in a 0.201 inch hole gives about 75.5 percent. Each thousandth of an inch of hole diameter moves the percentage by several points, which is why a drill that is slightly off matters.
More engagement is stronger up to a point, and beyond it the tap is harder to turn.
Measure the drilled hole with a pin gauge if the strength matters, and use the percentage the application needs. A bolt in a soft material needs more thread than in steel.
Choose the drill on the tap drill size calculator, set cutting speeds on the feeds and speeds page and check the surface speed on the surface speed calculator. The length converter changes inch and metric drill sizes, and the torque converter handles fastener torque.
What is percent thread engagement?
The share of a full thread profile that the tap cuts, determined by the size of the drilled hole relative to the thread's major diameter.
What happens if the drilled hole is too small?
The tap has to cut much more material, needs a great deal more torque and is more likely to snap off in the hole.
What happens if the hole is too large?
The cut thread will be shallow and weak, so it can strip out under load even though tapping feels easy.