Understanding Screw Threads
A screw thread is a helical ridge wrapped around a cylinder or cone. Its specifications are critical for ensuring that bolts, nuts, and tapped holes fit together correctly and provide the required strength.
This tool provides key dimensions for standard Unified National (UNC/UNF) and Metric threads. Using the correct tap drill size is especially important for creating strong internal threads that resist stripping.
Key Thread Terminology
| Term | Description |
|---|---|
| Pitch | The distance from the crest of one thread to the next. For inch threads, this is often expressed as Threads Per Inch (TPI). |
| Major Diameter | The largest diameter of the screw thread. For an external thread (bolt), it's the outer diameter. |
| Minor Diameter | The smallest diameter of the screw thread, measured from the root (bottom) of the thread. |
| Pitch Diameter | An effective diameter, roughly halfway between the major and minor diameters, where the thread and its mating part make contact. |
| Tap Drill Size | The specific drill bit size required to drill a hole before creating internal threads with a tap. This hole is slightly larger than the minor diameter. |
How to Use the Calculator
- Select the measurement system for your thread (Inch or Metric).
- Choose the nominal size of your thread from the dropdown list.
- Click "Get Specs" to see all the critical dimensions for your selected thread.
Frequently Asked Questions
What's the difference between UNC and UNF?
UNC stands for Unified National Coarse, and UNF stands for Unified National Fine. For a given diameter, UNC threads are deeper and have fewer threads per inch. They are more common and more resistant to stripping in softer materials. UNF threads are finer, stronger in tension, and allow for more precise adjustments.
What if my thread isn't listed?
This calculator contains a list of the most common standard sizes. There are many other less common or specialized thread sizes (like UNEF or special metric pitches). For these, you would need to consult a comprehensive machinist's handbook or an engineering resource like the Machinery's Handbook.