Measurement Format for Size Names
Nut and bolt size names, regardless of the measurement system, are always given as the OD followed by the TPI or thread pitch. The names within each measurement system will look slightly different.
Inch Format
The measurement of an inch nut or bolt will be listed as the thread size — a number between one and 12 or an inch increment followed by the threads per inch (TPI). If the diameter is less than 1/4 inch, the diameter will be listed as a number between zero and 12; the larger the number, the closer it is to 1/4 inch. When the diameter is equal to or larger than 1/4 inch, it will be listed as an inch increment.
The TPI, or number of threads within an inch of the thread length, will immediately follow the diameter.
- Example 1: #6-24 (#6 = 0.138 in. OD; 24 = 24 threads in one inch of thread length)
- Example 2: 1/4"-28 (1/4 = 1/4 in. OD; 28 = 28 threads in one inch of thread length)
Metric Format
The thread size of a metric nut or bolt is listed as the letter "M" followed by a number that indicates the number of millimeters across the diameter and then the thread pitch.
- Example: M4 x 0.7 (M4 = 4 mm OD; 0.7 = 0.7 mm between thread crests)
Coarse and Fine Threads
Nut and bolt size names will sometimes also factor in whether the thread is coarse or fine. Simply put, coarse threads are thicker and farther apart from each other, while fine threads are thinner and closer together.
There are a few acronymns that are used to designate each thread type:
Standard Coarse Threads
- UNC: Unified National Coarse threads comparable to ISO metric threads
- UNRC: Unified National Coarse threads; the "R" indicates "rolled" external threads that have a rounded root contour. They are full interchangeable with UNC fasteners.
- UNJC: Unified National Coarse threads with an increased minor diameter and controlled root radius that disperses tensile strength over a broader area. Derived from a military specification (MIL-S-8879), they are designed for high stress applications. However, they are not interchangeable with other UNC fasteners.
Metric Coarse Threads
- ISO metric threads will simply use the word "coarse."
Standard Fine Threads
- UNF: Unified National Fine threads.
- UNRF: Unified National Fine threads; the "R" indicates "rolled" external threads that have a rounded root contour. They are fully interchangeable with other UNF fasteners.
- UNJF: Unified National Fine threads with an increased minor diameter and controlled root radius that disperses tensile stress over a broader area. Derived from a military specification (MIL-S-8879), they are designed for high stress applications. However, they are not interchangeable with other UNF fasteners.
Metric Fine Threads
- ISO metric threads will simply use the words "fine" or "super fine."
Sometimes, you will see the same thread size listed twice, each with a different thread pitch or TPI next to them. For example:
- M2.3 x 0.45 vs. M2.3 x 0.4: These have the same diameter, but the thread pitch in the first measurement is larger than the second one, so the distance between threads will be bigger.
- #10 - 24 vs. #10 - 32: These have the same diameter, but the TPI in the first measurement is lower than the second one, so there will be fewer threads per inch of thread length.
Because TPIs and thread pitches are calculated differently, remember the following:
- Inch Threads: A higher TPI indicates finer threads as there are more threads within an inch of thread length.
- Metric Threads: A lower thread pitch indicates finer threads as there is less space between each thread crest.