Thread Types Explained
UNC, UNF, and metric coarse and fine threads: what they are and when to use each for fasteners.
Imperial: UNC and UNF
UNC (Unified National Coarse) is the default coarse pitch for imperial threads. UNF (Unified National Fine) has more threads per inch for the same diameter. Coarse is common for general use; fine is used where finer adjustment or strength in thin sections is needed.
| Type | Use | Example |
|---|---|---|
| UNC | General, coarse | 1/4-20 |
| UNF | Fine pitch | 1/4-28 |
Metric: coarse and fine
Metric coarse is the standard pitch (e.g. M8 x 1.25). Metric fine has a smaller pitch for the same diameter. Fine threads provide more engagement in thin material and allow finer adjustment.
| Size | Coarse pitch | Fine pitch example |
|---|---|---|
| M6 | 1.0 mm | 0.75 mm |
| M8 | 1.25 mm | 1.0 mm |
| M10 | 1.5 mm | 1.25 mm |
Identifying thread type
Measure diameter and pitch (or TPI) with calipers and a pitch gauge. Enter the values in our Thread Identifier to get likely thread designations. Use the Thread Pitch to TPI Converter to switch between metric and imperial pitch.
FAQ
What is UNC vs UNF?
UNC is Unified National Coarse (fewer threads per inch, larger pitch). UNF is Unified National Fine (more threads per inch, smaller pitch). Same diameter, different pitch.
What is metric coarse vs fine?
Metric coarse is the default pitch (e.g. M8 x 1.25). Metric fine has a smaller pitch (e.g. M8 x 1.0). Fine is used where strength in thin material or fine adjustment is needed.
How do I know which thread type I have?
Measure diameter and pitch or TPI. Use a pitch gauge or our Thread Identifier tool with caliper measurements to get the likely standard.
Related
Tools: Thread Identifier, Thread Pitch to TPI Converter
Charts: Universal Screw & Bolt Size Chart, Tap Drill Chart
Guide: Thread Pitch Explained