Thread pitch determines how a fastener engages with its mating part. Getting it wrong means stripped threads or parts that don't fit. This guide covers thread pitch for both unified (imperial) and metric threads, how to measure pitch, and common thread series.
What Is Thread Pitch?
Thread pitch is the distance between adjacent thread crests:
- Imperial threads: Expressed as TPI (Threads Per Inch)
- Metric threads: Expressed as pitch in millimeters
Higher TPI = finer thread = more threads per inch = smaller pitch distance.
Imperial Thread Series
UNC (Unified National Coarse)
The default for general-purpose fastening. Larger pitch, faster assembly, more tolerant of damaged threads.
UNF (Unified National Fine)
Finer pitch for vibration resistance, precise adjustment, and thin-walled applications. Requires more care to avoid cross-threading.
| Size | UNC TPI | UNF TPI |
|---|---|---|
| 1/4" | 20 | 28 |
| 5/16" | 18 | 24 |
| 3/8" | 16 | 24 |
| 7/16" | 14 | 20 |
| 1/2" | 13 | 20 |
| 5/8" | 11 | 18 |
| 3/4" | 10 | 16 |
| 1" | 8 | 12 |
Look Up Any Thread Pitch
Find TPI, pitch, tap drills, and thread dimensions for UNC, UNF, and metric threads.
Metric Thread Standards
Metric threads are designated as M × pitch. M8×1.25 means 8mm major diameter with 1.25mm pitch.
Coarse vs. Fine Metric
When only "M8" is specified, it implies the coarse pitch (1.25mm). Fine pitches must be explicitly stated (M8×1.0).
| Size | Coarse Pitch | Fine Pitch |
|---|---|---|
| M4 | 0.7 | 0.5 |
| M5 | 0.8 | 0.5 |
| M6 | 1.0 | 0.75 |
| M8 | 1.25 | 1.0 |
| M10 | 1.5 | 1.25/1.0 |
| M12 | 1.75 | 1.5/1.25 |
| M16 | 2.0 | 1.5 |
| M20 | 2.5 | 1.5 |
How to Measure Thread Pitch
Thread Pitch Gauge
The most reliable method. Match the gauge blade to the thread until it sits flush with no light gaps. Imperial gauges show TPI; metric gauges show pitch in mm.
Counting Threads
For imperial: count threads over one inch (or half-inch and double it). For metric: measure the distance over 10 threads and divide by 10.
Caliper Method
Measure across multiple thread crests. For 10 threads on a 1/4-20: 10 ÷ 20 = 0.500". If your measurement matches, you've confirmed the pitch.
When to Use Coarse vs. Fine
Use Coarse Threads When:
- General assembly with frequent disassembly
- Threading into softer materials (aluminum, cast iron)
- Threads may be exposed to contamination
- Speed of assembly matters
Use Fine Threads When:
- Vibration resistance is critical
- Precise adjustment is needed (set screws, adjusters)
- Threading into thin-walled material
- Maximum clamping force in minimal length
Special Thread Forms
- NPT/NPTF: Tapered pipe threads for pressure-tight joints
- ACME: Trapezoidal threads for power screws and lead screws
- Buttress: Asymmetric threads for high axial loads in one direction
Common Problems
- Cross-threading: More common with fine threads; start by hand
- Mixed standards: Metric and imperial can be close but don't interchange
- Class fit: 2A/2B is standard; 3A/3B is precision fit
Thread identification becomes second nature with experience. When you're unsure, test with a known nut or bolt before cutting new threads or ordering fasteners.
