US Threads Per Inch (TPI) Table – Monster Bolts
US (inch) screws are defined by Threads Per Inch (TPI)—the number of threads in one inch of the fastener. Metric fasteners use a different system, Metric Thread Pitch, and the two systems are not interchangeable.
How to measure TPI (fast, accurate)
- ½-inch method: Place a ruler along the threads, count the number of thread peaks across ½″, then double that number. (Counting a longer distance reduces error.)
- Thread gauge: Use a TPI gauge if available for a quick confirm.
- Don’t mix systems: If your part spec is metric (e.g., M8×1.25), do not substitute a US size; the fit and strength can be compromised.
UNC vs UNF (and UNEF)
- UNC (Coarse): fewer threads per inch, better for general use, softer materials, and faster assembly.
- UNF (Fine): more threads per inch, better for thinner walls, higher clamp load control, vibration resistance.
- UNEF (Extra Fine): niche applications; rarely used in general hardware (not included in the chart below).
What does “#8-32” or “1/4-20” mean?
#8-32 = #8 screw size with 32 TPI (fine vs coarse depends on the size). 1/4-20 = 1/4″ diameter with 20 TPI (UNC). When in doubt, check the table and your application needs.
US Threads Per Inch (TPI) Chart
| Screw Size | Coarse / UNC | Fine / UNF |
|---|---|---|
| #0 | – | 80 |
| #1 | 64 | 72 |
| #2 | 56 | 64 |
| #3 | 48 | 56 |
| #4 | 40 | 48 |
| #5 | 40 | 44 |
| #6 | 32 | 40 |
| #8 | 32 | 36 |
| #10 | 24 | 32 |
| #12 | 24 | 28 |
| 1/4″ | 20 | 28 |
| 5/16″ | 18 | 24 |
| 3/8″ | 16 | 24 |
| 7/16″ | 14 | 20 |
| 1/2″ | 13 | 20 |
| 9/16″ | 12 | 18 |
| 5/8″ | 11 | 18 |
| 3/4″ | 10 | 16 |
| 7/8″ | 9 | 14 |
| 1″ | 8 | 12 |
| 1-1/8″ | 7 | 12 |
| 1-1/4″ | 7 | 12 |
| 1-1/2″ | 6 | 12 |
Note: In addition to UNC (coarse) and UNF (fine), a much less common UNEF (extra fine) series exists but is rarely used in general hardware, so it’s not included here.
Quick math: approximate metric pitch (mm) = 25.4 ÷ TPI. Use for comparison only—do not mix systems.
Frequently Asked Questions
What is TPI in screws?
TPI (Threads Per Inch) is the number of thread crests along one inch of a screw. Higher TPI = finer thread; lower TPI = coarser thread.
How do I measure TPI on short screws?
Use the ½-inch method: count thread peaks across ½″ and double the count. Or use a TPI gauge to confirm. Measuring a longer span reduces error.
What’s the difference between UNC and UNF?
UNC (coarse) has fewer threads per inch—good for general use and softer materials. UNF (fine) has more threads per inch—better for thinner walls, finer clamp control, and vibration resistance. UNEF (extra fine) exists but is uncommon in general hardware.
Can I convert between TPI and metric pitch?
You can convert numerically (pitch in mm ≈ 25.4 ÷ TPI), but systems are not interchangeable. Similar numbers don’t guarantee fit; mixing systems can damage parts.
Are “number sizes” (#4, #8, #10) actual diameters?
No—“number sizes” are nominal designations. For exact major diameters, see US Screw Sizes – Decimal & mm Equivalents.
When should I choose UNC vs UNF?
Use UNC for general assembly, speed, and softer materials. Use UNF when you need greater thread engagement in thin sections, better clamp load control, or improved vibration resistance—provided alignment and cleanliness are good.