Thread Pitch Chart: UNC, UNF & Metric Reference
This thread pitch chart covers standard UNC (coarse) and UNF (fine) imperial thread sizes with threads per inch, major diameter, and recommended tap drill sizes. Use this reference to identify thread pitch, select the correct tap drill, or confirm bolt-to-nut compatibility.
UNC (Unified National Coarse) Thread Chart
UNC is the default thread standard for most imperial fasteners. These are the most commonly stocked sizes.
| Size | Major Dia. (in) | TPI | Tap Drill |
|---|---|---|---|
| #4-40 | 0.112 | 40 | #43 (0.089) |
| #6-32 | 0.138 | 32 | #36 (0.107) |
| #8-32 | 0.164 | 32 | #29 (0.136) |
| #10-24 | 0.190 | 24 | #25 (0.150) |
| 1/4-20 | 0.250 | 20 | #7 (0.201) |
| 5/16-18 | 0.3125 | 18 | F (0.257) |
| 3/8-16 | 0.375 | 16 | 5/16 (0.313) |
| 7/16-14 | 0.4375 | 14 | U (0.368) |
| 1/2-13 | 0.500 | 13 | 27/64 (0.422) |
| 9/16-12 | 0.5625 | 12 | 31/64 (0.484) |
| 5/8-11 | 0.625 | 11 | 17/32 (0.531) |
| 3/4-10 | 0.750 | 10 | 21/32 (0.656) |
| 7/8-9 | 0.875 | 9 | 49/64 (0.766) |
| 1-8 | 1.000 | 8 | 7/8 (0.875) |
UNF (Unified National Fine) Thread Chart
Fine thread fasteners are used where vibration resistance, higher strength, or precision adjustment is needed.
| Size | Major Dia. (in) | TPI | Tap Drill |
|---|---|---|---|
| #4-48 | 0.112 | 48 | 3/32 (0.094) |
| #6-40 | 0.138 | 40 | #33 (0.113) |
| #8-36 | 0.164 | 36 | #29 (0.136) |
| #10-32 | 0.190 | 32 | #21 (0.159) |
| 1/4-28 | 0.250 | 28 | #3 (0.213) |
| 5/16-24 | 0.3125 | 24 | I (0.272) |
| 3/8-24 | 0.375 | 24 | Q (0.332) |
| 7/16-20 | 0.4375 | 20 | 25/64 (0.391) |
| 1/2-20 | 0.500 | 20 | 29/64 (0.453) |
| 5/8-18 | 0.625 | 18 | 37/64 (0.578) |
| 3/4-16 | 0.750 | 16 | 11/16 (0.688) |
How to Identify Thread Pitch
The fastest way to identify thread pitch is with a thread pitch gauge — a fan of thin metal blades, each cut with a specific thread profile. Place each blade against the bolt threads until one seats perfectly into the grooves. The stamped number on that blade is your TPI (imperial) or pitch in mm (metric).
Without a gauge, you can count threads: hold a ruler against the bolt and count how many complete thread crests fit in exactly one inch. That count is your TPI. For metric bolts, measure the distance between two adjacent crests with calipers — that measurement in millimeters is the pitch.
When to Use Coarse vs Fine Thread
Use UNC (coarse) when: you need fast assembly, the bolt threads into soft or cast material, cross-threading is a concern, or you need the most commonly available replacement fastener. Coarse thread is the default for construction, structural, and general-purpose fastening.
Use UNF (fine) when: you need maximum strength in a given diameter, the joint is subject to vibration, fine adjustment is required, or the application specifies fine thread (common in automotive and aerospace). Fine thread fasteners require more care during installation to avoid cross-threading.
Frequently Asked Questions
What is thread pitch?
Thread pitch describes the distance between threads on a fastener. For imperial (inch) fasteners, it is expressed as threads per inch (TPI) — for example, 20 TPI means 20 threads in one inch of length. For metric fasteners, pitch is the distance in millimeters between adjacent threads — for example, M8x1.25 has 1.25mm between threads.
How do I determine the thread pitch of a bolt?
Use a thread pitch gauge (a set of thin metal blades with tooth profiles) and match it against your bolt threads. Alternatively, count the number of threads in one inch of bolt length for imperial fasteners. For metric, measure the distance between two adjacent thread crests with calipers.
Can I use a UNF nut on a UNC bolt?
No. UNC and UNF threads are not interchangeable even when the diameter is the same. A 1/4-20 (UNC) bolt requires a 1/4-20 nut, not a 1/4-28 (UNF) nut. Attempting to force mismatched threads will strip the threads and ruin both fasteners.
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