Imperial Thread Size Chart
Every standard Unified thread size has a fixed major diameter and one or two standard thread counts — coarse (UNC) and fine (UNF). A 1/2" thread is 0.500" across, with 13 threads per inch in the coarse series and 20 in the fine series.
UNC and UNF, Side by Side
Unified National Coarse (UNC) and Unified National Fine (UNF) are the two standard US thread series, and almost every diameter from #0 to 1 1/2" is defined in both.
UNC is the default choice for general-purpose fastening — it assembles faster, resists stripping in softer materials, and is less sensitive to minor damage or dirt in the threads. UNF has more threads per inch at the same diameter, which gives a finer adjustment, slightly higher clamping force for a given torque, and better performance in thin-walled parts where more thread engagement per unit length matters.
From Threads Per Inch to Pitch
A thread's pitch — the distance from one thread crest to the next — is simply the inverse of its TPI.
P, the thread pitch in inches, equals 1 divided by the threads-per-inch count.
- TPIThreads per inch, such as 20 for 1/4-20.
- PThe pitch: the inch distance between adjacent thread crests.
Worked calculation — 1/4-20 UNC
- 1 ÷ 20 = 0.05Invert the TPI
1/4-20 has a pitch of 0.05" between threads
UNC & UNF Thread Sizes, #0 to 1 1/2"
| Size | Major diameter | UNC (coarse) TPI | UNF (fine) TPI |
|---|---|---|---|
| #0 | 0.06" | — | 80 |
| #1 | 0.073" | 64 | 72 |
| #2 | 0.086" | 56 | 64 |
| #3 | 0.099" | 48 | 56 |
| #4 | 0.112" | 40 | 48 |
| #5 | 0.125" | 40 | 44 |
| #6 | 0.138" | 32 | 40 |
| #8 | 0.164" | 32 | 36 |
| #10 | 0.19" | 24 | 32 |
| #12 | 0.216" | 24 | 28 |
| 1/4" | 0.25" | 20 | 28 |
| 5/16" | 0.3125" | 18 | 24 |
| 3/8" | 0.375" | 16 | 24 |
| 7/16" | 0.4375" | 14 | 20 |
| 1/2" | 0.5" | 13 | 20 |
| 9/16" | 0.5625" | 12 | 18 |
| 5/8" | 0.625" | 11 | 18 |
| 3/4" | 0.75" | 10 | 16 |
| 7/8" | 0.875" | 9 | 14 |
| 1" | 1" | 8 | 12 |
| 1 1/8" | 1.125" | 7 | 12 |
| 1 1/4" | 1.25" | 7 | 12 |
| 1 3/8" | 1.375" | 6 | 12 |
| 1 1/2" | 1.5" | 6 | 12 |
For the matching tap drill size for any of these threads, see the tap drill chart.
Frequently Asked Questions
What is the major diameter of a 1/2 inch thread?
0.500" exactly — a fractional thread size names its own major diameter directly.
What is the smallest standard UNC/UNF thread size?
#0, with a major diameter of 0.060". It's only defined in the UNF (fine) series, as #0-80.
Why does #0 only have a UNF version?
#0 threads are used mainly in delicate, small-scale work like eyewear and electronics, where a fine 80-threads-per-inch pitch suits the application better than a coarser one — no standard UNC version was ever defined for this size.
How many threads per inch does a 3/4-10 have?
10 threads per inch — the "10" in "3/4-10" is the TPI, and "3/4" is the major diameter.
What's the difference between major diameter and pitch diameter?
Major diameter is measured across the outermost points of the external thread — the largest diameter the fastener has. Pitch diameter is measured at an imaginary cylinder roughly halfway down the thread's depth, where the groove width equals the ridge width — it's the dimension that actually determines fit between a bolt and a nut, and it's always slightly smaller than the major diameter.
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