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Thread callout decoder
The thread callout decoder explains a callout such as 1/4-20 UNC-2A, M10x1.5-6g or 1/4-18 NPT part by part: the series, the size, the threads per inch or pitch, the class, whether the thread is internal or external, its hand and its basic diameters, from ASME B1.1, ISO 261 and ISO 965-1.
Unified inch thread
The callout 1/4-20 UNC-2A
1/4Size- A nominal size of 1/4 inch: a basic major diameter of 0.2500 in.
20Threads per inch- 20 threads per inch: a pitch of 0.0500 in from one thread to the next.
UNCSeries- The coarse series: the usual choice for general fasteners.
2AClass- An external thread (A), class 2: the general-purpose tolerance, by far the most common on fasteners.
| Thread | Unified inch screw thread (ASME B1.1), 60° thread angle |
|---|---|
| Series | Unified coarse (UNC) |
| Nominal size | 1/4 in |
| Basic major diameter | 0.2500 in (6.35 mm) |
| Threads per inch | 20 |
| Pitch | 0.0500 in (1.27 mm) |
| Basic pitch diameter | 0.2175 in (5.525 mm) |
| Basic minor diameter of the internal thread | 0.1959 in (4.975 mm) |
| Class | 2A: the general-purpose tolerance, by far the most common on fasteners |
| External or internal | External (a bolt or stud) |
| Hand | Right-hand: a callout says LH only for a left-hand thread |
How to read it
How to read a thread callout.
A callout names the thread in a fixed order, left to right: size, then pitch, then series, then tolerance, then anything special.
Unified inch threads: 1/4-20 UNC-2A
- Size. 1/4 is the nominal diameter in inches. Below 1/4 inch, sizes are numbered: #10 is 0.190 in, which is 0.060 + 0.013 × 10.
- Threads per inch. 20 threads in every inch of length, so the pitch is 1/20 = 0.050 inch.
- Series. UNC is coarse, UNF fine and UNEF extra-fine; UN is a constant-pitch series and UNS a special thread. An R (UNR, UNRC) asks for a rounded root on the external thread, and a J (UNJ) for a controlled root radius.
- Class. 1, 2 or 3 for a loose, general-purpose or close tolerance; A for an external thread, B for an internal one.
- Hand. LH at the end marks a left-hand thread. A thread with no LH is right-hand.
ISO metric threads: M10x1.5-6g
- Diameter. M10 is an ISO metric thread 10 mm in nominal diameter.
- Pitch. x1.5 is a 1.5 mm pitch, the distance from one thread to the next. Left out, it means the coarse pitch: M10 alone is M10x1.5.
- Tolerance class. 6g is tolerance grade 6 (a smaller number is tighter) at position g (how much allowance). A lowercase letter is an external thread and a capital an internal one; 6H/6g gives the fit of a pair, and 5g6g gives the pitch diameter and the major diameter separately.
- Hand and engagement. LH marks a left-hand thread; S or L a short or long length of engagement.
Pipe threads: 1/4-18 NPT
The first number is a nominal pipe size, not a diameter: 1/4 NPT is cut on pipe 0.540 in across. 18 is the threads per inch. NPT tapers 1 in 16 on the diameter, so the joint tightens as it is screwed together; it seals with a sealant on the threads, while NPTF (dryseal) is made to seal without one.
Sources: ASME B1.1 (unified inch threads), ISO 261 and ISO 965-1 (ISO metric threads and their tolerances), ISO 68-1 (the basic profile behind the pitch and minor diameters), ASME B1.20.1 (taper pipe threads).
Common sizes
Common thread sizes.
The sizes the decoder checks a callout against. A size beyond them is still decoded from what the callout itself says.
Unified inch threads, #0 to 1 inch
| Size | Basic major diameter, in | Basic major diameter, mm | Coarse (UNC), threads per inch | Fine (UNF), threads per inch |
|---|---|---|---|---|
| #0 | 0.0600 | 1.524 | none | 80 |
| #1 | 0.0730 | 1.854 | 64 | 72 |
| #2 | 0.0860 | 2.184 | 56 | 64 |
| #3 | 0.0990 | 2.515 | 48 | 56 |
| #4 | 0.1120 | 2.845 | 40 | 48 |
| #5 | 0.1250 | 3.175 | 40 | 44 |
| #6 | 0.1380 | 3.505 | 32 | 40 |
| #8 | 0.1640 | 4.166 | 32 | 36 |
| #10 | 0.1900 | 4.826 | 24 | 32 |
| #12 | 0.2160 | 5.486 | 24 | 28 |
| 1/4 in | 0.2500 | 6.35 | 20 | 28 |
| 5/16 in | 0.3125 | 7.938 | 18 | 24 |
| 3/8 in | 0.3750 | 9.525 | 16 | 24 |
| 7/16 in | 0.4375 | 11.112 | 14 | 20 |
| 1/2 in | 0.5000 | 12.7 | 13 | 20 |
| 9/16 in | 0.5625 | 14.287 | 12 | 18 |
| 5/8 in | 0.6250 | 15.875 | 11 | 18 |
| 3/4 in | 0.7500 | 19.05 | 10 | 16 |
| 7/8 in | 0.8750 | 22.225 | 9 | 14 |
| 1 in | 1.0000 | 25.4 | 8 | 12 |
Source: ASME B1.1. A numbered size's basic major diameter is 0.060 + 0.013 × its number, in inches.
ISO metric threads, M1.6 to M24
| Size | Coarse pitch, mm | Common fine pitches, mm | Basic major diameter, in |
|---|---|---|---|
| M1.6 | 0.35 | - | 0.0630 |
| M2 | 0.4 | - | 0.0787 |
| M2.5 | 0.45 | - | 0.0984 |
| M3 | 0.5 | - | 0.1181 |
| M3.5 (second choice) | 0.6 | - | 0.1378 |
| M4 | 0.7 | - | 0.1575 |
| M5 | 0.8 | - | 0.1969 |
| M6 | 1 | - | 0.2362 |
| M8 | 1.25 | 1 | 0.3150 |
| M10 | 1.5 | 1.25, 1 | 0.3937 |
| M12 | 1.75 | 1.5, 1.25 | 0.4724 |
| M14 (second choice) | 2 | 1.5 | 0.5512 |
| M16 | 2 | 1.5 | 0.6299 |
| M18 (second choice) | 2.5 | 1.5 | 0.7087 |
| M20 | 2.5 | 1.5 | 0.7874 |
| M22 (second choice) | 2.5 | 1.5 | 0.8661 |
| M24 | 3 | 2 | 0.9449 |
Source: ISO 261. Any pitch finer than the coarse one is a fine pitch; ISO 261 lists more for each diameter than the common ones here. Second-choice diameters are ISO 261's second choice: use the first choice where you can.
NPT taper pipe threads, 1/16 to 2
| Nominal pipe size | Threads per inch | Pipe outside diameter, in | Pipe outside diameter, mm |
|---|---|---|---|
| 1/16 | 27 | 0.313 | 7.938 |
| 1/8 | 27 | 0.405 | 10.287 |
| 1/4 | 18 | 0.540 | 13.716 |
| 3/8 | 18 | 0.675 | 17.145 |
| 1/2 | 14 | 0.840 | 21.336 |
| 3/4 | 14 | 1.050 | 26.67 |
| 1 | 11.5 | 1.315 | 33.401 |
| 1-1/4 | 11.5 | 1.660 | 42.164 |
| 1-1/2 | 11.5 | 1.900 | 48.26 |
| 2 | 11.5 | 2.375 | 60.325 |
Source: ASME B1.20.1. The thread tapers 1 in 16 on the diameter.
Questions
What people ask about thread callouts.
What does 1/4-20 UNC-2A mean?
A thread 1/4 inch in nominal diameter with 20 threads per inch, in the Unified coarse series (UNC), made to class 2A: the general-purpose tolerance, on an external thread such as a bolt. Its basic major diameter is 0.2500 in, and with no LH it is right-hand.
What does M10x1.5-6g mean?
An ISO metric thread 10 mm in nominal diameter with a pitch of 1.5 mm, the coarse pitch for M10, made to tolerance class 6g: tolerance grade 6 at position g, a small allowance, on an external thread. A metric callout gives the distance from one thread to the next in millimeters, not threads per millimeter.
What is the difference between 2A and 2B?
Both are class 2, the general-purpose tolerance of ASME B1.1. A marks an external thread, such as a bolt or a stud, and B an internal thread, such as a tapped hole or a nut. Class 1 is looser and class 3 is closer.
What size is a #10 screw?
A numbered size's basic major diameter is 0.060 + 0.013 times the number, in inches, so #10 is 0.190 in (4.826 mm). #10-24 is its coarse thread (UNC) and #10-32 its fine one (UNF).
Why do some metric callouts have no pitch?
A metric callout with no pitch, such as M6, means the coarse pitch for that diameter: 1 mm for M6 and 1.5 mm for M10 (ISO 261). A fine pitch is always written, as in M10x1.25.
Is 1/4 NPT a quarter-inch thread?
No. In a pipe thread the number is a nominal pipe size, not a diameter: 1/4 NPT is cut on pipe 0.540 in across, at 18 threads per inch, and it tapers 1 in 16 on the diameter (ASME B1.20.1).
From SpecCapture
Reading a whole drawing?
A hole table's size column is often a list of thread callouts like these, and SpecCapture reads it row by row, each callout as printed. SpecCapture reads the title block, the revision history, the bill of materials, the hole table, the application block, the tabulated dash numbers and the general notes off 2D engineering drawings, with a box on the drawing around every value it reads, so checking it takes a glance.