UNC Thread Size Chart & Master Reference: Complete UNC, UNF, UNEF Dimensions per ASME B1.1
Your definitive engineering source for Unified Inch Screw Threads. Featuring interactive charts for Threads Per Inch (TPI), Decimal Equivalents, and precision Tap Drill selection for standard manufacturing.
Authority Note: This UNC Thread Size Chart is based on the ASME B1.1 Unified Inch Screw Threads standard. It provides critical data for Unified Coarse (UNC) series fasteners, commonly used in mass-production and general engineering. For custom manufacturing or Grade 8 specifications, please consult our Technical Team.
Quick Reference: The UNC (Unified National Coarse) thread series is the industrial standard for fasteners requiring rapid assembly and high stripping resistance in soft materials. This guide provides the full UNC thread size chart (from #0-80 to 4"-4), alongside comparisons with UNF and UNEF series to ensure precision in mechanical design and procurement.
Note: This visualization demonstrates the progressive increase in thread density from UNC to UNEF. While UNC (left) is optimized for rapid assembly and shear resistance in soft materials, UNEF (right) provides the maximum number of engaged threads in thin-walled components, significantly increasing resistance to vibration-induced loosening.
1. What is Characteristics of Unified National Coarse Threads
Out of the two Unified Thread Standards (UTS), UNC threads are the most commonly used, the other being UNF (United National Fine Threads). UNC threads have specific characteristics that make them more suitable for the applications they are used for. Some such features are as follows.
Unified Coarse Threads are less likely to cross-thread between themselves cue to their texture.
UNC threads enable faster assembly and disassembly of fittings. They are used in mass production due to this feature.
UNC threads enable a thicker plating and coating of fittings as they have large thread allowances.
When compared to UNF threads (fine threads), UNC threads have a fewer number of threads per distance.
UNC threads are subjected to a classification system which helps to make it easy to manufacture them. The thread class indicates the range of pitch diameter for any specific range of thread. These classes are specified according to their tolerances and are called the United Screw Thread Standard Series. UNC can be found in five different classes. They are as follows:
- Class 1 UNC threads are used for applications which require a liberal tolerance is required in order to ensure an easy assembly. This is common to instances where slightly nicked threads are used. They have a loosely fitting set of threads that are used in easy assembly points or in a dirty environment.
- Class 2 UNC threads are the types of UNC threads are the most commonly used class for general-purpose applications. These UNC threads are designed and manufactured to maximise the strength in relation to machine practice.
- Class 3 UNC threads are used in applications where it is important to use close fit and accurate thread elements.
- Class 4 UNC threads are manufactured to use in a closer fit than a Class 3 UNC thread.
- Class 5 UNC threads are designed nd manufactured to be used on item,s like spring shackles in automobiles. Therefore, class 5 UNC threads are not manufactured with loose threads,
UNC threads can be identified as either internal or external threads. The letter suffix along with their class indicates whether they are internal or external threads. The letter suffix of either ‘A’ or ‘B’ is used for this purpose, where ‘A’ indicates external threads, meanwhile ‘B’ indicates internal threads. Thereby, classes 1A, 2A, and 3A indicate external threads, while Classes 1B, 2B, and 3B apply to internal threads.
The thread pitch is the distance between the threads that is indicated in millimetres. Usually, smaller fasteners have finer threads, which means they have a relatively lower thread pitch. The typical designation of a UNC thread is ¼” x 20 Unified National Coarse Thread.
Unified Thread Standard (UTS) Master Directory
This interactive repository integrates three critical screw thread series: UNC (Coarse), UNF (Fine), and UNEF (Extra Fine) per ASME B1.1 standards. Our unified table provides precise data on TPI (Threads Per Inch), major diameters, and recommended tap drill sizes, optimized for high-precision engineering and industrial procurement.
| Size (UNC) | TPI | Major Dia (in) | Tap Drill | Application |
|---|---|---|---|---|
| Numbered Sizes | ||||
| #4 - 40 | 40 | 0.1120 | #43 | General electronic assembly |
| #6 - 32 ⭐ | 32 | 0.1380 | #36 | PC Case / Computer screws |
| Fractional Sizes | ||||
| 1/4" - 20 ⭐ | 20 | 0.2500 | #7 | Construction / General Purpose |
| 1/2" - 13 | 13 | 0.5000 | 27/64" | Heavy machinery bolts |
| Size (UNF) | TPI | Major Dia (in) | Tap Drill | Advantage |
|---|---|---|---|---|
| Precision Range | ||||
| #10 - 32 ⭐ | 32 | 0.1900 | #21 | Aviation / Rack mounting |
| 1/4" - 28 | 28 | 0.2500 | #3 | High tensile strength needs |
| 1/2" - 20 ⭐ | 20 | 0.5000 | 29/64" | Automotive engine parts |
| Size (UNEF) | TPI | Major Dia (in) | Tap Drill | Best For |
|---|---|---|---|---|
| 1/4" - 32 ⭐ | 32 | 0.2500 | 7/32" | Thin-walled tube connectors |
| 1/2" - 28 ⭐ | 28 | 0.5000 | 15/32" | Precision adjustment / Muzzles |
| 1" - 20 | 20 | 1.0000 | 61/64" | Optical instruments |
Technical Rationale for UNC Selection: The Unified National Coarse (UNC) series is strategically selected for these industries because its larger pitch and deeper thread profile allow for higher clamping loads and faster installation speeds compared to UNF. In the Gas and Oil industry, UNC's resistance to corrosion-induced seizing makes it the authoritative choice for outdoor infrastructure, while in Furniture and DIY tools, it provides the necessary thread engagement for softer substrates like aluminum or cast iron.
The UNC threads that have sizes below are given a number that is defined using the standard. This type of smaller-sized thread is commonly referred to using numbers and pitch. This can be put out as 10-24 which would be UNC No. 10 or 10-32 UNF No. 10.
It is important to note thast the basic profile of all the UTS threads, may it be UNC or UNF, is similar to that of the ISO Metric Scrrew thread standards. The only difference in vaalues is exhibited in the Dmaj (outer diameter) and P (pitch).
Below given is a UNC size chart according to the ANSI B1.1 size chart.
4. Conclusion
UTS or United Thread Standard is a standard widely used in the United States and Canada in order to assign designations and allowances to nuts and bolts that are used in manufacturing and industrial applications. The UTS can be divided into two classifications, i.e. the UNC (United National Coarse Thread) and UNF (United National Fine Thread). UNC differs from UNF due to the coarse threads it possesses.
UNC is commonly used in various applications due to their ease of use and therefore has a wide range of applications. They are used in many industries like manufacturing, oil and gas industry as well as hydraulic systems. It is important to maintain a standard like UTS to ensure smooth functioning of machinery and equipment.
Need High-Quality UNC Fasteners?
We manufacture standard and custom UNC bolts to meet your project's exact strength and coating requirements.
Get a Quote in 24hWhy is UNC preferred over UNF in heavy machinery and construction?
UNC (Coarse) threads have a larger pitch and deeper thread form, which provides a significantly larger shear area per thread. This makes them far more resistant to stripping when used in lower-strength materials like cast iron, aluminum, or structural steel. Additionally, their rugged design allows for much faster assembly and disassembly, even when parts are slightly contaminated by dirt or debris.
In the Oil and Gas industry, how does UNC help with corrosion management?
In harsh environments, fasteners are prone to oxidation and seizing. UNC threads have wider gaps between the thread ridges, which provides more space for protective coatings (like Hot Dip Galvanizing or Zinc-Nickel) without causing interference. This extra clearance also makes it easier to break loose a seized bolt during maintenance compared to fine threads.
Does UNC offer better performance in soft materials like those found in the Furniture or DIY industries?
Yes. When threading into wood, composite materials, or soft alloys, UNC's deeper thread engagement distributes the clamping load over a wider area of the substrate. This prevents the "pull-out" failure common with fine threads, ensuring a more secure and durable joint in consumer goods and furniture.