Imperial Thread Chart for Imperial Bolt Sizes

17 August, 2026
11:12 pm

This chart provides complete dimensional data for imperial bolt threads per the Unified Thread Standard (UTS / ASME B1.1). It covers Unified National Coarse (UNC), Unified National Fine (UNF), and Unified National Extra Fine (UNEF) series for bolt sizes from #0 through 4 inches. Each entry includes major diameter, threads per inch (TPI), minor diameter, pitch diameter, pitch, tap drill size, and clearance hole diameter.

Thread Size Lookup

Bolt Size

Thread Series

Imperial Thread Anatomy

Imperial Thread Size Chart

All dimensions in inches. Data per ASME B1.1 Unified Thread Standard. Tap drill sizes are approximate and may vary based on material and desired thread class (2B standard).

Bolt Size	Major Dia.	Series	TPI	Minor Dia.	Pitch	Pitch Dia.	Tap Drill	Clearance
Numbered Sizes (#0 – #3)
#0	0.0600	UNF	80	0.0447	0.012500	0.0496	0.0520	0.0700
#1	0.0730	UNC	64	0.0538	0.015625	0.0603	0.0625	0.0810
#1	0.0730	UNF	72	0.0560	0.013888	0.0615	0.0635	—
#2	0.0860	UNC	56	0.0641	0.017857	0.0717	0.0730	0.0960
#2	0.0860	UNF	64	0.0668	0.015625	0.0733	0.0760	—
#3	0.0990	UNC	48	0.0734	0.020833	0.0825	0.0860	0.1100
#3	0.0990	UNF	56	0.0771	0.017857	0.0845	0.0890	—
Numbered Sizes (#4 – #6)
#4	0.1120	UNC	40	0.0813	0.025000	0.0925	0.0960	0.1285
#4	0.1120	UNF	48	0.0864	0.020833	0.0954	0.0980	—
#5	0.1250	UNC	40	0.0943	0.025000	0.1054	0.1094	0.1360
#5	0.1250	UNF	44	0.0971	0.022727	0.1070	0.1100	—
#6	0.1380	UNC	32	0.0997	0.031250	0.1141	0.1160	0.1495
#6	0.1380	UNF	40	0.1073	0.025000	0.1184	0.1200	—
Numbered Sizes (#8 – #10)
#8	0.1640	UNC	32	0.1257	0.031250	0.1399	0.1440	0.1770
#8	0.1640	UNF	36	0.1299	0.027778	0.1424	0.1470	—
#10	0.1900	UNC	24	0.1389	0.041667	0.1586	0.1610	0.2010
#10	0.1900	UNF	32	0.1517	0.031250	0.1658	0.1695	—
Numbered Sizes (#12) & Fractional (1/4")
#12	0.2160	UNC	24	0.1649	0.041667	0.1890	0.1890	0.2280
#12	0.2160	UNF	28	0.1722	0.035714	0.1930	0.1935	—
#12	0.2160	UNEF	32	0.1777	0.031250	0.1957	0.1960	—
1/4"	0.2500	UNC	20	0.1887	0.050000	0.2127	0.2188	0.2660
1/4"	0.2500	UNF	28	0.2062	0.035714	0.2225	0.2280	—
1/4"	0.2500	UNEF	32	0.2117	0.031250	0.2287	—	—
Fractional Sizes (5/16" – 3/8")
5/16"	0.3125	UNC	18	0.2443	0.055556	0.2712	0.2770	0.3320
5/16"	0.3125	UNF	24	0.2614	0.041667	0.2806	0.2812	—
5/16"	0.3125	UNEF	32	0.2742	0.031250	0.2912	0.2900	—
3/8"	0.3750	UNC	16	0.2983	0.062500	0.3287	0.3320	0.3970
3/8"	0.3750	UNF	24	0.3239	0.041667	0.3430	0.3480	—
3/8"	0.3750	UNEF	32	0.3367	0.031250	0.3537	0.3580	—
Fractional Sizes (7/16" – 1/2")
7/16"	0.4375	UNC	14	0.3499	0.071428	0.3850	0.3906	0.4687
7/16"	0.4375	UNF	20	0.3762	0.050000	0.3995	0.4062	—
7/16"	0.4375	UNEF	28	0.3937	0.035714	0.4132	0.4130	—
1/2"	0.5000	UNC	13	0.4056	0.076923	0.4435	0.4531	0.5312
1/2"	0.5000	UNF	20	0.4387	0.050000	0.4619	0.4688	—
1/2"	0.5000	UNEF	28	0.4562	0.035714	0.4757	—	—
Fractional Sizes (9/16" – 5/8")
9/16"	0.5625	UNC	12	0.4603	0.083333	0.5016	0.5156	0.5938
9/16"	0.5625	UNF	18	0.4943	0.055556	0.5205	0.5312	—
9/16"	0.5625	UNEF	24	0.5514	0.041667	0.5303	—	—
5/8"	0.6250	UNC	11	0.5135	0.090909	0.5589	0.5625	0.6562
5/8"	0.6250	UNF	18	0.5568	0.055556	0.5828	0.5938	—
5/8"	0.6250	UNEF	24	0.5739	0.041667	0.5967	—	—
Fractional Sizes (3/4" – 1")
3/4"	0.7500	UNC	10	0.6273	0.100000	0.6733	0.6875	0.7812
3/4"	0.7500	UNF	16	0.6733	0.062500	0.7029	0.7031	—
3/4"	0.7500	UNEF	20	0.6887	0.050000	0.7162	0.7188	—
7/8"	0.8750	UNC	9	0.7387	0.111111	0.7946	0.7969	0.9062
7/8"	0.8750	UNF	14	0.7874	0.071428	0.8216	0.8281	—
7/8"	0.8750	UNEF	20	0.8137	0.050000	0.8412	0.8438	—
1"	1.0000	UNC	8	0.8466	0.125000	0.9100	0.9219	1.0313
1"	1.0000	UNF	12	0.8978	0.083333	0.9382	0.9531	—
1"	1.0000	UNEF	20	0.9387	0.050000	0.9661	0.9688	—
Large Fractional Sizes (1-1/8" – 1-1/2")
1-1/8"	1.1250	UNC	7	0.9497	0.142857	1.0228	1.0313	1.1562
1-1/8"	1.1250	UNF	12	1.0228	0.083333	1.0631	1.0781	—
1-1/4"	1.2500	UNC	7	1.0725	0.142857	1.1476	1.1093	1.2850
1-1/4"	1.2500	UNF	12	1.1460	0.083333	1.1879	1.1718	—
1-3/8"	1.3750	UNC	6	1.1681	0.166667	1.2563	1.2187	1.4420
1-3/8"	1.3750	UNF	12	1.2709	0.083333	1.3127	1.2968	—
1-1/2"	1.5000	UNC	6	1.2931	0.166667	1.3812	1.3437	1.5650
1-1/2"	1.5000	UNF	12	1.3959	0.083333	1.4376	1.4218	—
Heavy Fractional Sizes (1-3/4" – 4") — UNC Only
1-3/4"	1.7500	UNC	5	1.5019	0.200000	1.6085	1.5468	1.8750
2"	2.0000	UNC	4-1/2	1.9751	0.222222	1.8528	1.7812	2.1250
2-1/4"	2.2500	UNC	4-1/2	2.2251	0.222222	2.1028	1.9614	2.3750
2-1/2"	2.5000	UNC	4	2.4731	0.250000	2.3345	2.1752	2.6250
2-3/4"	2.7500	UNC	4	2.7230	0.250000	2.5844	2.4252	2.8750
3"	3.0000	UNC	4	2.9730	0.250000	2.8344	2.6752	3.1250
3-1/4"	3.2500	UNC	4	3.2229	0.250000	3.0843	2.9252	3.3750
3-1/2"	3.5000	UNC	4	3.4729	0.250000	3.3343	3.1752	3.6250
3-3/4"	3.7500	UNC	4	3.7228	0.250000	3.5842	3.4252	3.8750
4"	4.0000	UNC	4	3.9728	0.250000	3.8342	3.6752	4.1250

Data source: Accu Components technical reference per ASME B1.1. Entries marked “—” indicate values not published in the source. Tap drill sizes are approximate for 75% thread engagement.

Unified Thread Series Explained

UNC — Unified National Coarse

The default thread series for general-purpose bolting. Fewer threads per inch means a larger pitch depth, giving coarse threads better resistance to stripping in soft materials and faster assembly. UNC is the most widely available series.

UNF — Unified National Fine

More threads per inch than UNC for the same diameter. Fine threads provide higher tensile strength due to the larger minor diameter and offer finer adjustment in precision applications. They are less prone to loosening under vibration.

UNEF — Unified National Extra Fine

The finest standard series, used primarily in aerospace and instrumentation. UNEF threads offer the highest tensile strength and the finest adjustment resolution but are limited to specific diameter ranges and are less commonly stocked.

Thread Classes (2A / 2B)

Thread class defines the tolerance zone. Class 2A (external) and 2B (internal) are the standard commercial fit. Class 3A/3B provides a tighter fit for high-vibration environments. Class 1A/1B allows a looser fit for rapid assembly.

Thread Dimension Terminology

Major Diameter

The largest diameter of the thread, measured across the crests. This is the nominal size used to identify the bolt (e.g., a 3/8" bolt has a 0.3750" major diameter). Measure with calipers across the threaded portion.

Minor Diameter

The smallest diameter of the thread, measured at the roots. This determines the bolt's stress area and tensile strength. A larger minor diameter (as in fine threads) means a stronger bolt cross-section.

Pitch Diameter

The effective diameter where thread groove width equals ridge width. This is the most critical dimension for ensuring proper mating of bolt and nut. If pitch diameters don't match within tolerance, the joint will be too loose or too tight.

Pitch & TPI

TPI (Threads Per Inch) counts the number of threads along one inch of bolt length. Pitch is the axial distance per thread: Pitch = 1 / TPI. For example, 20 TPI means a pitch of 0.050". Metric uses pitch directly (mm per thread).

How to Read Imperial Thread Designations

Step-by-Step Guide

Read the diameter — The first part of the designation is either a gauge number (#0–#12) or a fractional inch (1/4"–4"). This tells you the nominal major diameter.
Read the TPI — The number after the dash is the threads per inch. Higher TPI means finer threads and a smaller pitch.
Read the series suffix — UNC = coarse (default), UNF = fine, UNEF = extra fine. If no suffix is given, UNC is assumed.
Look up the dimensions — Use the chart above to find the minor diameter, pitch diameter, tap drill size, and clearance hole for your specific combination.
Verify the thread class — Standard commercial fasteners use Class 2A (external) or 2B (internal). Aerospace or precision applications may require Class 3A/3B.

Example: A designation of 1/4-20 UNC means: 1/4 inch major diameter, 20 threads per inch, Unified National Coarse series. From the chart: minor diameter = 0.1887", pitch diameter = 0.2127", tap drill = 7/32" (0.2188"), clearance hole = 0.2660".

Applicable Standards

ASME B1.1

The primary standard defining Unified Inch Screw Threads (UN, UNR, UNJ forms). It covers UNC, UNF, and UNEF series with all dimensional data, tolerance classes, and thread design specifications.

ASME B1.2

Defines the gauging practices for unified screw threads. Specifies how to use thread ring gauges and plug gauges to verify that external and internal threads meet the required tolerance class.

ANSI B94.1

Covers drill sizes including tap drill recommendations. Provides the standard fractional, letter, and number drill bit sizes used for pilot holes before tapping internal threads.

ISO 965-1

While primarily for metric threads, ISO 965-1 defines the general principles for tolerance classes that parallel the UTS system. Useful when converting between metric and imperial thread specifications.

Frequently Asked Questions

What does an imperial thread size designation mean?

An imperial thread designation such as 1/4-20 UNC encodes three pieces of information: the nominal major diameter (1/4 inch), the number of threads per inch (20 TPI), and the thread series (UNC = Unified National Coarse). For numbered sizes like #10-24 UNC, the number before the dash refers to the screw gauge number rather than a fractional diameter.

What is the difference between UNC, UNF, and UNEF threads?

UNC (Unified National Coarse) has fewer threads per inch, providing stronger resistance to stripping and faster assembly. UNF (Unified National Fine) has more threads per inch, offering higher tensile strength and finer adjustment. UNEF (Unified National Extra Fine) has even more threads per inch than UNF and is used in precision applications such as aerospace instrumentation. Coarse threads are the default choice for general-purpose bolting.

How do I convert TPI (threads per inch) to pitch?

Pitch is the reciprocal of TPI. The formula is: Pitch (in inches) = 1 / TPI. For example, a 1/4-20 UNC thread has 20 threads per inch, so its pitch is 1/20 = 0.050 inches. In metric terms, this equals 0.050 × 25.4 = 1.27 mm per thread. The pitch represents the axial distance the screw advances in one full revolution.

What is the pitch diameter and why does it matter?

The pitch diameter (also called the effective diameter) is the diameter of an imaginary cylinder that passes through the thread profile at the point where the widths of the groove and the thread ridge are equal. It is the most critical dimension for determining whether two threaded parts will mate correctly. If the pitch diameters of a bolt and nut do not match within tolerance, the assembly will be either too loose or too tight.

What is the difference between a tap drill and a clearance hole?

A tap drill creates the pilot hole for internal threads. Its diameter is close to the minor diameter of the thread, leaving enough material for the tap to cut. A clearance hole is drilled larger than the bolt's major diameter so the bolt passes through without engaging threads. In a typical bolted joint, one piece has a clearance hole and the mating piece has a tapped (threaded) hole.

Are metric threads interchangeable with imperial threads?

No, metric and imperial threads are not interchangeable. They use different diameter measurements (millimeters vs. inches), different pitch conventions (mm per thread vs. threads per inch), and different thread angles (60 degrees for both, but the profile proportions differ). Attempting to mate a metric nut with an imperial bolt will result in cross-threading or incomplete engagement, which can cause joint failure.

What is the major diameter of a thread?

The major diameter is the largest diameter of the thread, measured from the crest (top) of one thread to the crest of the opposite thread. It is the nominal size used to identify the bolt. For example, a 3/8-inch bolt has a major diameter of 0.3750 inches. You can measure it with calipers across the threaded portion of the bolt.

What standards govern imperial bolt threads?

Imperial bolt threads in the United States are governed by the Unified Thread Standard (UTS), defined in ASME B1.1. The UTS defines three thread series: UNC (coarse), UNF (fine), and UNEF (extra fine), each with specific TPI values for each nominal diameter. Thread classes 1A/1B (loose), 2A/2B (standard), and 3A/3B (tight) define the tolerance zones for external and internal threads respectively.

How do I read imperial bolt thread sizes on a chart?

Each row on an imperial thread chart lists a specific bolt size and thread series combination. The key columns are: major diameter (nominal size), TPI (threads per inch), minor diameter (root of thread), pitch diameter (effective mating diameter), pitch (axial distance per thread), tap drill size (for internal threads), and clearance hole size (for bolt passage). The thread series (UNC, UNF, or UNEF) determines which TPI applies to each bolt diameter.

Why do some bolt sizes not have UNEF thread data?

UNEF (Unified National Extra Fine) threads are not available for all bolt diameters. Very small sizes (#0 through #4) and very large sizes (above 1-1/2 inch) typically only have UNC and UNF series defined in the standard. UNEF is primarily used in the intermediate range (#5 through 1-1/2 inch) where fine thread control is needed for precision applications. Additionally, tap drill and clearance hole data may not be published for all combinations.

Disclaimer: The data on this page is sourced from Accu Components technical reference per ASME B1.1 for general informational purposes. Actual thread dimensions may vary within specified tolerance classes. Tap drill sizes are approximate and assume 75% thread engagement; adjust based on material and application requirements. GUXIANG does not guarantee the accuracy or completeness of this data for any specific application. Always verify dimensions against the governing standard before production use.

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Imperial Thread Chart for Imperial Bolt Sizes

Thread Size Lookup

Imperial Thread Anatomy

Imperial Thread Size Chart