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UNC Thread Chart

Look up UNC size, TPI, pitch, and basic thread dimensions—or match measured diameter and TPI.

UNC lookup

Find a UNC Thread Size

Select a size, or enter outside diameter and TPI to find the closest listed UNC thread.

Basic 60° profile

UNC Thread Profile and Basic Diameters

The diagram links pitch and the basic diameter levels to the selected thread. It is schematic, not an inspection-limit drawing.

Basic Unified 60-degree thread profile for 1/4-20 UNC Schematic external Unified thread profile showing pitch P and horizontal reference levels for nominal major diameter D, basic pitch diameter E, basic internal minor diameter, and basic external minor diameter. P = 0.050000 in D E 60°
Basic geometry only. Thread class, allowance, tolerance, coating, lead error, and gaging determine actual acceptance limits.
Selected thread
1/4-20 UNC
Pitch P
0.050000 in
Major D
0.2500 in
Pitch diameter E
0.2175 in
External minor
0.1887 in
Internal minor
0.1959 in

Reference chart

UNC Thread Size and Basic Dimensions Chart

A limited common-size UNC reference from #1 through 1-1/2 in. Basic geometry is calculated from nominal diameter and pitch.

Search by designation, fractional size, decimal diameter, or TPI. Sortable headers become active when JavaScript is available.

Scroll horizontally to view additional columns and vertically to view additional rows. The UNC designation column and column headers remain visible while scrolling.

Limited common-size UNC reference. Basic dimensions are theoretical Unified-thread geometry and are not class-specific manufacturing or inspection limits.
Identification Thread pitch Calculated basic geometry Copy
Nominal size
#1-64 UNC #1 in 0.0730 64 0.015625 0.397 0.0629 0.0538 0.0561
#2-56 UNC #2 in 0.0860 56 0.017857 0.454 0.0744 0.0641 0.0667
#3-48 UNC #3 in 0.0990 48 0.020833 0.529 0.0855 0.0734 0.0764
#4-40 UNC #4 in 0.1120 40 0.025000 0.635 0.0958 0.0813 0.0849
#5-40 UNC #5 in 0.1250 40 0.025000 0.635 0.1088 0.0943 0.0979
#6-32 UNC #6 in 0.1380 32 0.031250 0.794 0.1177 0.0997 0.1042
#8-32 UNC #8 in 0.1640 32 0.031250 0.794 0.1437 0.1257 0.1302
#10-24 UNC #10 in 0.1900 24 0.041667 1.058 0.1629 0.1389 0.1449
#12-24 UNC #12 in 0.2160 24 0.041667 1.058 0.1889 0.1649 0.1709
1/4-20 UNC 1/4 in 0.2500 20 0.050000 1.270 0.2175 0.1887 0.1959
5/16-18 UNC 5/16 in 0.3125 18 0.055556 1.411 0.2764 0.2443 0.2524
3/8-16 UNC 3/8 in 0.3750 16 0.062500 1.587 0.3344 0.2983 0.3073
7/16-14 UNC 7/16 in 0.4375 14 0.071429 1.814 0.3911 0.3499 0.3602
1/2-13 UNC 1/2 in 0.5000 13 0.076923 1.954 0.4500 0.4056 0.4167
9/16-12 UNC 9/16 in 0.5625 12 0.083333 2.117 0.5084 0.4603 0.4723
5/8-11 UNC 5/8 in 0.6250 11 0.090909 2.309 0.5660 0.5135 0.5266
3/4-10 UNC 3/4 in 0.7500 10 0.100000 2.540 0.6850 0.6273 0.6417
7/8-9 UNC 7/8 in 0.8750 9 0.111111 2.822 0.8028 0.7387 0.7547
1-8 UNC 1 in 1.0000 8 0.125000 3.175 0.9188 0.8466 0.8647
1 1/8-7 UNC 1 1/8 in 1.1250 7 0.142857 3.629 1.0322 0.9497 0.9704
1 1/4-7 UNC 1 1/4 in 1.2500 7 0.142857 3.629 1.1572 1.0747 1.0954
1 3/8-6 UNC 1 3/8 in 1.3750 6 0.166667 4.233 1.2667 1.1705 1.1946
1 1/2-6 UNC 1 1/2 in 1.5000 6 0.166667 4.233 1.3917 1.2955 1.3196

Basic dimensions are theoretical values. Thread class, allowance, tolerance, coating/plating, length of engagement, gaging method, and the applicable product specification determine acceptable manufactured limits.

How to Read a UNC Thread Designation

A designation such as 1/4-20 UNC combines the nominal thread size, the pitch count, and the thread series. The first part is the nominal major diameter, 20 means 20 threads per inch, and UNC identifies the Unified National Coarse series.

1/4
Nominal major diameter of 0.2500 in.
20
Twenty threads per inch, so pitch P = 1/20 = 0.050000 in.
UNC
Unified coarse-thread series designation.

Numbered screw sizes such as #6-32 UNC use a numbered nominal diameter rather than a fraction. The designation is an identifier; it is not a measured tolerance limit.

UNC Basic Thread Dimensions Explained

The chart calculates the basic 60° Unified profile from nominal major diameter D and pitch P. These relationships describe ideal geometry before class-specific allowance and tolerance are applied.

Major diameter D
The nominal outside diameter used to identify the thread size.
Pitch P
Axial distance from one thread to the corresponding point on the next; P = 1/TPI.
Pitch diameter E
The basic diameter where thread and groove widths are equal in the ideal profile.
Minor diameters
Basic external root and internal minor diameters are different because the Unified profile truncations differ.

Basic Dimensions vs. 2A, 2B, 3A, and 3B Limits

A basic diameter is a theoretical starting value, not a pass/fail inspection limit. ASME B1.1 separately defines thread classes, allowance, tolerance, limits of size, coating accommodation, and designation rules.

Do not use this chart as a GO/NO-GO inspection table. External A-class and internal B-class threads require the applicable class limits and gaging method from the governing standard and product specification.

Wear, plating, manufacturing tolerance, lead error, flank-angle error, and measurement technique can all make a real thread differ from the ideal basic dimensions shown here.

How to Identify an Unknown UNC Thread

  1. Measure the external major diameter across the thread crests with calipers.
  2. Measure TPI with an inch thread-pitch gauge; do not infer thread pitch from wrench size.
  3. Enter both measurements in the finder. An exact TPI match is weighted more strongly than diameter proximity.
  4. Compare the returned nominal diameter and residual. A close candidate is an identification aid, not proof of conformance.
  5. If the TPI does not match a listed UNC row, check UNF, UNEF, UNS, metric, pipe, ACME, or another thread family.

UNC vs. UNF threads

UNC and UNF can share the same nominal major diameter while using different pitch counts. For example, a 1/4 in thread identified only by outside diameter is incomplete; TPI is required to distinguish the series reliably.

Worked UNC Thread Examples

Example 1: 1/4-20 UNC

Given: D = 0.2500 in and n = 20 TPI.

Pitch: P = 1/20 = 0.050000 in = 1.270 mm.

Basic geometry: E ≈ 0.2175 in, external minor ≈ 0.1887 in, internal minor ≈ 0.1959 in.

Interpretation: These are ideal basic-profile values; use the required ASME class limits for production acceptance.

Example 2: identify a measured thread

Given: outside diameter ≈ 0.500 in and pitch gauge = 13 TPI.

Lookup: the supported list points to 1/2-13 UNC.

Sanity check: a 0.500 in diameter alone is not enough—1/2 in Unified threads exist in other series with different TPI.

Limitation: caliper and pitch-gauge identification does not establish thread class or acceptance.

Standards, Scope, and Data Rights

Dataset T2E-UNC-BASIC-v1.0 · common UNC sizes #1 through 1-1/2 in · calculated basic geometry · no class-limit table reproduction.

ASME B1.1-2024 is the controlling current standard for Unified inch thread form, series, class, allowance, tolerance, designation, formulas, and basic dimensions. Because ASME B1.1 is copyrighted, this page does not reproduce its complete dimensional or tolerance tables. The displayed geometry is independently calculated from nominal diameter and pitch and cross-checked against the open historical U.S. National Bureau of Standards Unified-thread publication.

Dataset and Source-Check Details

Publisher
Turn2Engineering
Controlling edition
ASME B1.1-2024
Dataset
T2E-UNC-BASIC-v1.0
Source checked
August 19, 2026
Canonical calculation unit
Inches
Thread form
60° Unified basic profile
Included scope
Limited common UNC designations #1 through 1-1/2 in
Excluded
Class-specific limits, gage dimensions, coating allowances, UNF/UNEF/UNS, pipe and ACME threads
Rights treatment
Original calculations and limited transformed reference; complete ASME tables not reproduced
CSV
Disabled

UNC Thread Chart FAQs

It identifies a nominal 1/4 in Unified coarse thread with 20 threads per inch; the designation does not by itself specify the external/internal class or inspection limits.

Use the Chart for Identification, Then Verify the Required Class

Use nominal diameter and TPI to identify the UNC designation, and use the calculated basic dimensions to understand the ideal thread geometry. For manufacturing or inspection, continue to the governing ASME B1.1 class limits and the applicable product/gaging requirements.

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