This comprehensive internal screw threads dimension chart provides major, pitch, and minor diameter tolerances for US standard internal (tap) threads from #0 through 2 inch in both UNC (coarse) and UNF (fine) series. All values conform to ANSI/ASME B1.1 for Class 2B and Class 3B fits. Use the interactive lookup tool below to quickly find complete dimensional data for any internal thread size.
Select a screw size and thread series from the dropdowns to view the complete internal thread dimensional specifications including major, pitch, and minor diameter tolerances for each thread class.
The diagram below illustrates the three critical diameter measurements for internal (tapped) threads. The major diameter defines the basic hole size, the pitch diameter determines thread fit quality, and the minor diameter represents the thread root — which corresponds to the tap drill size.
The table below covers ANSI standard internal thread sizes from #0 through 2 inch in both UNC (coarse) and UNF (fine) series. Each entry includes the basic major diameter, pitch diameter tolerance range, and minor diameter maximum for both Class 2B (general purpose) and Class 3B (precision) fits. All values are in inches per ASME B1.1.
Internal thread classes define the tolerance range for manufactured threads. The ANSI/ASME B1.1 standard specifies five classes for internal threads, with 2B and 3B being the most commonly encountered in fastener applications. Understanding the difference helps you select the right tap and gauge for your application.
Class 1B — Loose Fit
Widest tolerances, designed for quick assembly and disassembly where precision is not critical. Rarely used in modern fastener applications except for temporary or adjustable fittings.
Class 2B — General Purpose
The default class for nearly all commercial fasteners, nuts, and tapped holes. Provides a good balance of ease of assembly, strength, and manufacturing cost. Specified unless drawing calls otherwise.
Class 3B — Precision Fit
Tight tolerance class for aerospace, instrumentation, and high-vibration environments. Pitch diameter and minor diameter tolerances are approximately 30–40% tighter than Class 2B for the same thread size.
Thread Fit Designation
The letter "B" designates internal (blind or through) threads. External threads use the letter "A" (e.g., 2A, 3A). A complete thread specification includes the diameter, TPI, series, and class — e.g., 1/4-20 UNC-2B.
Internal threads must always be dimensionally larger than their mating external threads at every diameter point. The major diameter of the internal thread equals the major diameter of the external thread (basic size), but the internal thread's minor diameter is larger than the external thread's minor diameter. This ensures the external thread can enter the internal thread without interference.
Major Diameter Match
The basic major diameter is identical for both internal and external threads of the same size. For example, both a 1/4-20 internal thread and a 1/4-20 external thread share a basic major diameter of 0.2500 inches.
Pitch Diameter Clearance
The internal thread pitch diameter minimum is always smaller than the external thread pitch diameter maximum, creating the necessary clearance for assembly. This is the primary determinant of thread fit quality.
Minor Diameter Gap
The internal thread minor diameter (root) is larger than the external thread minor diameter (crest), creating a radial gap at the root. This gap prevents interference and allows for manufacturing variations.
Thread Engagement
For steel-to-steel joints, minimum thread engagement should equal at least 1.0× the nominal diameter. For aluminum or brass tapped holes, increase to 1.5×–2.0× diameter to compensate for lower shear strength of the softer material.
Internal thread dimensions and tolerances are governed by several key ANSI/ASME standards. Understanding which standard applies ensures proper tap selection, gauge verification, and final assembly fit.
ASME B1.1
Unified Thread Standard — the primary reference for all inch-based thread dimensions. Defines the geometry, tolerance classes (1B–5B), and measurement methods for UNC, UNF, and UNEF series internal threads.
ASME B1.2
Gauging and Gages — defines the specifications for GO/NO-GO thread plug gages used to inspect internal threads. Covers gage tolerances, wear allowances, and inspection procedures.
ANSI B94.1
Drill Sizes — standardizes the diameters of twist drills including tap drills. Provides the decimal equivalents for numbered drill sizes (#1–#80) and letter drill sizes (A–Z).
ISO 965-1
Metric thread tolerances — the metric equivalent of ASME B1.1 for ISO metric threads (M-series). Provides internal thread tolerance classes 4H–7H for major, pitch, and minor diameters.
What are internal thread dimensions?
Internal thread dimensions are the standardized measurements that define the size and fit of a threaded hole. They include the major diameter (basic hole size), pitch diameter (where thread width equals groove width), and minor diameter (the root of the thread). These dimensions determine whether a mating bolt will properly fit and are defined by ANSI/ASME B1.1 for Unified threads. The major diameter is the theoretical exact size, while the pitch and minor diameters have tolerance ranges that vary by thread class (2B or 3B).
How do I determine the correct tap drill size for an internal thread?
The tap drill diameter approximately equals the minor diameter of the thread. A practical formula: Tap drill = Major diameter − (1/TPI × 1.0825 × % of thread). For a 75% thread (common general-purpose target): Tap drill ≈ Major diameter − 0.81 × pitch. For example, a 1/4-20 UNC thread (major dia. 0.250, 20 TPI, pitch = 0.050): Tap drill ≈ 0.250 − 0.81 × 0.050 = 0.2095 inches, which corresponds to a #7 drill (0.201"). Standard tap drill charts are published in ANSI B94.1.
What is the difference between 2B and 3B internal thread classes?
2B is the standard general-purpose internal thread class used in most commercial fasteners, offering medium tolerance for a good balance of ease of assembly and strength. 3B is a tight-tolerance class for precision applications like aerospace and instrument assembly, with both pitch diameter and minor diameter tolerances roughly 30–40% tighter than 2B. Both classes share the same basic major and pitch diameters; only the tolerance ranges differ. Unless your drawing specifically calls for 3B, default to 2B.
What is the minor diameter of an 8-32 internal thread?
For an 8-32 UNC internal thread in Class 2B, the minor diameter maximum is 0.1369 inches. The minor diameter represents the root of the internal thread and determines the minimum material condition — essentially the size of the hole at the bottom of the thread. This value is critical for verifying that a tap has cut to the correct depth and diameter. For Class 3B, the minor diameter maximum is slightly tighter at 0.1364 inches.
What is the pitch diameter in an internal thread?
The pitch diameter is the imaginary cylinder diameter where the thread groove width equals the thread ridge width. It is the most critical dimension for ensuring proper thread fit and engagement. For internal threads, the pitch diameter falls between the major and minor diameters. For example, a 1/4-20 UNC 2B thread has a pitch diameter range of 0.2278" (min) to 0.2309" (max). Pitch diameter is verified using thread plug gauges or the three-wire method with a micrometer.
Can I use the same tap drill for 2B and 3B threads of the same size?
Yes, the tap drill size is the same for both 2B and 3B classes of the same thread size. The tap drill is determined by the basic major diameter and thread pitch, which are identical regardless of class. The difference between 2B and 3B lies in the tolerance of the finished thread, not the tap drill size. However, the quality of the tap and the drilling precision may need to be higher for 3B to consistently achieve the tighter tolerances.
How do I measure internal thread dimensions?
Internal thread major diameter is checked with a calibrated plug gauge or coordinate measuring machine (CMM). Pitch diameter is verified using a thread plug gauge (GO/NO-GO) or the three-wire method with a micrometer. Minor diameter is measured with a small-hole gauge or CMM. For production inspection, GO/NO-GO thread plug gauges per ANSI B1.2 are the standard method: the GO gauge must thread in fully, while the NO-GO gauge should not exceed 2 turns of engagement.
What is the major diameter of a 6-32 internal thread?
The basic major diameter of a 6-32 internal thread is 0.1380 inches. This is the nominal diameter of the thread, equal to the gauge size calculation: 0.060 + (6 × 0.013) = 0.138 inches. For a 6-32 2B internal thread, the major diameter tolerance ranges from 0.1380" (min) to 0.1420" (max). The basic value is the theoretical exact size from which tolerance limits are derived.
What are the standard internal thread classes per ASME B1.1?
ASME B1.1 defines five internal thread classes: 1B (loose fit, quick assembly), 2B (standard general purpose, most common), 3B (tight tolerance, precision applications), 4B (extra fine adjustment), and 5B (special applications). Class 1B is rarely used except where rapid, loose assembly is needed. Class 2B is the default for nearly all commercial fasteners. Class 3B is specified for aerospace, instrument, and high-vibration environments where precise fit is critical.
How does internal thread size affect tap selection?
The tap must match the thread's major diameter, threads per inch (TPI), and thread class. The tap's pitch diameter tolerance is designed to produce threads within the specified class range. Using a 2B tap for a 3B requirement will produce threads outside the 3B tolerance. Additionally, the minor diameter determines the tap drill size. For materials that expand or contract (like aluminum or stainless), slightly different drill sizes may be needed to account for material springback during and after tapping.
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