UNC & UNF Thread Size Chart

UNC thread size chart

UNC: 28 rows, nominal diameters 0.073 in to 3.75 in, thread counts 4 to 64 per inch.

UNF thread size chart

UNF: 24 rows, nominal diameters 0.06 in to 1.5 in, thread counts 12 to 80 per inch.

UNEF thread size chart

UNEF: 26 rows, nominal diameters 0.216 in to 1.6875 in, thread counts 18 to 32 per inch.

UN thread size chart (constant pitch)

UN: 155 rows, nominal diameters 0.3125 in to 3 in, thread counts 4 to 32 per inch.

UN holds the thread count constant across a range of diameters, so the same thread count appears many times with a different nominal diameter.

UNS thread size chart (specials)

UNS: 8 rows, nominal diameters 0.25 in to 2.4375 in, thread counts 12 to 36 per inch.

UNC vs UNF on the same diameter

The same nominal diameter exists in both series at a different thread count — which is why the thread count, not the diameter, has to be quoted. Pitch difference is the fine pitch minus the coarse pitch, in millimetre.

What UNC, UNF, UNEF, UN and UNS mean

The series letter tells you how the thread count was chosen for that diameter.

How to choose between them

The standard's own guidance, at clause level: for new design use the standard series, and treat coarse and fine as the preferred ones — depart from them only with approval. The practical split is that coarse threads are easier to tap and less likely to cross-thread, while fine and extra fine threads carry more shear area on a given wall thickness and hold better against vibration. Where the choice is genuinely open, coarse is the cheaper and more widely available option.

Metric vs unified: the closest metric size

For each UNC size below, the metric coarse size with the nearest nominal diameter, with both differences stated. This is a conversion reference for reading drawings and catalogues that switch systems.

It is not an interchangeability statement. ISO metric and unified inch threads are separate systems: a nominal diameter that is close does not make the parts assemble.

Thread classes: 1B and 2B, or 3B

Internal unified threads come in classes 1B, 2B and 3B, and external threads in the matching 1A, 2A and 3A. The standard's guidance is to start from the general-purpose pair — a 2A external thread with a 2B internal thread — and to justify the tighter 3A / 3B classes before specifying them, because they cost more to produce. Where a looser fit than 1A / 1B is wanted, the standard points to a non-preferred class rather than to a coarse thread. The class also changes the hole: our source prints the tap drill size and percent of thread in two separate tables for classes 1B/2B and for class 3B, which is why a class 3B hole is a different instruction from a class 2B hole on the same thread.

The class changes the limits of size, not the basic dimensions in this table: on a given class the hole and the part are made to different limits while the nominal diameter, thread count and pitch stay as tabulated. Since the limits differ, a class 3B tapped hole needs more care than a class 2B one, which is what the tap drill chart quantifies.

How pitch, pitch diameter and minor diameter are derived

Only two values per row come from the source: the nominal major diameter and the threads per inch. Everything else on this page is computed from them with the definitions below.

Which values are read and which are calculated

• Pitch P = 1 / TPI
• Basic pitch diameter d2 = d − 0.649519 × P
• Basic minor diameter d1 = d − 1.082532 × P

The two coefficients come from the 60° basic thread form: 0.649519 = 0.75H and 1.082532 = 2 × 0.625H, where H is the height of the basic triangle, H = (√3 / 2) × P.

Read from the source: nominal major diameter and threads per inch, row by row. Calculated here: pitch in inch and millimetre, basic pitch diameter, basic minor diameter and the closest metric size. Calculated values are marked in the tables and are reproducible from the formulas above.

The values in this table are quoted as nominal (basic) sizes for engineering and purchasing reference. Limits of size, tolerances and the class of fit to use are set by the standard and by the drawing you are working to, not by this page.

Thread data sources and row-level provenance

Every row is a diameter and thread count pair taken from an appendix table of FED-STD-H28/2B (Unified Inch Screw Threads UN and UNR), a US Federal Standard. The source prints that pair twice: in table II.A.1 for internal thread classes 1B and 2B, and in table II.A.2 for class 3B, because the drilled hole and the percent of thread differ by class while the thread size, thread count and series columns do not. This page takes the thread size, thread count and series from those tables and computes the pitch and the diameters from the thread geometry. Each row also carries the line number of our collection file, so a single value can be traced back to the row it was read from; the hole sizes and percent of thread are a separate family and are linked per size.

What this table covers

All 241 diameter and thread count combinations listed in the source appendix: 236 read directly, 5 accepted after the review described below. Nominal major diameters run from 0.06 in to 3.75 in and thread counts from 4 to 80 threads per inch.

Rows accepted after review

These rows could not be read cleanly from the source scan on the first pass and were re-read or recovered before being accepted.

• #4-48 UNF
• 3/8-36 UNS
• 9/16-12 UNC
• 9/16-24 UNEF
• 5/8-28 UN

5 rows out of 241 could not be read cleanly from the source scan or did not agree with its own arithmetic on the first pass. Each was either re-read against the surrounding table or recovered from an independent source and then accepted; no row was dropped.

What it does not cover

It is not a list of every unified combination that exists: only the combinations listed in the FED-STD-H28/2B appendix are included. Tolerances, limits of size and gauging are also out of scope — this table gives nominal sizes only. Tap drill sizes and percent of thread are a separate family and are linked per size.

Values on this page are compiled from the source below and computed here; the source's tables are not reproduced.

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