Inch threads: UNC, UNF and Whitworth

November 3, 2026  ·  Indumor

Why will a 1/2 Whitworth bolt not go into a 1/2 UNC nut, when a 3/8 Whitworth runs into a 3/8 UNC nut and a 5/8 does too? Because half an inch is the one common size where the two families picked a different count: 12 threads per inch in the British system, 13 in the American. Everywhere else they agree: 20 at 1/4, 18 at 5/16, 16 at 3/8, 11 at 5/8, 8 at one inch. They go together, but it is not a proper fit. One has 55° flanks and rounded crests; the other 60° flanks and flat crests, and contact ends up on the tips. Fine for hanging a sign; not for a bolt under load.

In a shop outside the US and the UK, inch threads arrive by three routes: American machinery and its spares (UNC and UNF), British machines and lathes built before 1970 (Whitworth: BSW and BSF), and the hardware-store "inch" bolt, which is Whitworth or UNC depending on where it came from and cannot be told apart in the common sizes except at 1/2. Pipe threads, BSP and NPT, have their own chapter 6; metric threads, chapter 4.

Threads per inch and pitch

Inch threads are not named by pitch but by threads per inch (n, or tpi): 1/4-20 is a quarter-inch bolt with 20 threads in every inch of length. The pitch is a division: 25.4 / 20 = 1.27 mm. The full callout adds the series and the class: 1/4-20 UNC-2A is a bolt (A) of commercial tolerance (2); the nut is 2B, and 3A and 3B are the precision classes. To measure, use a thread pitch gauge: the leaf that seats without light over four or five threads gives the count. Without one, count crests against one inch of a rule.

Small sizes are not given as fractions but by number, #0 to #12, each number 0.013 in bigger than the last on a base of 0.060 in: a #10 is 0.190 in, 4.83 mm, just over 3/16. These are the threads of panel screws, electronics and small American machinery; #6-32 and #8-32 are the ones you meet most.

Dash: no standard thread. Numbers 7, 9 and 11 do not exist in ASME B1.1, and #0 comes in fine pitch only.

UNC and UNF: the Unified thread

The Unified thread is the 1948-49 agreement between the United States, Britain and Canada, after a war in which one side's spares would not fit the other side's machines. It took the 60° Sellers profile of 1864 and the diameter and pitch series of the American National thread, and today it is ASME B1.1. UNC is the coarse series, for general use, castings and soft materials; UNF the fine, for automotive, aircraft and thin walls; UNEF the extra fine; and the constant-pitch series (8-UN, 12-UN, 16-UN) take over at large diameters, so flange bolts above one inch are 8-UN. A combination outside every series is called UNS, like 1-14. The R in UNR means a rounded root on the bolt, required in quality fasteners for fatigue, just as in metric.

The tap drill is calculated from the percentage of thread you want to leave, and 75 % is the figure behind every chart: for 3/8-16 it gives 0.375 − 0.974 / 16 = 0.314 in, and the nearest drill is 5/16 (0.3125 in), which leaves 77 %. In the small sizes the drills are the number and letter series: #7 for 1/4-20, F for 5/16-18, Q for 3/8-24, all listed in chapter 10. In thou, the 75 % rule says the drill sits 974/n thousandths under the major diameter: 61 thou for 16 tpi, 49 thou for 20 tpi.

Find a UNC thread

Find a UNF thread

Thread %: what the listed drill leaves, measured the way American charts do (against 3/4·H). D₁ = D − 1.0825·P is the basic minor diameter of the nut.

Whitworth: BSW and BSF

Joseph Whitworth proposed his thread in 1841 and it became the first standardized thread anywhere: 55° between flanks, crests and roots rounded to a radius of 0.137·P, which leaves a thread depth of 0.640·P. BSW is the coarse series and BSF the fine one, from 1908. Britain dropped them for Unified and then for metric, but BS 84 is still in force for spares, and they live on in British lathes and machines, in old farm equipment and, by inheritance, in BSP pipe threads, which share the 55° form. The tap drill follows the same rule as metric, D − P in millimeters, which leaves about 78 % thread; the chart values are the ones tap makers publish.

D₁ = D − 1.2806·P (thread depth h = 0.6403·P per side). Thread %: what the drill leaves against that depth. Drills as published by tap makers, rounded to stock sizes.

The ones that get mixed up

Four inch-metric pairs cross paths in the bolt bin, plus the 1/2 UNC and 1/2 BSW pair from the top of the chapter. The rule for sorting them: the caliper gives the diameter and the pitch gauge gives the pitch. When a nut goes on a turn or two and binds, the pitch is different; when it runs loose to the bottom, the diameter is.

Calculators

Frequently asked questions

How many threads per inch is a 1/2 inch bolt?

13 in UNC, 20 in UNF and 12 in Whitworth (BSW). It is the one common size where UNC and Whitworth do not agree.

What drill size for a 1/4-20 tap?

A #7 drill, 0.201 in (5.1 mm), which leaves 75 % thread. A 13/64 (0.203 in) also works and leaves 72 %; a 5 mm metric drill leaves 82 % and makes the tap work hard.

Are Whitworth and UNC interchangeable?

Not properly. Whitworth has 55° flanks and rounded crests; UNC has 60° flanks and flat crests. They share the threads per inch in nearly every size except 1/2 (12 against 13), and they screw together with contact only on the tips.

Sources: ASME B1.1-2019 (Unified inch screw threads: UNC, UNF, UNEF, UN and UNR); ASME B94.11M (number, letter and fractional twist drills); BS 84:2007 (Whitworth form threads, BSW and BSF); ISO 68-2:1998 (basic profile of inch screw threads); ISO 5864 (ISO inch screw threads); Machinery's Handbook (tap drill formula by percentage of thread).