#4-40 UNC thread pitch and dimensions

The #4-40 UNC thread has 40 threads per inch, a pitch of 0.0250 in (0.64 mm), on a 0.112 in major diameter (ASME B1.1). Its basic pitch diameter is 0.0958 in and its basic minor diameter is 0.0849 in.

Threads per inch (UNC)

40 TPI

pitch 0.0250 in · 0.64 mm

Major diameter (D)

0.112 in (2.84 mm)

Pitch (1 / TPI)

0.0250 in (0.64 mm)

Basic pitch diameter (E)

0.0958 in (2.43 mm)

Basic minor diameter (internal)

0.0849 in (2.16 mm)

Tensile stress area (As)

0.0060 in² (3.9 mm²)

Tap drill (75% chart)

#43 (0.0890 in)

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#4-40 UNC basic dimensions (ASME B1.1)
Dimension	Formula	inch	mm
Major diameter D	nominal	0.1120	2.84
Pitch P	1 / TPI	0.0250	0.64
Pitch diameter E	D − 0.649519 / n	0.0958	2.43
Minor diameter (internal)	D − 1.082532 / n	0.0849	2.16
Thread height (basic)	0.541266 / n	0.0135	0.34
Tensile stress area As	0.7854 (D − 0.9743 / n)²	0.0060 in²	3.9 mm²

#4 threads: UNC vs UNF
Thread	TPI	Pitch Ø E	Stress area As	Tap drill
#4-40 UNC (this page)	40	0.0958 in	0.0060 in²	#43
#4-48 UNF	48	0.0985 in	0.0066 in²	#42

Neighbouring UNC threads
Thread	TPI	Pitch (mm)	Minor Ø	Stress area As
#2-56 UNC	56	0.45	0.0667 in	0.0037 in²
#4-40 UNC (this page)	40	0.64	0.0849 in	0.0060 in²
#6-32 UNC	32	0.79	0.1042 in	0.0091 in²
#8-32 UNC	32	0.79	0.1302 in	0.0140 in²
#10-24 UNC	24	1.06	0.1449 in	0.0175 in²

Practical notes

Unified threads are named diameter–TPI–series: #4-40 UNC is a #4 nominal diameter (number 4 screw, 0.112 in) with 40 threads per inch in the coarse series.

These are basic (zero-tolerance) dimensions. Most screws are made to class 2A and nuts to 2B (ASME B1.1), so measured sizes sit slightly below the basic values on the screw.

40 TPI is a pitch of 0.64 mm, which does not match a metric ISO pitch exactly; metric and Unified fasteners of similar size are not interchangeable.

Tensile stress area As is the area used for bolt strength (proof load = As × proof stress for the SAE J429 or ASTM grade). For #4-40 UNC it is 0.0060 in².

FAQ

How many threads per inch is #4-40 UNC?

40 TPI, a pitch of 0.0250 in (0.64 mm), in the Unified coarse series (ASME B1.1).

What is the minor diameter of #4-40 UNC?

The basic minor diameter of the internal thread is D − 1.082532/n = 0.0849 in (2.16 mm).

What is the tensile stress area of #4-40 UNC?

As = 0.7854 × (D − 0.9743/n)² = 0.0060 in² (3.9 mm²).

What tap drill does #4-40 UNC need?

The standard chart drill is #43, 0.0890 in (2.26 mm), about 71% thread.

What is the difference between #4-40 UNC and #4-48 UNF?

Same #4 diameter, different pitch: 40 TPI versus 48 TPI. They are not interchangeable.

Sources: ASME B1.1 — Unified inch screw threads (UN and UNR thread form): basic dimensions, tensile stress area · ASME B18.2.1 — square, hex, heavy hex and askew head bolts and hex, heavy hex, hex flange screws (width across flats)

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