Thread Pitch Chart: UNC, UNF and Metric Threads

Threads per inch for every common US bolt in coarse, fine and extra fine, metric coarse and fine pitches, how to convert between TPI and mm, the US and metric threads that are easy to confuse, and how to measure a thread.

Updated October 13, 2026

Two bolts side by side, a 1/4-20 with coarse threads and a 1/4-28 with fine threads, with the distance between threads marked on each: 1.27 mm and 0.91 mm

Thread size has two parts: the diameter and how close together the threads are. US threads count threads per inch (TPI), so a bigger number means finer threads. Metric threads give the pitch, the distance from one thread to the next in millimeters, so a smaller number means finer threads. A 1/4-20 bolt has 20 threads per inch; an M6 × 1.0 bolt has threads 1 mm apart.

US thread chart: UNC, UNF and UNEF

Size	Diameter	UNC (coarse)	UNF (fine)	UNEF (extra fine)
#4	0.112 in	40	48	—
#6	0.138 in	32	40	—
#8	0.164 in	32	36	—
#10	0.190 in	24	32	—
1/4	0.250 in	20	28	32
5/16	0.3125 in	18	24	32
3/8	0.375 in	16	24	32
7/16	0.4375 in	14	20	28
1/2	0.500 in	13	20	28
9/16	0.5625 in	12	18	24
5/8	0.625 in	11	18	24
3/4	0.750 in	10	16	20
7/8	0.875 in	9	14	20
1	1.000 in	8	12	20

UNC is what hardware stores sell by default. UNF is common on cars, aircraft and anywhere vibration could loosen a bolt. UNEF is for thin-walled parts and fine adjustment, like bike and instrument fittings.

Metric thread chart: coarse and fine pitch

Size	Coarse pitch	Fine pitch	Extra fine
M3	0.5 mm	0.35 mm	—
M4	0.7 mm	0.5 mm	—
M5	0.8 mm	0.5 mm	—
M6	1.0 mm	0.75 mm	—
M8	1.25 mm	1.0 mm	0.75 mm
M10	1.5 mm	1.25 mm	1.0 mm
M12	1.75 mm	1.5 mm	1.25 mm
M14	2.0 mm	1.5 mm	—
M16	2.0 mm	1.5 mm	—
M18	2.5 mm	2.0 mm	1.5 mm
M20	2.5 mm	2.0 mm	1.5 mm
M24	3.0 mm	2.0 mm	—

When a metric size has no pitch, as in M8 × 30, it’s coarse. Car wheel studs and brake parts are the usual exceptions: M12 × 1.5 and M12 × 1.25 are both common lug nut threads.

TPI to pitch conversion

Pitch (mm) = 25.4 ÷ TPI, and TPI = 25.4 ÷ pitch.

TPI	Pitch	Pitch	TPI
8	3.18 mm	0.5 mm	50.8
10	2.54 mm	0.7 mm	36.3
11	2.31 mm	0.8 mm	31.8
12	2.12 mm	1.0 mm	25.4
13	1.95 mm	1.25 mm	20.3
14	1.81 mm	1.5 mm	16.9
16	1.59 mm	1.75 mm	14.5
18	1.41 mm	2.0 mm	12.7
20	1.27 mm	2.5 mm	10.2
24	1.06 mm	3.0 mm	8.5
28	0.91 mm
32	0.79 mm
40	0.64 mm

Threads that look alike but don’t mix

These pairs are close in diameter and pitch, so a nut can start on the wrong bolt before it binds. Forcing it strips the threads.

US thread	Metric thread	Diameter	Thread spacing
#10-32	M5 × 0.8	4.83 vs 5.00 mm	0.79 vs 0.80 mm
1/4-20	M6 × 1.0	6.35 vs 6.00 mm	1.27 vs 1.00 mm
5/16-18	M8 × 1.25	7.94 vs 8.00 mm	1.41 vs 1.25 mm
3/8-16	M10 × 1.5	9.53 vs 10.00 mm	1.59 vs 1.50 mm
1/2-13	M12 × 1.75	12.70 vs 12.00 mm	1.95 vs 1.75 mm

#10-32 and M5 are the trickiest: the thread spacing is almost identical, so the only tell is the 0.17 mm difference in diameter. If a nut gets hard to turn after a couple of turns, stop and check.

How to measure thread pitch

With a thread gauge: press the leaves against the threads until one sits perfectly with no light showing. Thread gauges come in TPI and metric sets.
With a ruler, metric: measure across 10 thread peaks and divide by 10. Over 10 threads, the difference between 1.25 and 1.5 mm is a clear 2.5 mm.
With a ruler, US: count the peaks in exactly 1 inch, or count over 1/2 inch and double it.
Then measure the diameter across the outside of the threads to tell, for example, a 5/16 in bolt (7.9 mm) from an M8 (8.0 mm). A calibrated mm ruler on your screen works for both steps. With both numbers, the thread identifier can identify a bolt from its diameter and thread count.

Pipe threads are a separate system with tapered threads: see the pipe size chart. For the drill size to use before tapping a thread, see the tap drill chart; for bolt diameters and wrench sizes, the screw and bolt size chart and the wrench size chart.

More DIY charts: tap drill sizes · bolt torque · washer sizes · drill bit sizes.

Frequently asked questions

How do I convert threads per inch to pitch in mm?

Divide 25.4 by the threads per inch. 20 TPI is 25.4 ÷ 20 = 1.27 mm pitch. To go the other way, divide 25.4 by the pitch: a 1.25 mm pitch is about 20.3 threads per inch.

What is the difference between UNC and UNF?

Same diameters, different thread counts. UNC (Unified National Coarse) is the standard thread, quicker to assemble and less likely to strip in soft materials. UNF (Fine) has more threads per inch, holds better under vibration and allows finer adjustment. A 1/4 in bolt is 20 TPI in UNC and 28 TPI in UNF.

Will an M8 nut fit a 5/16 bolt?

No. The diameters are nearly the same (8 mm vs 7.94 mm), but an M8 has a 1.25 mm pitch and a 5/16-18 bolt has 1.41 mm between threads. The nut may start a turn or two, then jam, and forcing it ruins both threads.

What is the standard pitch of an M10 bolt?

1.5 mm. Metric bolts written without a pitch, like M10 × 40, have the coarse thread. M10 fine threads are 1.25 mm (common on cars) and 1.0 mm.

How do I measure thread pitch without a gauge?

For metric threads, measure the length of 10 threads (peak to peak) with a ruler and divide by 10: 12.5 mm over 10 threads is a 1.25 mm pitch. For US threads, count the peaks along exactly 1 inch: that's the threads per inch.