What does M8×1.25 mean?

8 mm nominal diameter with a 1.25 mm pitch (1.25 mm between threads). 1.25 is the coarse pitch for M8, so it's often written just "M8". "M8×1.0" would be the fine-pitch version.

How is TPI different from pitch?

They measure the same thing inversely. TPI is threads per inch (inch system); pitch is millimetres per thread (metric). Pitch (mm) ≈ 25.4 ÷ TPI.

My bolt is close to both a metric and an inch size — how do I tell them apart?

Use the pitch gauge: only the matching system's leaf will seat with no gaps. Inch and metric pitches almost never coincide, so the gauge disambiguates sizes that look similar on calipers.

Reference & Charts

Thread Pitch Chart

Standard thread pitches for metric (ISO) coarse and fine threads in millimetres, and Unified inch (UNC/UNF) threads in threads-per-inch — with how to use a thread pitch gauge to identify an unknown thread.

Last verified: 2026-07-09

Pitch is the distance between adjacent threads: millimetres per thread for metric (ISO), or threads per inch (TPI) for Unified inch. To identify an unknown thread, measure the diameter, then match the pitch/TPI with a thread pitch gauge — press the leaf that seats cleanly into the threads with no gaps or rocking.

Metric (ISO) thread pitch — mm

Metric coarse & fine pitch (ISO 261 / ISO 262)
Size	Coarse pitch (mm)	Fine pitch (mm)
M3	0.5	0.35
M4	0.7	0.5
M5	0.8	0.5
M6	1	0.75
M8	1.25	1
M10	1.5	1.25
M12	1.75	1.25
M14	2	1.5
M16	2	1.5
M20	2.5	1.5
M24	3	2

Coarse is the default (e.g. "M8" = M8×1.25). Fine pitches (e.g. M8×1.0) are common in automotive/precision work; some sizes have more than one fine pitch — this shows the most common.

Unified inch thread — TPI

Unified Coarse (UNC) & Fine (UNF) — threads per inch
Size	UNC (TPI)	UNF (TPI)
1/4"	20	28
5/16"	18	24
3/8"	16	24
7/16"	14	20
1/2"	13	20
9/16"	12	18
5/8"	11	18
3/4"	10	16

Using a thread pitch gauge

Pick metric or inch: try the metric (mm) gauge for M-threads, the inch (TPI) gauge for UN threads.
Press each leaf against the threads and hold it to the light — the correct leaf seats fully with no light gaps between teeth and doesn't rock.
Read the stamped value on that leaf (e.g. 1.25 mm, or 20 TPI).
Confirm the diameter separately (calipers on the crest for an external thread), then combine: e.g. 8 mm × 1.25 = M8×1.25, or 1/2" × 13 TPI = 1/2"-13 UNC.

Frequently asked

What does M8×1.25 mean?: 8 mm nominal diameter with a 1.25 mm pitch (1.25 mm between threads). 1.25 is the coarse pitch for M8, so it's often written just "M8". "M8×1.0" would be the fine-pitch version.
How is TPI different from pitch?: They measure the same thing inversely. TPI is threads per inch (inch system); pitch is millimetres per thread (metric). Pitch (mm) ≈ 25.4 ÷ TPI.
My bolt is close to both a metric and an inch size — how do I tell them apart?: Use the pitch gauge: only the matching system's leaf will seat with no gaps. Inch and metric pitches almost never coincide, so the gauge disambiguates sizes that look similar on calipers.

Sources

ISO 261 / ISO 262 — ISO general-purpose metric screw threads
ASME B1.1 — Unified Inch Screw Threads