What is thread pitch versus TPI?

Pitch is the distance in millimetres between adjacent thread crests, used by the metric system. Threads per inch, or TPI, is the imperial measure counting crests in one inch. They are reciprocals scaled by 25.4, so TPI equals 25.4 divided by the pitch in millimetres.

What is the difference between coarse and fine threads?

Coarse threads such as UNC or metric coarse have larger pitch and fewer threads per length. Fine threads such as UNF or metric fine have smaller pitch, give finer adjustment and better vibration resistance, but strip more easily in soft materials like aluminium.

What does M8 x 1.25 mean?

M8 is an ISO metric thread with an 8 mm major diameter, and 1.25 is the pitch in millimetres. That is the standard coarse pitch for M8. A fine M8 thread would be written M8 x 1.0.

Are these the nominal or actual diameters?

The diameter shown is the nominal major diameter, which is the outside diameter of the male thread crest. The actual pitch diameter and minor diameter are smaller and depend on the thread form and tolerance class.

Which standards are these tables based on?

Metric pitches follow ISO 261 for general-purpose threads. Unified UNC and UNF threads-per-inch follow ASME B1.1, the standard for inch-series screw threads used across North America.

Screw & Bolt Thread Chart

Picking the right screw or bolt means matching three things: the major diameter, the thread pitch, and the thread system. This chart lists the standard ISO metric coarse and fine pitches alongside the Unified UNC and UNF threads-per-inch so you can identify a fastener or specify a tapped hole.

How it works

Threads are described by how far apart their crests sit. The metric system measures this directly as pitch in millimetres — an M6 coarse thread has a 1.0 mm pitch. The imperial system instead counts threads per inch (TPI) — a 1/4-20 UNC bolt has 20 crests in one inch. The two are linked because one inch is 25.4 mm, so:

TPI = 25.4 / pitch(mm)
pitch(mm) = 25.4 / TPI

The tool computes TPI for metric rows and pitch for imperial rows on the fly, so you can cross-reference either system. Each row also shows the nominal major diameter, the outer crest-to-crest size of the male thread.

Coarse vs fine, and a worked example

Coarse threads (UNC, metric coarse) are the default for general assembly: they tolerate damaged threads, thread faster, and hold better in soft metals and plastics. Fine threads (UNF, metric fine) have a smaller pitch, giving more turns per millimetre, finer torque control, and stronger resistance to loosening under vibration — but they strip more easily in aluminium.

Example: an M10 x 1.5 bolt has a 10 mm major diameter and 1.5 mm pitch. Its equivalent TPI is 25.4 / 1.5 ≈ 16.9. The fine version, M10 x 1.25, has 25.4 / 1.25 ≈ 20.3 TPI — noticeably more threads packed into the same length.

Notes

Always confirm the system before mixing fasteners: an M6 and a 1/4-inch bolt look nearly identical but will cross-thread and destroy each other. The diameters here are nominal; precise fits depend on the tolerance class (6g/6H for metric, classes 1A–3B for Unified).

Screw & Bolt Thread Chart

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How it works

Coarse vs fine, and a worked example

Notes