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, cross-reference between systems, or specify the correct tapped hole.
How thread pitch and TPI work
Threads are described by the spacing between their crests. The metric system measures this spacing directly as pitch in millimetres — an M8 coarse thread has a 1.25 mm pitch, meaning each crest is 1.25 mm from the next. The Unified imperial system counts threads per inch (TPI) instead — a 5/16-18 UNC bolt has 18 thread crests in one inch of length. The two describe the same geometry from different angles and are directly convertible because one inch is 25.4 mm:
TPI = 25.4 / pitch (mm)
pitch (mm) = 25.4 / TPIThe chart computes TPI for every metric row and pitch for every imperial row on the fly, so you can cross-reference either system without a separate calculator.
Common standard sizes at a glance
A few reference points that are useful to know without looking them up:
| Fastener | Diameter | Coarse pitch / TPI |
|---|---|---|
| M3 | 3 mm | 0.5 mm (≈ 50.8 TPI) |
| M6 | 6 mm | 1.0 mm (≈ 25.4 TPI) |
| M10 | 10 mm | 1.5 mm (≈ 16.9 TPI) |
| M12 | 12 mm | 1.75 mm (≈ 14.5 TPI) |
| 1/4-20 UNC | 6.35 mm | 20 TPI (1.27 mm) |
| 3/8-16 UNC | 9.53 mm | 16 TPI (1.59 mm) |
| 1/2-13 UNC | 12.7 mm | 13 TPI (1.95 mm) |
Coarse vs fine — when each is right
Coarse threads (UNC, metric coarse) are the default for general-purpose assembly. They thread faster, tolerate slightly damaged threads, and hold reliably in soft metals, plastics, and through-holes where the full engagement length may be short. For most bolted joints, coarse is the right choice unless there is a specific reason to use fine.
Fine threads (UNF, metric fine) pack more turns per unit length. That gives finer adjustment increments for precision assemblies (adjusting wheel bearings, for example), better resistance to loosening under vibration (which is why aerospace and automotive fasteners often specify fine), and slightly higher tensile strength at the same nominal diameter. The trade-off is that fine threads strip more easily in soft materials and are more sensitive to cross-threading.
Worked example: an M10 × 1.5 coarse bolt has 1.5 mm between crests, or about 16.9 TPI. The fine equivalent, M10 × 1.25, has 20.3 TPI — 20% more thread engagement per millimetre of bolt length. For a precision instrument adjustment, the fine version gives finer positional control. For a structural steel joint, the coarse version is easier and faster to assemble correctly.
Avoiding the most common mistake
An M6 bolt and a 1/4-inch (6.35 mm) bolt are visually almost indistinguishable — both about 6 mm in diameter — but they are incompatible thread systems and attempting to mate them will cross-thread and damage both parts. Always confirm both the thread standard (metric or Unified) and the specific pitch or TPI before assembly. Mixing metric and imperial fasteners in the same application is one of the most common causes of stripped threads on bicycles, European machinery, and imported equipment.
The diameters in this chart are nominal major diameters — the crest-to-crest diameter of the external thread. The actual minor diameter (at the root) and the pitch diameter (midway, where most load is carried) are smaller and depend on the tolerance class: 6g/6H is standard for metric general purpose, while Unified uses 2A/2B for most commercial applications and 3A/3B for close-fit precision work.