Metric to inch thread conversion

How to use the calculator

Enter the outside diameter and pitch of the metric thread. The calculator will instantly show a table of the nearest inch analogues (UNC, UNF, UNEF) with deviations ΔØ and ΔP. Quick selection buttons (M6×1.0, M8×1.25 and others) allow you to set the most common threads with one click. To search for pipe analogues (NPT, BSPP, BSPT), enable the appropriate filter.

When do you need to convert metric to inch

The most common situation is imported equipment with inch fasteners, where metric bolts and taps are not available in the required size. Also: restoration of threads in the body of an American or Japanese machine tool, selection of seals for hydraulics with pipe threads, identification of fasteners without markings.

The closest pair is M8×1.25 and 5/16-18 UNC: the difference in diameter is only 0.06 mm. In this case, the pitch is different (1.250 vs 1.411 mm), which excludes direct alignment.

Why is there no “exact” analogue?

The metric and inch systems are initially incompatible: their steps and diameters are calculated according to different standards. Even with close values ​​of Ø and P, the profile angle of both systems is the same (60°), but the fits, tolerances and strength markings are different. Therefore, “geometrically close” ≠ “interchangeable.”

Acceptable applications of the analogue: cutting new threads in soft materials (aluminum, plastic) with the nearest tap, installation without load, approximate selection of tools.

It is unacceptable to screw inch fasteners into a metric thread “because it fits the diameter” - the thread will be cut off or leak.

Frequently Asked Questions

Are M10 and 3/8" the same thing? No. M10 = 10.00 mm, 3/8" = 9.53 mm. The difference is 0.47 mm in Ø and the pitch is different - the parts are incompatible.

How to distinguish metric fasteners from inch fasteners without markings? Using a comb template: see at what step the profile fits perfectly. If the step is a multiple of 0.5 mm - metric. If close to fractional values ​​(for example 1.27, 1.41, 1.58 mm) - inch.

Taps, dies, gauges for metric and inch threads - RUKO, VÖLKEL, DAPPRICH - in our catalog.