Inch to metric thread conversion

How to use the calculator

Select an inch thread from the list (UNC, UNF, UNEF, NPT, BSP) or enter the parameters manually: outer Ø in mm and TPI or pitch. The calculator will show a table of the nearest metric threads with deviations ΔØ and ΔP. The best analogue is highlighted. The “Coarse / Fine Only” filter narrows the search to the desired thread type.

ΔØ values ​​indicate how much the outer diameter of a metric thread differs from an inch thread. For most design purposes, an agreement of ±0.5 mm is considered "close" but not directly interchangeable.

Is it possible to replace an inch thread with a metric one?

It depends on the purpose of the connection. Geometric matching in Ø and pitch does not guarantee compatibility - due to different profile angles (UNC/UNF = 60°, like Metric, but a different fit standard), tolerances and strength classes.

What is acceptable: replacement by matching Ø for gap installation in soft materials, use of metric taps for cutting new threads to replace worn inch threads.

What is unacceptable: direct screwing of metric fasteners into inch threads without checking - even with “close” parameters, the connection will be cut off or leak.

Features of pipe threads (NPT, BSPP, BSPT)

For pipe threads, the calculator shows geometrically close metric analogs, but direct replacement is not possible in most cases: NPT has a profile angle of 60° and a conical shape, BSP has a 55° profile (cylindrical G or conical R). Metric pipe and hydraulic threads (M×P) have different sealing geometries. Use adapters of the appropriate standard.

Frequently Asked Questions

Are 1/4-20 UNC and M6×1.0 interchangeable? They are close in Ø (6.35 vs 6.00 mm), but the pitch is different (1.27 vs 1.00 mm). You can screw it in, but the thread will be cut off. Not interchangeable.

Why do you need a Coarse/Fine filter? If the part has a small thread (for example, a brake caliper), there is no point in looking for an analogue among Coarse - the pitch will not match. Fine gives a more precise selection for thin-walled joints.

Taps, dies and gauges - RUKO, VÖLKEL, DAPPRICH - in our catalog.