What does a thread callout like M10 x 1.5 mean?

M10 means a metric thread with a 10 mm major (outer) diameter; 1.5 is the pitch, the distance in mm between adjacent threads. Both must match between a bolt and the nut or tapped hole it fits.

What is the difference between UNC and UNF threads?

UNC (Unified Coarse) has fewer, deeper threads per inch and is the common general-purpose choice; UNF (Unified Fine) packs more threads per inch for a shallower, higher-strength engagement, common in precision or high-vibration assemblies.

Does Gibraltar use metric or UTS/UNC threads?

Gibraltar follows UK/EU practice and uses ISO metric threads as the standard for fasteners; imported UTS/UNC hardware does turn up, so check the actual callout on a drawing or part before ordering.

Thread Calculator — Pitch, Minor Diameter & Tolerance | Construction Calculators

Thread Size Calculator: Metric and UTS Screw Thread Dimensions

A thread callout (like "M10 x 1.5" or "3/8"-16 UNC") packs a major diameter and a pitch into a few characters, and both numbers have to match on a bolt and its nut or tapped hole — even a slightly wrong pitch will bind or strip instead of threading smoothly. This calculator turns a callout, in metric ISO or inch UTS/UNC/UNF form, into its actual pitch diameter and minor diameter, plus the tolerance allowance for a standard external fit, so you can check a fastener against a drawing or a mating part before you buy or machine it.

How to use the thread calculator

Pick the standard: ISO metric or UTS/UNC/UNF, then choose a common size preset (M3-M24, or 1/4"-20 up to 3/4"-10 UNC) or enter your own major diameter and pitch.
Select an external tolerance position (e, f, g, or h) and grade — h/6g is the typical general-purpose fit for a standard bolt.
Read off the basic pitch diameter, minor diameter, and the maximum pitch/major diameter once the allowance is applied.

The thread geometry formula

From ISO 68-1, the fundamental triangle height is H = P x sqrt(3)/2, where P is the pitch. The basic pitch diameter is d2 = d - 2 x (3/8) x H and the basic minor diameter is d1 = d - 2 x (5/8) x H, where d is the major diameter.

Example, M10 x 1.5: H = 1.5 x 0.866 = 1.299 mm, so d2 = 10 - 0.75 x 1.299 = 9.026 mm and d1 = 10 - 1.25 x 1.299 = 8.376 mm. For a standard "g" allowance at this pitch, ISO 965-1's closed-form deviation is about -0.271 mm, putting the maximum pitch diameter near 8.755 mm.

Tips

Pitch matters more than diameter for compatibility: an M10 x 1.0 fine-pitch bolt will not thread into an M10 x 1.5 coarse-pitch nut even though the major diameter is identical.

This tool computes basic and allowance-adjusted sizes only — it does not include the grade-dependent tolerance zone width from ISO 965-1's tables, so treat the minimum-size figures as informational, not a go/no-go gauge replacement.

In Gibraltar, a standard metric bolt or screw of the size you look up typically retails for around £0.12-£2.40 at a hardware store, depending on diameter, length, and material — this is a per-fastener retail cost, not a machining or service fee.