Metric Pipe Threads Explained: M-Threads, ISO and DIN vs NPT and BSP

6 min read · updated Oct 2026

The short answer: Household plumbing in Europe does not use metric threads — it uses British BSP threads, which the world adopted as the international (ISO) standard. True metric threads, written like "M18×1.5", are used on cars, machines and hydraulic equipment, and they seal with an O-ring or washer rather than on the threads.

The European paradox

Europe measures almost everything in millimetres, so it's surprising that a German or French water pipe fitting is still sized in inches. The reason is history: by the time metric plumbing could have been invented, British pipe threads were already everywhere. Instead of replacing them, the International Organization for Standardization adopted them:

ISO 228 = straight British thread, marked G (for example "G 1/2").
ISO 7 = tapered British thread, marked R, Rp or Rc.

So if a European tap says "G 3/4", that's a BSP thread wearing an ISO label.

Where true metric threads are used

A metric thread is named by its outside diameter and the distance between threads, both in millimetres. "M18×1.5" means 18 mm across, with 1.5 mm from one thread to the next. You'll find them in:

Hydraulics and machinery (ISO 6149, DIN 3852): tractors, excavators, presses, pumps.
Cars: brake lines, oil drain plugs, sensors, fuel fittings.
Compression fittings (DIN 2353): metal tube fittings with a 24° cone and cutting ring.
Tapered metric (DIN 158, marked "keg"): grease nipples and small plugs on German equipment.

How metric fittings seal

Most metric fluid threads are straight, so the threads themselves don't make the seal. Instead, one of these does:

An O-ring sitting in a small groove or chamfer at the base of the thread (ISO 6149).
A copper or bonded washer squeezed between two flat faces (DIN 3852).
A metal cone pressed into a matching seat (24° compression fittings).

That's why PTFE tape is usually the wrong choice on a metric fitting — it can stop the washer seating properly.

Metric vs NPT vs BSP at a glance

	Metric (M)	NPT	BSP (G / R)
Named in	Millimetres	Inches	Inches
Thread angle	60°	60°	55°
Shape	Usually straight	Tapered	Straight (G) or tapered (R)
How it seals	O-ring, washer or cone	Threads + tape	Washer (G) or threads + tape (R)
Typical use	Hydraulics, cars, machines	North American plumbing	Plumbing almost everywhere else

How to identify a metric thread

Measure the outside diameter with calipers. A whole number in millimetres (14, 16, 18, 22 mm) suggests metric.
Check the pitch. A metric pitch gauge (1.0, 1.5, 2.0 mm) will sit perfectly in the threads; an inch gauge won't.
Look for an O-ring groove or a washer face at the base.
Consider the source. European cars and hydraulic machines are very likely metric.

Easy mix-ups to watch for

Some sizes are dangerously close. M20×1.5 is near 1/2" BSP, and M26×1.5 is near 3/4" BSP. They may start threading but will leak or strip. Always measure both the diameter and the pitch before connecting anything under pressure.

Connecting metric to NPT or BSP

Use a proper adapter with the correct thread on each end, and seal each side the way that thread is designed to seal — washer or O-ring on the metric side, tape on tapered pipe threads. For low-pressure water jobs, a printed adapter can bridge the gap when a store-bought one isn't available.

Design your own adapter

Choose a thread for each end, set the lengths, and download a ready-to-print model with a full water passage.

Open the designer →