Measure the existing pipe before buying the transition – why 21.3 and 22 mm are not the same

Rule of thumb

Almost every project starts at a pipe that is already there. Which transition fits is decided not by the nominal size and not by eye, but by the measured outside diameter together with the pipe type – read the stamp or print, measure with a calliper or a string, then order.

Where does the rule come from?

From sales practice: most queries and exchanges of transitions arise because people ordered by feel – a “half-inch pipe” can be copper 15 mm, copper 18 mm or threaded steel tube 21.3 mm. Manufacturers therefore list their transitions by outside diameter (copper 15 mm) or by thread (½, ¾, 1 inch).

What is technically correct?

Copper tube is designated by outside diameter (15, 18, 22 mm), threaded steel tube by thread size – ½ inch has 21.3 mm, ¾ inch 26.9 mm, 1 inch 33.7 mm outside diameter. Multilayer pipes carry their size in the print (“16x2”). Measure with a calliper directly on the pipe or via the circumference: circumference divided by 3.14 – a circumference of 67 mm gives 67 ÷ 3.14 = 21.3 mm, i.e. threaded steel tube ½ inch. The main risk of confusion is between steel 21.3 mm and copper 22 mm – stamp, colour and a magnet help. Only with pipe type and size is it clear whether the transition is direct (iFIT onto copper 15 mm via the press transition module) or via a thread.

Where does the rule end?

• If there is already a thread on the existing installation (valve, fitting), the thread size decides – then the thread is determined, not the pipe.
• Other manufacturers’ multilayer and plastic pipes are never pushed directly into the iFIT adapter, even if the size matches: the route is via the press transition of the other system with male thread.
• On heavily corroded old pipe, measure at a bright spot or cut back to sound pipe.
• The rule says nothing about whether the existing installation is still sound – that is a separate question.

What does the standard say?

The dimensions are in the pipe standards: EN 1057 for copper tube, EN 10255 for threaded steel tube, EN 10226-1 for the pipe thread. There is no standard requiring you to measure before buying a transition – the rule is an empirical value.

DIN/EN versus ASME/ASTM

In North America the dimensions differ: copper tube to ASTM B88 is named by nominal size, ½ inch has 15.88 mm outside diameter; steel pipe to ASME B36.10M and thread to NPT. Anyone working on an installation with US dimensions must measure all the more – the metric transitions do not fit.

Worked example

A customer wants to move a washing machine and orders “a transition to ½ inch copper”. He has not measured. On site the calliper shows 21.3 mm, the pipe is grey and magnetic: galvanized threaded steel tube ½ inch. The right route is an iFIT transition module with female thread ½ onto a thread on the existing pipe – not the copper transition. One minute of measuring would have saved the second trip.

Zickwolff reference

The existing-pipe finder on the page on transitions to other systems takes the measured outside diameter and names the matching transitions with example article and availability. If you are unsure, tell us the stamp or print and the measured size – we find the transition and supply it from stock.

Checked today

Verdict: applies (class A). The rule follows the dimensions of the pipe standards and the designation of the transitions in the delivery programme; it is an ordering rule, not an interpretation of a standard.

Further reading

• How to measure pipe flanges, pipes and fittings correctly – bolt circle, diameter, wall thickness – How to measure
• Copper tubes to EN 1057: tempers, dimensions and applications – Copper tubes
• Threaded tubes EN 10255: dimensions – table, dimensional drawing and tolerances – Threaded tubes to EN 10255