Rule of thumb · applies
Measure the outside diameter, do not guess the nominal size – why a compression fitting only knows the pipe it clamps
Measure the outside diameter – do not guess the nominal size.
A rule of thumb replaces neither the design calculation nor the applicable standard.
- Category
- robust rule
- Standards referenced
- EN 10255, EN 10220 (series 1 to 3), EN 1057, DIN 16836 / EN ISO 21003 – manufacturer catalogue dimensions (Gebo, GF)
Rule of thumb
A compression fitting, a repair clamp, a repair band clamp on the outside diameter of the pipe – and “DN 25” or “1 inch” names five different outside diameters, depending on whether you have threaded pipe, boiler tube, copper tube, PE pipe or multilayer pipe in front of you.
Where does the rule come from?
The rule comes from the manufacturers’ installation instructions: every series is built for one outside diameter with a narrow tolerance band. For the same 1 inch thread size Gebo therefore offers three fittings – 33.7 mm for threaded pipe to EN 10255, 31.8 mm for boiler tube to EN 10220 series 2/3 and 32 mm for PE pipe. GF states the tolerance band for every PRIMOFIT dimension (steel pipe 1 inch: 33.3 to 34.2 mm). Whoever orders by nominal size will quite likely receive the wrong part.
What is technically correct?
The nominal size DN is an index number, not a dimension (rule of thumb “DN is not a diameter”). What the fitting needs is the measured pipe: outside diameter with a calliper at two points 90° apart (ovality), for multilayer pipe also the inside diameter (Multigebo puts a gauge in the kit because the same outside diameter comes with different wall thicknesses). Then the pipe type: black or galvanised, copper, plastic, multilayer – it decides series and seal. Only then follows the order designation.
Where does the rule end?
- Stainless repair bands (gebounifix) cover ranges (21–25, 26–30 mm …) and tolerate slight deviations – measuring to the millimetre is enough there.
- Wide-range couplings are built for several materials and diameters in one size – the rule applies, but with a wide band.
- On heavily corroded or out-of-round pipe the measurement does not help either: then the question is whether the fitting will hold at all (rule of thumb FR-18).
What does the standard say?
EN 10255 (threaded pipe) and EN 10220 (series 1 to 3) fix the outside diameters, EN 1057 the copper tubes, DIN 16836 and EN ISO 21003 the multilayer pipes with outside and inside diameter. The manufacturers refer their series to exactly these series – the catalogue names the standard next to the dimension.
DIN/EN versus ASME/ASTM
In the ASME world the pipe is called NPS, and the outside diameter differs from the EN series at NPS 3/4, 1, 1 1/4, 2 1/2, 5 and 12 (33.4 against 33.7 mm at 1 inch). A compression fitting for EN threaded pipe therefore fits there only if the tolerance band includes the dimension – measure, do not translate.
Worked example
A maintenance fitter reports “1 inch line, leaking, heating”. Measured: 31.8 mm, black – a boiler tube. The right repair clamp is the one for boiler tube, not the threaded-pipe clamp for 33.7 mm; that one would have 1.9 mm of play and would not seal.
Zickwolff reference
From daily business: the most frequent query on compression fittings is not the type but the dimension – which is why our pages give the outside-diameter range and the pipe standard per series, not just the inch size.
Checked today
Verdict: applies (class A). The rule follows the manufacturer data; it is an installation truth, not an interpretation of a standard.
Further reading
- Joining, repairing, transitioning pipe – without welding and without press tools – Joining, repairing, transitioning without welding – the entry page
- Compression fittings for steel pipe and boiler tube: Gebo malleable iron, brass, HTB for gas, GF PRIMOFIT – Compression fittings for steel pipe and boiler tube
- Multigebo: joining, repairing, transitioning multilayer pipes of different makes – Multigebo: joining multilayer pipes of different makes
- Which solution fits? Enter pipe type, outside diameter, problem and medium – compression fitting, repair clamp or Multigebo – Tool: which solution fits?
- DN is not a diameter – why DN 50 never measures 50 mm – DN is not a diameter
- Steel pipes explained: EN 10216/10217, seamless or welded, test categories TC1/TC2 – Pipe dimensions to standard in the pipe finder
Frequently asked questions
Is “1 inch” enough for the order?
Only for threaded pipe to EN 10255 – there 1 inch is 33.7 mm. For boiler, copper, PE or multilayer pipe we need the measured outside diameter.
How do I measure a multilayer pipe correctly?
Outside with the calliper, inside with the gauge from the Multigebo kit – the same outside diameter comes with several wall thicknesses, and the support body must match the inside diameter.
What if the pipe is not round?
Two measurements 90° apart; if the larger one exceeds the tolerance band of the fitting, a repair band with a range is the better choice – or the question to us.
Your inquiry to Zickwolff
Send us what you have – designation, standard, quantity, date. We will clarify the rest with you.
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Designation, standard and quantity are enough.
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Attach it as a file.
Helpful if already known: project and country, date needed, certificates, packaging and language of the packing lists. If anything is missing, we will ask.
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When do you need it? Express delivery, to the site if required.
Tell us by when you need the material – we will tell you what is possible.