Aluminum Buis 26mm is like many inches

Conversion from inches to millimeters in heating and sanitary areas

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The common DIN and ISO values ​​indicate standards for a wide variety of units of measurement and conversion factors. As a rule, one inch can therefore be converted to 25.4 millimeters.

However, there are deviations from this standard in the heating and sanitary sector
The definition of 1 inch in this area is still based on historical reasons. These different dimensions developed as early as the Middle Ages. The reason for this is that there are always two dimension lines in a pipe. The inside diameter of a pipe is differentiated from the outside diameter of the thread.

Why is 1 inch not 25.4 millimeters in heating and plumbing?

In the Middle Ages, the internal dimensions of a pipe were always used. Therefore, the dimensions always referred to the inside diameter of a pipe. Over the years, the materials could be continuously improved, whereby the wall thickness of the metal pipes was reduced. The difference between the inside diameter and the outside diameter changed and became smaller and smaller. A standardized dimension was therefore not possible from the point of view of technical development.

The other DIN and ISO values ​​are not subject to such a change, which means that no precise conversion factor for pipe dimensions can be specified. The heating and sanitary sector therefore calculates in thread sizes, which can be read out using a table. In this way, values ​​can be converted from inches to millimeters or inches to centimeters. For example, the value of 1 inch is around 33 millimeters in the outer diameter.

How do I correctly measure a pipe / thread?

When buying new pipes or connecting pieces, the question often arises as to which size is the right one. If you want to measure inches from a pipe or thread, the outer diameter must always be taken into account. If you have a single pipe or a pipe end in front of you, you can measure the millimeters at the widest point.

When measuring in inches, make sure that there is no cladding on the pipes. This outside diameter can be converted into inches using the conversion table for heating and sanitary applications. In this way you can calculate inches in millimeters and inches in centimeters and read off exactly how wide the respective pipe is. If there is no visible end on the pipe, the best way to determine the outside diameter is with a small pipe wrench. This can be held tightly to the pipe and the value can then be read off the pipe wrench opening with the ruler.

What do you have to pay attention to when buying?

If you have correctly taken off the dimensions of the pipe, you can read off the inch unit using the conversion table for the heating and plumbing area. In the hardware stores, pipes and threaded pieces are usually labeled with a small engraving, which can be easily recognized. You can then read off the measure in inches. If you are unsure, it is advisable to simply ask an employee from the respective specialist shop.

📌 Table for converting inches to millimeters in heating and sanitary areas

In this table, lengths from 1/8 to 5 inches are converted into millimeters (mm) and can therefore be read off quickly for various purposes. These dimensions are predominantly common in the sanitary and heating sector. For pipes, connecting pieces, valves, nipples, adapters, faucets, etc., information in fractions and inches is standard in households, trade and industry. Only the conversion when measuring and re-measuring in millimeters or centimeters is not always easy.

Outside diameter according to DIN EN ISO 228-1 and DIN 2999

Inch, fractionInch, decimalmin. outside diametermax. outside diameter
1/8″0,125″9.5 mm9.7 mm
1/4″0,25″12.9 mm13.1 mm
3/8″0,375″16.4 mm16.7 mm
1/2″0,5″20.7 mm21.0 mm
5/8″0,625″22.6 mm22.9 mm
3/4″0,75″26.2 mm26.4 mm
7/8″0,875″29.9 mm30.2 mm
1″1″32.9 mm33.3 mm
1 1/8″1,125″37.5 mm37.9 mm
1 1/4″1,25″41.5 mm41.9 mm
1 1/2″1,5″47.4 mm47.8 mm
1 3/4″1,75″53.4 mm53.8 mm
2″2″59.2 mm59.6 mm
2 1/4″2,25″65.3 mm65.7 mm
2 1/2″2,5″74.7 mm75.2 mm
3″3″87.4 mm87.9 mm
3 1/2″3,5″100.0 mm100.3 mm
4″4″112.6 mm113.0 mm
4 1/2"4,5″125.3 mm125.7 mm
5″5″138.0 mm138.4 mm
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