Part 3 of our guide series "Converting units" is about converting inches to mm or cm. Would you like to know how to convert inches to millimetres or centimetres? Then you've come to the right place! In this guide article, we will show you everything you need to know about converting inches to millimetres, including conversion tables and how to convert inches to the metric system of measurement. We will also give you some useful tips and tricks to help you convert inches to millimetres or centimetres. Whether you work in construction or industry, this knowledge will certainly come in handy.
Inch to mm conversion table
Even today, the question often arises: How many millimetres is 1 inch and is inch the same as the length measurement inch, which originated in the Anglo-Saxon world? In fact, the term inch is the German equivalent of the British inch. Both measures of length are based on a value of 2.54 cm per inch, which is why both terms can be used equally.
Note: To convert inches to mm, it is advisable to convert the inch measurements, which are often shown as a fraction, into a decimal value and then multiply by 25.4.
For example, a pipe with a diameter of 3/8 inch corresponds to a decimal value of 0.375. Multiplying by 25.4 results in a metric measurement of 9.525 mm or 0.9525 cm.
The common inch sizes and their metric equivalent in mm are listed below.
| Inch dimensions displayed as a fraction | Inch dimensions converted to mm (rounded) | Decimal values of imperial dimensions (rounded) |
|---|---|---|
| 1/64 inch | 0.397 mm | 0.016 |
| 1/32 inch | 0.794 mm | 0.031 |
| 1/16 inch | 1.587 mm | 0.063 |
| 1/8 inch | 3.175 mm | 0.125 |
| 1/4 inch | 6.350 mm | 0.25 |
| 3/8 inch | 9.525 mm | 0.375 |
| 1/2 inch | 12,700 mm | 0.5 |
| 5/8 inch | 15.875 mm | 0.625 |
| 3/4 inch | 19.050 mm | 0.75 |
| 7/8 inch | 22.225 mm | 0.875 |
| 1 inch | 25,400 mm | 1 |
| 1 1/4 inch | 31.750 mm | 1.25 |
| 1 1/2 inch | 38,100 mm | 1.5 |
| 1 3/4 inch | 44,450 mm | 1.75 |
| 2 inch | 50,800 mm | 2 |
| 2 1/4 inch | 57,150 mm | 2.25 |
| 2 1/2 inch | 63,500 mm | 2.5 |
| 2 3/4 inch | 69.850 mm | 2.75 |
| 3 inch | 76,200 mm | 3 |
| 3 1/4 inch | 82,550 mm | 3.25 |
| 3 1/2 inch | 88,900 mm | 3.5 |
| 3 3/4 inch | 95,250 mm | 3.75 |
| 4 inch | 101.60 mm | 4 |
| 4 1/4 inch | 107.95 mm | 4.25 |
| 4 1/2 inch | 114.30 mm | 4.5 |
| 4 3/4 inch | 120.65 mm | 4.75 |
| 5 inch | 127.00 mm | 5 |
| 6 inch | 152.40 mm | 6 |
| 7 inch | 177.80 mm | 7 |
| 8 inch | 203.20 mm | 8 |
| 9 inch | 228.60 mm | 9 |
| 10 inch | 254.00 mm | 10 |
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Inch threaded sizes for pipes
Common pipe threads are labelled 3/8 inch, for example, but are defined in their nominal diameter in mm according to the German industrial standard. The following applies: Inches are only used today for pipes with a diameter of up to 40 mm. Pipes above this size are always specified in their metric sizes in mm. It should also be noted that while the diameter of American screw threads can be easily calculated from inches to mm by multiplying the inch dimension by 2.54, this is not possible for pipe threads. Below we have listed the most common inch external threads with their respective dimensions in mm.
| Common pipes with inch dimensions | Pipe inner diameter in mm | Outer thread diameter in mm | Internal thread diameter in mm |
|---|---|---|---|
| 3/8" | 10 | 16.4 | 15.1 |
| 1/2" | 15 | 21.5 | 19 |
| 3/4" | 20 | 26.3 | 24.5 |
| 1" | 15 | 33 | 31.1 |
| 1 1/4" | 32 | 41.2 | 39 |
| 1 1/2" | 40 | 48 | 45.5 |
| 2" | 50 | 59 | 57 |
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Why is an inch threaded pipe not equal to 25.4 mm?
It is generally accepted today that there are no standardised dimensions for pipe threads in the heating and plumbing sector. In fact, the sanitary inch dimensions are apparent sizes that do not refer to the actual inside diameter of the respective threaded pipe.
This is primarily due to the material used. For example, pipes with an internal thread of 25.4 mm diameter usually had an external diameter of 33 mm. Due to the steel used, which was not as robust and high quality as it is today, the pipe walls were thicker than they are today. However, the quality and strength of steel alloys improved significantly at the beginning of the 19th century. As a result, the thickness of the pipe walls decreased. As a result, pipes currently available for sanitary and heating applications have much thinner walls, some of which are only half as thick as they used to be.
However, as people wanted to stick to the usual external dimension of 33 mm, the internal diameter of the pipe threads increased. This meant that a 1 inch internal thread had a larger diameter than 25.4 mm. Depending on the manufacturer, we are now at around 30 mm and therefore well over an inch.
| Nominal pipe diameter DN | Nominal pipe diameter in inches |
|---|---|
| 12 | 3/8" |
| 16 | 1/2" |
| 20 | 3/4" |
| 25 | 1" |
| 32 | 1 1/4" |
Whitworth pipe threads in accordance with DIN/ISO 228-1 are cylindrical internal and external threads for connections that do not seal in the thread. This type of thread is mainly used to firmly connect fittings and valves.
In comparison, the EN 10226 standard specifies how pipe threads for thread-sealing connections with a conical external thread and a cylindrical internal thread should be constructed. Threads according to DIN EN 10226 - also known as Whitworth BSPT threads (British Standard Pipe Taper) - replace the old DIN 2999 standard. However, regardless of whether DIN EN ISO 228-1 or DIN 2999: the outer diameter of a 1 inch pipe thread is between 32.89 mm and 33.25 mm.
The diameter of pipe threads conforming to DIN EN ISO 6708 and EN 10255 also deviates upwards, so that an actual "inch" threaded pipe of the size 1 inch (2.54 cm) has an outer diameter of 33.7 mm.
The thread sizes described in the current standard also correspond to the internationally valid ISO 7/1.
Despite careful editing and checking of the content, Stecker Express assumes no liability for the topicality, correctness, completeness and quality of the information provided.
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