Imperial to metric: converting thread sizes

Let's start with a quick look at how standard sizes came to be. In plumbing, as in many other fields, there are two main standards when it comes to pipe sizes:

• European metric sizes where values are indicated in millimetres for dimensions such as 12/17, 15/21, 20/27, 26/34, and so on. Diameters are expressed with two figures: the first indicates the inner diameter of the pipe; the second refers to the outer pipe dimensions.

• British imperial sizes or British Standard Pipe (BSP) where values are given in inches for dimensions such as 3/8", 1/2", 3/4", 1", and so on. In fact, this list of dimensions is so long it goes right up to the point where pipes are large enough to fit an adult human! BSP is indicated by the sign (") which, of course, stands for inches. 

Metric sizes

European metric sizes are noted in millimetres, but the abbreviation 'mm' is rarely used. It's worth noting that the inner and outer diameters are approximate values. These dimensions were created in the days of the industrial revolution when most pipes were made of steel. The role of these pipes was to transfer hot water and steam to power the industrial machines of the time. Pipe technology has come a long way since then and pipe materials have thinned out which means less material is required to make them.

Today, it's possible to find thinner pipe (such as copper pipe) and thicker pipe (such as steel pipe). As a result, the inner to outer diameters do not correspond exactly to the two figures as seen above. The metric unit DN or nominal diameter came about as a way to navigate this issue with common dimensions including DN10, DN15, DN25, DN25, DN32, and so on.

While a little mental gymnastics is required to juggle the three values, bear in mind that the three most commonly found dimensions are as follows: 3/8", 1/2" and 3/4". 

Most domestic plumbing and heating installations will feature 1/2" or 3/4" fittings.

The thread sizes for reducers or adapters are usually indicated as two figures separated by an 'x', hyphen or the word 'to'. For example: 3/4" x 1/2".

A forward slash will rarely be used to separate the sizes as it would render the dimensions illegible. Here's an example of metric reducer dimensions: M-F 20/27-15/21.

So you can see that if a forward slash was used to separate the dimensions (i.e. M-F 20/27/15/21), it would be very difficult to read. It's worth noting that the dimensions may not be given in any particular order – the larger or the smaller diameter may be written first. However, if the M (for male) is written first, the first figure will correspond to the male diameter. With this in mind, the above example can be noted in two ways: 

1. F-M 15/21-20/27 reducer

2. 20/27-15/21 M-F reducer

Of course the double quote mark (") used to indicate inches only applies to BSP dimensions and nothing else. This makes things a little clearer and means you aren't likely to mix up imperial and metric dimensions. 

That said, if you happen to be noting metric sizes, be careful not to use quote marks! For example, if you are looking for a compression fitting for a 15mm pipe (meaning a 1/2" or DN15 pipe), be careful – If you write "DN15-DN15 female copper compression fitting", you'll leave room for confusion. It's best in this case to write either:

• DN15-1/2" female compression fitting or

• DN15-15/21 female compression fitting.

Considering the number of different plumbing fittings out there, it's easy to get mixed up so be careful about how you note down values.