There has to be a system that allows different pipes to be used in many different guises so that everything was of a standard measurement. That system was called the Imperial measurement and was used for many years in the engineering world until around 1971 when something called metric was introduced. Below is some brief information regarding the metric and imperial measurements for the pipework that plumbers use:

All of the copper and stainless steel pipework that is used in plumbing today are made and supplied in metric sizes, however there is a large amount of pipework that is already installed in the many most older houses will have been made to the imperial standard measurements. Neasden Plumbers know the different sizes and types of pipes. If you compare the equivalent dimensions:

15 mm=1/2in,
22 mm=3/4in
28 mm=1in

the difference seems obvious, however, metric pipe is measured by its external diameter while imperial pipework is measured internally. The difference is very small, but can cause some real problems when trying to join one type of pipe to a different one. If you are creating a soldered joint, an exact fit is critical. There are imperial to metric adaptors available and are necessary when joining 22mm pipe to its imperial opposite and these adaptors are convenient when you are working with 28mm pipes or with thick walled 1/2in pipework. A Neasden Plumber is trained to know the difference between metric and imperial. These type of adaptors are not required when using compression fittings If you were joining 22mm to 3/4in plumbing, slip an imperial olive on the 3/4in pipework. Typically, 15mm (1/2in) pipework is used for the water supply to basins, kitchen sinks, washing machines, some showers, and radiator flow and returns. However, 22mm (3/4in) pipework is used to supply baths, high output showers, hot water cylinders and main central heating circuits, and 28mm (1in) pipe for larger heating installations.