IMPERIAL PIPE AND METRIC PIPE ARE MEASURED AND REFERENCED DIFFERENTLY SO MAKING COMPARISONS IS NOT AS STRAIGHT FORWARD AS IT FIRST APPEARS

In general metric and imperial pipe & fittings are not interchangeable. As demonstrated in the chart, there is a 1.4 mm difference between the O/D of the 20mm Metric and the O/D of the 1/2", which are close equivalents from the two systems. This means that the 20mm pipe would rattle around in a 1/2" fitting, to avoid leaks and pressure losses you need a good close fit, so this would not suffice. You can convert from one system to another using a converter or adaptor, which is why our size comparison chart is useful as well as for identifying existing pipe.