European sizes are a metric-based French system originally known as Paris Points; in the 14th century, King Edward II developed UK sizes based in inches and the sizes of barley corn. It's no wonder size conversion can be tricky. A Paris Point is 2/3 centimetre (17/64 inch). European sizes are calculated by the length of your foot in Paris Points, plus 2 centimetres (25/32 inch). UK sizes go up a whole every 8.5 mm (1/3 inch), with half-sizes being 4.25 mm (1/6 inch). European sizes are not gender specific, while UK sizes are different for men and women.
Determine your shoe size. Foot size can change throughout your life, so having a professional measurement will help with your calculations.
Add 33 to your shoe size if you wear a whole size. If you wear a UK size 6, you would wear a European size 39.
Add 33.5 to your shoe size if you wear a half-size. If you wear a women's UK size 6.5 you would wear a European size 40.
Factor in the difference in the metric and imperial measurements. A Paris Point is 2/3 cm. Since the measurements are not exact, you will find a slight difference in size conversions as you go up or down sizes, meaning that you may need to try on a few European sizes to truly find a good fit for you.
European half-sizes are quite rare, so round up to the nearest whole size when converting. For men's UK sizes, add an extra half-size.