It’s best to measure your feet in the evening or at night time, when they’re the most swollen. You should measure both feet and use the biggest one for reference.
If you can get someone’s aid, it is advised you measure your feet while standing. Otherwinse, you can do it while sitting, provided you put in some pressure so your feet are flat on the floor.
If you’ve measured your feet in centimeters, all you must do is check the manufacturer’s tables.
Your shoe size should also depend on the type of shoe and the season it is meant to be worn in.
During the winter, as it is common to wear thick socks, it is advised to order larger numbers, whereas for lighter shoes the number indicated in the tables should be enough.
Usually boots’ sizes are bigger than regular shoes so, for example, if you wear 41.5 size shoes then you should order 42 size boots.
Some shoes have an inner pad which can be removed or replaced by a thinner one.
If you’re wearing leather shoes you should expect them to swallow a bit with use, however, you can take them to a shoemaker who can enlargen them by placing an appropriate mold inside it for a few hours. This is especially useful when it is too tight on the sides but is the correct length. It it’s too tight lengthwise, its best to exchange it for a larger number.
Try to put on some thicker socks or an extra padding.