Uh, not exactly. This is a rather complex subject but I'll give you the general details as best I understand them: The French size (42, 43, 44, 45 etc.) is based on the Parisan stitch measurement - one stitch being equivalent to 6.667 mm. You get your French size by measuring the length of your foot at rest, converting to stitches, and then adding 1.5 stitches (about 1cm) because that is thought to be roughly how much the average foot stretches when walking. So the: FOOT LENGTH TO FRENCH SIZE EQUATION IS: ((foot length in cm x 10)/6.67) + 1.5 = French size For instance, my foot measures 29 cm long so my French size is theoretically a 45. English shoe sizes are based on a different measurement - the inch. The inch is supposed to be equivalent to three grains of (admittedly rather uniform :-) barley. So the basic unit of measurement for English shoemakers is one grain of barley, which for practical purposes is equivalent to one-third of an inch. One-third of an inch equals 1 size, one-sixth of an inch equals one-half size. English sizes can be determined by starting with a foot-length (at rest) of eight inches for a theoretical size 0, and adding one-half size for each one-sixth of an inch your foot measures over eight inches. So the: FOOT LENGTH TO ENGLISH SIZE EQUATION IS: (((Foot length in inches - 8")/.167")/2) = English size Using my foot as an example again, my foot length is about 11.5" so my theoretical English size would be 10.5. Problem is that there are more measurements to a foot than length. The volume and shape of the different parts of the foot are just as important. Shoemakers have ways of calculating increased (average) girth that can be expected with increased length. One of the simple formulas is to take the French size, add to it the width number (5 for E, 6 for F, 7 for G and 8 for H) and divide by two to get the girth of the metatarsals (the widest portion of the forepart of your foot.) By that formula a size French size 45 in an E fitting would be designed for a person with a metarsal girth of 25 cm. My metarsels measure 29.5 cm - no wonder I often wear a size 12 1/2 English/47-47.5 French for sufficient width.. I'm not sure how to convert foot length to American size. Apparently the American sizing scale is similar to the English but it starts at a lower point. In my experience the American size is 1/2 size larger than the equivalent English size, 1 full size in some brands. Confused yet??
In view of the complicated nature of this sort of thing, I recommend that you dispense with measuring your own foot altogether and get measured by a professional with a Brannock device at his disposal. It also would be advisable to buy shoes from a maker who has a variety of lasts available - hopefully you will find one that approximates to some degree the shape of your foot. To make matters worse, different brands have different ideas of how, say, a size 45 should fit. With experience, you may be able to figure out the different fitting characteristics of each brand. If you like to buy shoes on ebay or online like me, I would suggest that you take every opportunity to try on different brands of shoes when you are shopping. Even if you don't buy a pair, keep a record of the size and width that fit you correctly - that way if a pair crops up on ebay later you will have an idea as to whether they will fit or not.