30 mm and 40 mm are the two widths you will see most
The 30 mm reversible belt has a 3 cm strap and a buckle about 3.5 cm high — a proportion that sits comfortably through the belt loops of dress trousers and reads as a slimmer, more formal belt. The 40 mm reversible belt has a 4 cm strap and a buckle measuring roughly 4.5 × 4.5 cm; the extra centimetre of strap width and the larger frame make it the bolder of the two and the one that pairs better with denim and wider loops. Those figures are published on Louis Vuitton's own product pages, so they are the numbers to quote when you ask about a specific belt.
How Louis Vuitton belt sizing actually works
Louis Vuitton publishes belt sizes as strap lengths in centimetres — commonly 75, 80, 85, 90, 95, 100 and 110 cm, depending on the model. That is the length of the strap, not your waist measurement, and it is not the same as a trouser size. The practical consequence is that a size you wear in a different brand's belt will often not transfer. A second detail matters just as much: these belts are reversible, which means the strap is finished on both faces and the buckle turns to expose the second side. Reversibility typically means the size cannot be shortened at the buckle end the way a conventional belt can, so getting the strap length right matters more than it would on a single-face belt.
Reversible construction and buckle types
A reversible belt has a rotating buckle: the frame is mounted on a pivot so that after the strap is fed through, the buckle can be turned to show either face. That is what allows one belt to present two leathers. The trade-off is weight and thickness — the mechanism and the double-faced strap make a reversible belt stiffer and slightly heavier than a single-layer belt of the same width, which is noticeable on a thin strap and less so on the 40 mm. When you ask about a specific belt, the two useful questions are which two faces the strap carries and whether the buckle rotates or is fixed.
Measuring before you ask
Measure a belt that fits you the way you want, rather than measuring your waist. Lay the belt flat, from the fold at the buckle end to the hole you actually use — that distance is the figure comparable to a published centimetre size. Measuring the full length end to end gives a number several centimetres longer and is the most common reason a size conversation goes wrong. If you do not own a belt in the width you are considering, note the loop height on the trousers you intend to wear it with: a 40 mm strap needs roughly 4.5 cm of loop to sit flat, and forcing a wider strap through a narrow loop distorts both the belt and the trouser.