Size guide
Every retailer and every brand has its own size chart.
Always use the retailer’s chart
A size M at one brand is not an M at another. The product page at the retailer has a size chart, and that is the one to trust.
When in doubt
Between two sizes, take the larger. For relaxed pieces and knitwear that is almost always better, and tailored pieces can be adjusted by a tailor.
Unisex pieces
Unisex pieces are usually cut wide. For a closer fit, size down; for a looser one, size up.