How do I choose the right pattern size?

Measure yourself, then use the pattern envelope’s measurement chart. Ignore your ready-to-wear size entirely.

Pattern sizing and shop sizing are different systems and have been for decades. Someone who buys a size 12 in shops routinely cuts a 16 or 18 from a pattern. This is not a comment on anyone’s body; it is two unrelated numbering conventions.

Measure three things, over the underwear you will wear with the garment: full bust, natural waist (the narrowest point), and full hip.

Choose which measurement leads. For a top, dress or jacket, go by the bust — the upper chest is the hardest area to alter. For a skirt or trousers, go by the hip. Waist is the easiest to adjust and should not decide the size.

You will almost certainly span sizes, and that is normal. Cut the larger where you need it and blend between the lines with a curved ruler.

Check the finished garment measurements too, which most patterns print on the pattern piece itself. The difference between your body measurement and the finished measurement is the ease — how loose the garment is designed to be. A pattern with 15 cm of ease at the bust is meant to be loose; sizing down because it sounds big will produce something that does not fit.

And make a toile for anything fitted. A calico version costs an evening and saves the fabric you actually care about.