Size labels differ between brands and collections. Start with the measurements on the specific product rather than assuming your usual letter or number will match.
Compare like with like
If a chart lists garment measurements, lay a similar item flat and measure it in the same way. A flat chest width is not the same measurement as body circumference. Check the chart’s instructions before comparing.
Allow for how you want it to fit
Consider room for movement and any layers you plan to wear underneath. Stretch, cut and fabric weight also affect the result.
If key measurements are missing, contact the store before ordering. A clear answer is more useful than guessing from a model photograph.