Start with your foot, not the label
Measure both feet while standing, in the socks you expect to wear. Use the larger result. A seller chart may refer to foot length, insole length or an outsole measurement; these are not interchangeable.
Ask the precise question
Confirm which measurement the chart uses and whether it belongs to the exact colour and variant. Compare it with a comfortable pair, measured in the same way. Avoid a blanket “size up” rule.