Converting between Zara and Shein trips people up because the two brands are cut differently. Zara runs small with a slim cut, while Shein tends to run noticeably small. Shein generally runs smaller than Zara, so you'll often need to size up when switching from Zara. The converter above corrects for this automatically by reading the body-measurement range behind each size rather than matching the letter on the tag.
As a quick guide: a size S in Zara converts to about a S in Shein, a M lands around a M, and a L is roughly a L. When your measurement falls near a boundary the tool flags it as a between-sizes case — worth checking the garment's own measurements before you order.