Converting between Shein and Temu trips people up because the two brands are cut differently. Shein tends to run noticeably small, while Temu tends to run noticeably small. Shein and Temu fit fairly similarly, though cut and fabric still shift things style to style. 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 Shein converts to about a S in Temu, 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.