Converting between H&M and Zara trips people up because the two brands are cut differently. H&M runs slightly small, while Zara runs small with a slim cut. Zara generally runs smaller than H&M, so you'll often need to size up when switching from H&M. 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 H&M converts to about a S in Zara, 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.