Allow me to input something I know, e.g. I wear size 12 Adidas. Then tell me what size I need to buy of the shoes you're selling me, be they Nike, Reebok, or something new.
Shoes is one example, applies for many other types of clothing.
Euro sizing has been pretty reliable in my experience (except with gloves, but my hands are very slender and my knuckles rather broad). You could try translating the US size to Euro sizing based on Adidas' chart, and then converting Euro sizing to Nike using Nike's chart.
Allow me to input something I know, e.g. I wear size 12 Adidas. Then tell me what size I need to buy of the shoes you're selling me, be they Nike, Reebok, or something new.
Shoes is one example, applies for many other types of clothing.