Here’s a common confusion worth clearing up; if a product costs $80 and is sold for $100, what is the Margin and Mark-up of the product?

Margin - is the sale price ($100) minus the cost price ($80) i.e.  $100-$80 = $20 Margin, or, as a percentage: ($100-$80)/$100 = 20% (profit/sale)

Markup - is the amount by which the cost price ($80) of a product is increased from the sale price ($100). ie ($100-$80)/$80 = 25% (profit/cost)

A useful site: Markup Calculator