What is the difference between markup and margin?

Markup is measured against your cost; margin is measured against your selling price. A part that costs 1,000 and sells for 1,500 carries a 50% markup but only a 33% margin. Confusing the two is how shops end up with less money than they planned for — you set what feels like a 40% margin, but you actually applied a 40% markup and kept 29%. This calculator reports both so the gap is visible.