Margin vs markup — the crucial difference
They're easy to confuse but very different. Markup is profit as a percentage of cost (how much you add on). Margin is profit as a percentage of the selling price (how much of the sale you keep). A product costing $40 sold for $100 has a 150% markup but only a 60% margin.
Knowing both helps you price products correctly and compare profitability. Calculation is instant and private.