Formula and method
Selling price = cost × (1 + markup/100). Equivalent margin = (price − cost)/price × 100.
Worked example
A unit costing 70 with a 30% markup sells for 91. Its margin is about 23.08%, not 30%.
Other ways to solve
Cost from price and markup: cost = price ÷ (1 + markup/100), so a 91 price at 30% markup implies a cost of 70. Markup from cost and price: markup = (price − cost) ÷ cost × 100, so cost 60 and price 100 is a 66.67% markup.
Price from a target margin: price = cost ÷ (1 − margin/100). A 70 cost at a 30% margin needs a price of 100, not the 91 that a 30% markup gives.
List price after marketplace fees
Marketplaces usually take a percentage of the sale price plus a fixed amount per order. To keep a target profit T after fees, list price = (cost + fixed fee + T) ÷ (1 − fee %). To keep a target margin m of the list price after fees, list price = (cost + fixed fee) ÷ (1 − fee % − m).
Example: a 10 cost, a 15% fee and a 0.30 fixed fee need a list price of (10 + 0.30 + 5) ÷ 0.85 = 18 to keep 5 of profit. Fees on 18 are 18 × 0.15 + 0.30 = 3, leaving 18 − 3 − 10 = 5.