Profit = selling price − cost. Gross margin is profit divided by the selling price; markup is profit divided by the cost. The same sale therefore has a larger markup than margin: a $60 item sold for $100 makes $40 profit, which is a 40% margin and a 66.7% markup.
To reach a target margin, price = cost ÷ (1 − margin). To reach a target markup, price = cost × (1 + markup).
Both measure profit, but from different bases. Margin is a share of the selling price, so it can never reach 100%. Markup is a share of the cost and can be any size. A 50% markup equals a 33.3% margin.
You cannot: a 30% markup is only a 23.1% margin. For a 30% margin you need a 42.9% markup.
No. It is gross profit: selling price minus the direct cost of the item. Overheads, shipping, fees and taxes are not included unless you add them to the cost.
See also: Percentage Calculator · Discount Calculator · Sales Tax Calculator