What is the difference between margin and markup?

What they are measured against. Markup is the profit as a share of what it cost you. Margin is the profit as a share of what you sold it for. Buy at 100 and sell at 150 and that is a 50 percent markup and a 33.3 percent margin, from the same two numbers.

Why is a margin of 100 percent refused?

Because there is no price that produces it. A 100 percent margin means the cost is zero as a share of the price, which needs an infinite price for any real cost. Anything above 100 is the same problem.

Margin and Markup Calculator

Both numbers, because they are not the same one.

Cost

Work from

Selling price

Target percentage

Both numbers, because they are not the same one.

How to use it

Enter the cost. What the item cost you, before you add anything.
Choose what you know. A selling price, a target margin, or a target markup.
Read both percentages. Margin and markup are both shown on every result, always.

When you would use this

Margin and markup are calculated from the same two numbers and they are not the same number, which is a distinction that costs people money regularly. Markup is measured against what it cost you. Margin is measured against what you sold it for. Buy at 100, sell at 150, and that is a 50 percent markup and a 33.3 percent margin. Both describe the same transaction. The damage happens when a target is quoted in one and applied in the other. Someone who needs a 40 percent margin and instead adds a 40 percent markup ends up with a 28.6 percent margin, and the difference comes straight out of the job. It is a quiet error because both numbers look reasonable and the invoice still adds up. So both are on every result, always, rather than being a mode you select. You can work forwards from a price, or backwards from either target, and see immediately what the other one becomes.

Questions

What is the difference between margin and markup?: What they are measured against. Markup is the profit as a share of what it cost you. Margin is the profit as a share of what you sold it for. Buy at 100 and sell at 150 and that is a 50 percent markup and a 33.3 percent margin, from the same two numbers.
Why does it matter which one I use?: Because quoting one as the other underprices the work. Someone who needs a 40 percent margin and applies a 40 percent markup ends up with a 28.6 percent margin, and the shortfall comes out of the job. Both figures appear on every result here for exactly that reason.
Why is a margin of 100 percent refused?: Because there is no price that produces it. A 100 percent margin means the cost is zero as a share of the price, which needs an infinite price for any real cost. Anything above 100 is the same problem.