Ask a small business owner what margin they want on a product and what markup they apply, and many will give you the same number for both. That's the mistake: margin and markup measure the same profit dollar against two different denominators, and treating them as interchangeable means every price you set is wrong in the same direction, toward too cheap.
The math that trips people up
Markup is profit divided by cost: if something costs you $100 and you sell it for $150, your markup is $50 ÷ $100 = 50%. Margin is profit divided by selling price: that same $50 profit on a $150 sale is $50 ÷ $150 = 33.3%. The two numbers only match at 0%, and the gap between them widens as the percentage gets bigger, a 100% markup is only a 50% margin, and a 300% markup is only a 75% margin.
The costly version of this mistake happens in reverse: a business decides it needs a 40% margin to hit its profit targets, then applies a 40% markup to costs assuming that's the same thing. It isn't, a 40% markup on a $100 cost gives a $140 price, which is actually a 28.6% margin, well under the 40% the business needed. Run the numbers on the Margin vs. Markup Calculator to see exactly how much revenue that gap costs across a full year of sales.
Which one should drive your pricing?
Margin is almost always the number that matters for business planning, because it tells you what share of every sales dollar is actually profit, which is what funds rent, payroll, and everything else that isn't cost of goods. If you know the margin percentage you need, the correct way to set a price is to divide cost by (1 − margin), not to multiply cost by the margin percentage; the Profit Margin Calculator does this conversion directly so you're never accidentally pricing off markup math.
Markup is still a useful number for quick, informal pricing, especially in retail and wholesale where "cost plus X%" is a standard convention, and the Markup Calculator is built for exactly that use case. The key is knowing which one you're using at any given moment, and never assuming a target expressed in one automatically holds in the other.
Where this shows up on real invoices
This distinction matters most on quotes and invoices where the margin assumption feeds directly into whether a job is actually profitable once it's billed. When you're building out pricing for a quotation or invoice, it's worth double-checking that the percentage you typed in is the margin you actually want, not a markup number mislabeled as margin, since that single mislabel compounds across every line item and every client.