Markup starts from cost
Markup is the percentage added on top of what something costs you. If an item costs 20 and you add a 50% markup, the selling price becomes 30.
That sounds like a 50% profit margin, but it is not. The profit is 10, and 10 is one third of the 30 selling price.
Margin starts from selling price
Margin measures profit as a share of the final selling price. A 40% margin means 40% of the revenue remains as gross profit before overheads and other costs.
This is why teams can talk past each other. Suppliers may quote markup, while accountants and owners often care about margin.
Use both before changing prices
A markup calculator is useful when you know your cost and want to add a fixed percentage. A margin calculator is useful when you know the profitability you need and want to work backward.
For real pricing, include payment fees, tax treatment, shipping, discounts, returns, and overhead in the broader decision. The calculator gives the clean math, not a full business model.