Markup is profit measured against cost. Margin is profit measured against the selling price. They use the same two numbers -- cost and price -- but divide them differently, and mixing them up is one of the most common pricing mistakes a new business makes.
The numbers diverge fast
Say something costs you $50 and you sell it for $100. Markup is (100-50)/50 = 100%. Margin is (100-50)/100 = 50%. Same item, same profit dollar amount, two very different-looking percentages. The gap gets bigger the higher the percentage goes: a 200% markup is only a 67% margin, not a 200% margin.
This matters in practice because if you set a pricing target by 'margin' but calculate it using the markup formula, you will systematically underprice your products -- the error compounds across every sale.
Which one to use when
Margin is generally the more useful number for assessing overall business health, because it tells you what share of each sales dollar is actually profit -- which is what flows into your overall profitability. Markup is more useful at the point of pricing an individual item, because it is calculated directly from a cost you already know.
Retail and wholesale pricing conversations often default to markup language ('we mark up 40%'), while financial statements and investor conversations almost always use margin language ('we run a 25% margin'). Knowing which one is being discussed avoids a real and common misunderstanding.
Checking your own numbers
The margin calculator and markup calculator compute both directions from the same cost and price inputs, so you can see exactly how far apart the two numbers are for your own pricing. If you are looking at the business as a whole rather than a single item, the net profit margin calculator and contribution margin calculator work from revenue and expense totals instead.