How to use this profit margin calculator
Start by choosing what you already know. If you have a cost and a selling price, the first mode gives you the margin, the markup and the profit per unit straight away. If you know your cost and the margin you want to earn, the second mode tells you what to charge; the third does the same for a target markup. Cost means everything you pay to have one sellable unit in your hands: the wholesale or manufacturing price plus inbound freight, packaging and any duty.
The fees block is where the useful part happens. Type in the referral or commission percentage your marketplace charges, the card processing rate and its fixed per-transaction fee, the postage you pay and any other per-unit cost such as a poly mailer or a returns allowance. The results then show gross and net margin side by side, the net profit on each sale, and the break-even price below which you lose money. Add the number of units you expect to sell and you also get the total net profit for that batch.
Why margin, markup and fees are easy to get wrong
Margin and markup use the same profit figure but divide it by different things. Buy at $50 and sell at $80 and your $30 profit is a 37.5% margin (30 ÷ 80) but a 60% markup (30 ÷ 50). A supplier quoting "50% markup" and a retailer quoting "50% margin" are describing very different prices: $75 versus $100 on that same $50 cost. The reference table in the tool shows the pairs that come up most often so you can translate between the two without a calculator.
Marketplace sellers routinely underestimate how much of the sticker price never reaches them, because the deductions are spread across several line items on a statement rather than shown as one number. On the default example, a $29.99 listing with a 15% referral fee, 2.9% plus $0.30 processing and $4.50 of postage leaves $19.82 before the $12 product cost, so a headline 60% gross margin becomes a 26% net margin. The break-even price is the figure to remember when you run a promotion: because percentage fees shrink alongside the price, it is not simply cost plus postage, and discounting below it means every extra unit sold makes the loss bigger, not smaller.