Contribution Margin Explained

Updated 2026-08-31 · ProfitKit

Contribution margin is what one unit contributes toward covering your fixed costs, after its own variable costs are paid. It is the single most useful number for deciding which products to push — and it is not the same as gross margin.

The formulas

Contribution margin per unit = Price − Variable cost per unit

Contribution margin ratio = (Price − Variable cost) ÷ Price

Variable costs are the ones that exist only because you sold that unit: the item cost, the transaction and payment fees, the postage, the packaging. Your monthly software subscription is not one of them.

Worked example

A product sells for $32. Item cost $11, platform and payment fees $4.10, postage and packaging $5.40.

Variable cost = $20.50. Contribution margin = $11.50 per unit, a ratio of 35.9%.

If fixed costs are $920 a month, you need $920 ÷ $11.50 = 80 units a month to break even. That is exactly the break-even calculation, and contribution margin is the engine inside it.

Why it beats gross margin for decisions

Gross margin excludes selling costs, so it flatters products that are expensive to fulfil. Consider two items:

Item A has the better-looking margin percentage. Item B puts more than three times as much cash toward your fixed costs per sale. If your marketing effort is the constraint, B is the product to promote.

Per unit or per ratio?

Use contribution per unit when the limit is something countable — units you can make, orders you can pack, hours you can work. Use the ratio when the limit is revenue-shaped, such as deciding what a discount does to your break-even volume.

The trap in bundles and free shipping

Both move costs into the variable column. Free shipping does not reduce postage; it moves it from the buyer to your contribution margin. Recalculate before offering it, because a 35% contribution ratio can become 15% quietly, and the break-even volume more than doubles.

Do the calculation