Contribution margin: use each sale to test whether growth helps cover fixed costs
Calculate contribution margin per sale and test how discounts, commissions, shipping, and capacity change the result.
Bottom line
Contribution margin is revenue minus the costs that vary with that revenue. It shows how much remains to cover fixed costs and profit. The calculation is useful only when the cost classification matches the decision, so a discount test may need payment fees, shipping, commission, and support effort that a high-level product report leaves out.
Is it right for you?
- Choose the product, service, customer, or channel being tested
- Separate costs that change with the sale from fixed period costs
- Recalculate after discounts, refunds, fees, and commissions
- Check capacity before treating more volume as the answer
Use the formula at the level of the decision
Contribution margin per unit equals selling price minus variable cost per unit. Contribution margin ratio equals contribution margin divided by revenue. A company can calculate the measure by unit, order, customer, service line, or channel, provided revenue and variable costs use the same scope.
Do not force rent, executive salaries, and every shared system into a unit figure for a first discount decision. Those fixed costs matter for total profit, but inventing an allocation can hide whether the next sale itself contributes anything.
A discount changes more than revenue
| Illustrative case | Price | Variable cost | Contribution | Units to cover $12,000 fixed cost |
|---|---|---|---|---|
| Current offer | $80 | $32 | $48 | 250 |
| $10 discount, same variable cost | $70 | $32 | $38 | 316 after rounding up |
This example does not predict demand. It shows the question created by the discount: can the business sell roughly 66 additional units without adding capacity, overtime, returns, or acquisition cost that changes the variable-cost assumption?
Costs that often move between models
Card fees, marketplace commissions, freight, packaging, sales commission, usage-based software, and contract labor often change with volume. Labor can be fixed for one decision and variable for another. A salaried employee may be fixed this month, while an added shift required by the promotion is incremental.
Write the assumption beside each cost. Finance can then rerun the model when volume, channel, or service level changes instead of debating a label with no context.
Questions to ask before approving the offer
- Which customers receive the lower price?
- Will refunds or support contacts change?
- Does the channel add a fee or commission?
- Can current staff and equipment handle the volume?
- What result will cause the offer to stop?
Frequently asked questions
Does gross margin measure the same thing? Not always. The measures can include different cost categories, so document the definition used.
Can the figure go negative? Yes. Each additional sale then increases the shortfall before fixed costs.
Should fixed costs be ignored? No. The figure shows what is available to cover them; the business still needs a full profit view. For the exact unit count where contribution finally covers those fixed costs, see our break-even point guide.
Who should approve the assumptions? Finance should own the model while sales and operations verify price, volume, service, and capacity inputs.
Is a 30% margin good here? There is no universal cutoff. A 30% figure can be comfortable for a high-volume, low-fixed-cost retailer and dangerously thin for a business with heavy rent, salaries, and equipment costs to cover. A 50% figure reads the same way: half of each sale's revenue survives its own variable costs, but whether that counts as good depends entirely on the size of the fixed-cost base sitting on top of it. Run the number through the discount example above before judging any single percentage on its own.
The purchasing and inventory side of this calculation is covered in our COGS formula guide.