When a Discount Loses You Money, the Break-Even Math
How to use the SBA's own break-even formula to check whether a sale price actually makes a small shop money or quietly erases its margin.
Running a discount feels simple: knock a percentage off the price, sell more units, everyone wins. The math underneath that assumption is less forgiving. The U.S. Small Business Administration's own break-even guidance lays out the formula that tells you exactly how much extra volume a discount needs to generate before it actually helps your shop, rather than quietly costing it money.
The SBA's break-even formula
The SBA defines the break-even point as the level of sales where total cost equals total revenue, meaning no profit and no loss. Its standard formula for break-even point in units is:
Break-even point (units) = fixed costs ÷ (sales price per unit - variable cost per unit)
The denominator, sales price per unit minus variable cost per unit, is called the contribution margin: the amount each unit sold actually contributes toward covering fixed costs and, beyond that, profit. A discount changes the sales price per unit directly, which means it changes the contribution margin, which means it changes how many units you need to sell just to stay even, before you've made a single dollar of extra profit from the sale.
A worked example
Say a product costs $12 to make or source (the variable cost per unit) and normally sells for $20. The normal contribution margin per unit is $20 minus $12, or $8, which is a 40% margin on the sale price.
Here's what happens to that $8 of profit-per-unit as the discount gets deeper:
| Discount | New price | New contribution per unit | Units needed to match original total profit |
|---|---|---|---|
| 10% | $18.00 | $6.00 | 1.33x |
| 20% | $16.00 | $4.00 | 2.00x |
| 30% | $14.00 | $2.00 | 4.00x |
| 40% | $12.00 | $0.00 | every sale breaks even, zero profit |
| 50% | $10.00 | -$2.00 | every sale loses money |
At a 10% discount, you need about a third more unit sales than before just to earn the same total profit you were earning without the discount. At 20% off, you need to double your unit sales. At 30% off, you need four times the volume. And once the discount drops the price to or below your unit cost ($12 in this example), there is no volume of sales that makes the promotion profitable, because every single unit sold is now a loss or, at best, a wash.
Why this trips up small shops specifically
A larger retailer running a loss-leader promotion is often deliberately accepting a loss on one item to drive traffic that buys other, higher-margin items. A one-person shop selling a single product line usually doesn't have that cross-subsidy available. If the discounted item is the only thing in the cart, the math above is the whole story: no hidden upside is coming from somewhere else to rescue a discount that's priced below the break-even line.
This is also where markup and margin confusion compounds the mistake. If you calculated your "normal" price using a markup percentage on cost rather than a margin percentage of the sale price, you may already be running a thinner contribution margin than you think, which means a discount that looks modest on paper can push you underwater faster than the percentage suggests.
How to check a planned discount before running it
Before announcing any percentage off, run your own numbers through the SBA's formula:
- Confirm your actual variable cost per unit (what it really costs you to produce or acquire one unit, not including fixed overhead).
- Compute your contribution margin at the discounted price: discounted price minus variable cost.
- Divide your current contribution margin by the new, discounted contribution margin. That ratio is how many times more units you need to sell just to break even with the discount compared to not running it.
- Decide honestly whether the promotion is likely to drive that much extra volume. If it isn't, the discount is a net loss dressed up as a sale.
This same formula works for sales-dollar break-even too, using the SBA's alternate version: break-even point in sales dollars equals fixed costs divided by the contribution margin percentage. Either version gives you a number to check your gut feeling against before you commit to a price cut you can't easily walk back once customers expect it.
Key takeaways
- The SBA's break-even formula is fixed costs divided by contribution margin (price minus variable cost per unit); a discount shrinks that margin directly.
- A 10% discount on a 40%-margin product requires roughly 33% more unit sales just to match prior total profit; a 20% discount requires double the volume.
- Once a discount brings the price down to your unit cost, there is no sales volume that makes the promotion profitable; below cost, every sale actively loses money.
- Small, single-product shops usually can't rely on other items subsidizing a loss-leader discount the way larger retailers can.
- Run the break-even ratio before announcing a discount, and compare it honestly against how much extra volume you actually expect the promotion to generate.
The percentage on a sale sign is marketing. The contribution margin underneath it is accounting, and it's the one that decides whether the sale made money or gave it away.