What is discount break-even?
Run 20% off and your revenue per unit drops 20%. Your profit per unit drops far more, because the entire discount comes out of your contribution margin. Discount break-even is the volume lift that closes that gap: the point where the sale earns the same total profit as selling at full price.
Lift needed % = Discount % ÷ New margin % × 100
Units needed = Current units × (1 + Lift needed % ÷ 100)
Every percentage here is measured against the regular price. That matters: a discount does not scale with your margin, it eats it point for point. Take 20 points off the price and you take 20 points off the margin, no matter how healthy that margin was.
Worked example
The defaults model a product with a 55% contribution margin, a 20% off sale, and 500 units sold at full price over the sale window.
20 ÷ 35 × 100 = 57.1% more volume needed
500 × 1.571 = 786 units, rounded up to whole units
Each discounted unit now contributes 35 cents per dollar of regular price instead of 55. To hand you the same total profit, the sale has to move 786 units where you used to move 500. And because those units sell at 80% of the old price, revenue must come in roughly 26% higher just to tie. A sale that posts record revenue can still be a worse week than the quiet one before it.
Why discounts punish thin margins
The lift formula has a brutal denominator: margin minus discount. As the discount climbs toward your margin, the required lift does not grow linearly. It goes vertical.
At a 30% margin, a 10% discount needs 50% more volume: 10 divided by (30 minus 10). Push the same store to 20% off and the requirement becomes 200% more volume, triple the units. Meanwhile a 55% margin brand running that first 10% discount needs only 22.2% more volume. Same promo, wildly different bar.
That asymmetry is the whole story of discounting. High-margin brands can promo their way through a slow quarter. Thin-margin brands running the same calendar are trading profit for revenue and calling it momentum. And once the discount reaches the margin, the math stops entirely: no volume breaks even, and every extra unit digs the hole deeper.
What counts in contribution margin
The margin input is contribution margin: the share of the regular price left after every variable cost. Landed product cost, shipping and fulfillment, payment processing, and per-order extras like inserts and expected refunds all come out first. Fixed costs stay out. Salaries, software, and rent do not change when you sell one more unit, so they have no place in per-unit sale math.
If you only know your numbers in dollars, run the Contribution Margin Calculator first. It converts order value and per-order costs into the percentage this tool needs. Resist the shortcut of using gross margin: it ignores shipping and fees, flatters the sale, and makes a losing promo look like a coin flip.
Discounts vs alternatives: bundles, gifts, thresholds
A sitewide percentage off is the bluntest tool in the promo drawer. Three alternatives create urgency while protecting more of the margin you just calculated.
Bundles: discounting the second unit instead of the first cuts the effective discount rate while raising order value. Model the order-value side with the AOV Calculator.
Gifts with purchase: a gift costs you its landed cost, not a percentage of revenue. A $5 gift on an $80 order is a 6.25% margin hit that can read like a bigger deal than $10 off.
Spend thresholds: free shipping or a free add-on above a spend bar moves the average order up instead of the price down. Set the bar with the Free Shipping Threshold Calculator.
None of these are free. All of them tend to cost less per point of perceived value than a straight percentage off, because they spend dollars instead of margin points.
Frequently asked questions
Why does a small discount need so much extra volume?
Because the discount comes out of profit, not revenue. At a 55% margin, 20% off removes 20 of your 55 margin points, which is 36% of your profit on every unit. Volume then has to replace that missing profit at the new, thinner margin. The two effects stack, which is why the default sale needs 57.1% more units, not 20% more.
Does the traffic spike from a sale cover the gap?
Sometimes, so check your own history instead of hoping. Sales lift conversion for a few days, then hand some of it back as pull-forward: buyers who would have paid full price next week. Count only truly incremental units against the lift number. If your last promo lifted volume 40% and this calculator says you need 57%, that promo lost money.
Should I run ads into a sale?
Only after rerunning your break-even ROAS on sale-price margins. The default sale here drops margin from 55% to 35% of the regular price, which pushes break-even ROAS from about 1.8 to about 2.3. Campaigns that print at full price can lose money during the promo at the exact same ROAS. Check your numbers in the Break-Even ROAS Calculator before scaling spend into the sale.
When is a discount still worth it?
When you are deliberately buying something other than this month's profit. Clearing dead inventory converts stuck cash into working capital, and storage fees were eating that margin anyway. Discounting a first order can be an acquisition cost you earn back on repeat purchases. Those are LTV bets: fine to make, as long as you write down the payback window and audit it later.
Is 20% off ever free?
No. The margin math on this page applies to every discount, including the ones that feel like pure marketing. What changes is perception: a discount anchored to the regular price reads as a deal, while a permanently lower price just reads as cheap. Remember the anchor works on your dashboard too. Sale revenue always looks great; profit is the line to watch.
Where StefanBrain fits
This calculator tells you the volume a sale has to hit. Hitting it is a demand problem, and demand is creative plus offer. StefanBrain generates the promo ads, landing pages, and offer angles that drive sale traffic, launches them to Meta, and reads the results back. When the sale ends, score it like any campaign with the Marketing ROI Calculator.