A 20% discount feels reasonable. Customers love it. Sales volume jumps. The dashboard looks great.
Then the monthly P&L arrives and the profit number is lower than the month before, despite higher revenue.
This happens because discounting operates on margin, not on revenue. The math is straightforward once you see it — and most sellers never look at it until after the damage is done.
The Core Formula
The number of additional units you need to sell to maintain the same profit after a discount is determined by your contribution margin — the amount left over from each sale after variable costs.
Break-even volume increase = Discount% / (Original Margin% − Discount%)
Or in plain terms: if you currently make 40 cents on every dollar of revenue (40% margin) and you discount by 20%, you need to sell 100% more units just to make the same total profit.
The Discount × Margin Matrix
| Discount | Gross Margin 20% | Gross Margin 30% | Gross Margin 40% | Gross Margin 50% |
|---|---|---|---|---|
| 5% off | +33% more units | +20% more units | +14% more units | +11% more units |
| 10% off | +100% more units | +50% more units | +33% more units | +25% more units |
| 15% off | +300% more units | +100% more units | +60% more units | +43% more units |
| 20% off | N/A (loss) | +200% more units | +100% more units | +67% more units |
| 25% off | N/A (loss) | N/A (loss) | +167% more units | +100% more units |
| 30% off | N/A (loss) | N/A (loss) | N/A (loss) | +150% more units |
"N/A (loss)" = selling below variable cost — every additional unit loses money regardless of volume.
Example: A Shopify seller with 40% gross margin runs a 20% off sale. To make the same monthly profit as before the discount, they need to sell exactly double the units. If they typically sell 200 units/month, they need 400 units this month. If the discount drives 250 units instead — a 25% volume increase that any marketer would celebrate — they're still less profitable than the pre-sale baseline.
Why Sellers Consistently Underestimate This
Three cognitive biases make discount math chronically underappreciated:
Revenue anchoring: Revenue is the number that's reported, celebrated, and tracked on most dashboards. A 30% revenue increase from a sale feels like success — even if profit is flat or negative.
Volume optimism: Sellers consistently overestimate how much volume a discount will drive. Price elasticity varies enormously by product category, competitive environment, and customer base. A 20% discount on a commodity product in a crowded category might drive 10% more volume. The math requires 100%.
Hidden costs of volume: More volume at a discount means more fulfillment cost, more customer service load, and often more returns. These costs offset revenue gains further.
When Discounting Actually Works
Discounts are not inherently destructive. They create positive economics when:
Clearing aged inventory: Selling an item at 60% margin instead of 40% is bad. Selling it at 15% margin to recover cash from inventory that's otherwise heading to disposal is better than the alternative.
Customer acquisition: A first-purchase discount that converts a customer who will make repeat purchases at full margin has a positive LTV calculation — the discount is a CAC substitute.
Competitive response: When a direct competitor runs a deep discount, matching it may be a defensive necessity even at margin cost.
Flash sales with real scarcity: Limited-time offers with genuine scarcity (not manufactured urgency) can generate volume spikes large enough to justify the margin compression — but this requires the volume math to work.
The Pricing Alternative: Strategic Bundling
Bundling increases average order value without cutting per-unit margin on individual items. A $20 product bundled with a $15 complementary product at $30 (instead of $35 à la carte) offers a 14% perceived discount while maintaining or improving total margin per transaction.
Most sellers who run regular discounts haven't modeled bundling as an alternative. The math consistently favors bundling for margin preservation while still delivering a "deal" perception.
The 10% Rule
A practical heuristic from experienced e-commerce operators: Never discount more than 10% without modeling the required volume increase first.
At 10% discount with 40% margin, you need 33% more units — challenging but achievable in a real sale.
At 20% discount with 40% margin, you need 100% more units — rarely achievable from discounting alone, meaning the sale is almost certainly margin-negative.
Calculate It Yourself
The Discount Impact Calculator shows you exactly how many additional units you need to sell to break even on any discount, given your current margin. Enter your selling price, cost, and discount percentage — and see the true cost of your next promotion before you run it.