Promotion math
Flash sale break-even.
Enter your price, costs, discount and the units you would normally sell in the same window. See the units the sale must sell, the multiple of normal volume, and the deepest discount that still earns.
Break-even units = (full-price profit for the window + sale costs) / profit per unit at the sale price. The example below needs 543 units, 2.71 times normal volume, at 30% off.
Your numbers
Result
Worked example
A $50.00 item costs $20.00 and $4.00 to pick, pack and ship, with 3% in fees, so it earns $50.00 x 0.97 - $20.00 - $4.00 = $24.50 at full price. In the sale window the store would normally sell 200 units for $4,900. At 30% off the price is $35.00 and each unit earns $35.00 x 0.97 - $24.00 = $9.95. After $500 of sale costs the sale must earn $5,400, which takes 543 units (rounded up), 2.71 times normal volume. Selling 600 units earns 600 x $9.95 - $500 = $5,470, or $570 more than full price. Above a 50.52% discount each unit loses money.
How it works
Profit per unit = price x (1 - fee %) - unit cost - per-unit costs. Payment fees come off the price actually charged, so a deeper discount also shrinks the fee, but shipping and packaging do not shrink at all.
Break-even units = (units normally sold x full-price profit per unit + fixed sale cost) / sale profit per unit. This compares the sale against doing nothing in the same window, so the "normal volume" you enter is what you would have sold anyway at full price.
Deepest discount = 1 - (unit cost + per-unit costs) / (price x (1 - fee %)). Past that point every unit is sold at a loss before any fixed cost.
The tool does not model sales that are only pulled forward from next week, returns, or the shoppers who wait for the next sale. Those make the real break-even worse, so treat the result as a floor and, if you can, enter a lower normal volume only when you are sure the window really is slow.
Sources
Retrieved 2026-10. The formulas are arithmetic on your inputs; the sources below are the rules and standards this page refers to.
- FTC Guides Against Deceptive Pricing, 16 CFR Part 233: a "was" price used for a sale comparison should be the actual, bona fide price at which the item was offered to the public on a regular basis for a reasonably substantial period (section 233.1(a)). This tool does not set that reference price for you.
Everything runs in your browser. Nothing you type is sent or stored. This is a planning aid, not legal or accounting advice.
FAQ
How many units does a 30% off sale need?
It depends on your margin. At a 49% full-price margin and 30% off, each unit earns about 40% of what it did, so volume has to rise by roughly 2.5 times before fixed costs. Enter your figures above for the exact number.
What should I enter as normal volume?
The units you would sell in the same hours or days at full price, using recent history for the same weekday and season. A sale only creates profit on the units above what you would have sold anyway.
Does the tool count customers pulling purchases forward?
No. Some sale buyers would have bought next week at full price, which means the extra units are fewer than they look. Reduce the expected units you enter to reflect that.
Can I use any "was" price in the ad?
The FTC pricing guide says a former-price comparison should reflect the actual price offered for a reasonably substantial period. Check the rules that apply where you sell.