Discount & Promotion Margin Calculator

A 30% discount does not cost you 30% of your profit. On a typical marketplace product it costs closer to 47% — and on thin-margin items a discount that looks generous can put every order underwater.

The reason is arithmetic that most promotion planning skips: your costs do not discount themselves. Your product cost is fixed. Your fixed per-order fee is fixed. Only revenue falls, so the entire discount comes out of the profit line.

Why the damage is disproportionate

Take a $50 product with $15 COGS on a marketplace charging 13.25% + $0.40:

At full price:

• Revenue $50.00
• COGS −$15.00
• Fees −$7.03
• Profit $27.97 (55.9% margin)

At 30% off ($35.00):

• Revenue $35.00
• COGS −$15.00 (unchanged)
• Fees −$5.04 (falls slightly with price)
• Profit $14.96 (42.7% margin)

You cut the price by 30% and lost 46.5% of the profit. The percentage fee shrank by $1.99 — a small consolation — while COGS and the $0.40 fixed fee did not move at all.

The Discount Margin Calculator runs this for your own numbers, including the volume you would need to make up the difference.

The multiplier: how much extra volume you need

The real question is not "what does one discounted sale earn?" but "does the promotion earn more in total?"

At $27.97 profit per unit, 100 full-price sales earn $2,797. At $14.96 per unit you need 187 sales to match — 87 extra sales, an 87% volume lift, just to stand still.

Here is how the requirement escalates on the same product:

Discount	Profit/unit	Margin	Extra sales per 100 needed
0%	$27.97	55.9%	—
10%	$23.63	52.5%	19
20%	$19.29	48.2%	45
30%	$14.96	42.7%	87
40%	$10.62	35.4%	164
50%	$6.28	25.1%	345
60%	$1.94	9.7%	1,342

A 50% off sale needs to more than quadruple your volume to be profit-neutral. A 60% sale needs fourteen times the volume. Almost no promotion delivers that — which means most deep discounts are volume theater that trades profit for a bigger revenue number.

The maximum discount you can afford

There is a price below which every sale loses money:

Break-even price = (COGS + Fixed fee) ÷ (1 − Fee rate)

For the example: ($15 + $0.40) ÷ (1 − 0.1325) = $17.75. That is a 64.5% discount — the absolute floor. Anything deeper and you pay customers to take your product.

Note how much closer this floor sits than intuition suggests on thinner margins. A product with 30% contribution margin cannot survive past roughly a 30% discount. A 20% margin product breaks at about 20% off. Your maximum discount is roughly your contribution margin percentage — a rule of thumb worth memorizing before agreeing to any promotional calendar.

When discounting is genuinely the right call

The math above argues against casual discounting, not all discounting. Four cases justify it:

Clearing dead inventory. Product occupying shelf or FBA storage costs money every month. Selling at 60% off recovers cash and stops the bleed — the comparison is against writing it off, not against full price.

Customer acquisition with real repeat purchase. If a first-time buyer reliably returns, a loss-making first order can be rational. This requires actual cohort data, not optimism. Verify with the same logic as breakeven ROAS and lifetime value.

Volume tiers that reduce your costs. If selling more genuinely lowers your landed cost or shipping cost per unit, the discount partially funds itself. Model it with the Landed Cost Calculator.

Genuine competitive necessity. Sometimes a price is the price. Just be honest that you are defending share, not making money, and know how long you can sustain it.

Promotion tactics that cost less than a price cut

Most goals people use discounts for have cheaper solutions:

Free shipping threshold. "Free shipping over $75" raises average order value instead of cutting price. It costs you the shipping on larger orders while increasing basket size — usually far cheaper than an equivalent percentage off.

Bundles. Selling three units at a 15% bundle discount often nets more profit than one at full price, and the fixed per-order fee is spread across more revenue.

Gift with purchase. A $4 add-on feels like meaningful value and costs you $4 — far less than a 20% discount on a $50 product ($10).

Loyalty credit for the next order. Defers the cost, drives repeat purchase, and a meaningful share is never redeemed.

Time-limited over deep. A 15% flash sale with real urgency frequently outperforms 30% with no deadline, at half the margin cost.

Marketplace-specific traps

Amazon: coupons cost the discount plus a redemption fee per unit. Lightning Deals carry a separate fee. Both need adding to the calculation before you evaluate the promo.

Etsy: running sales through Etsy's tools does not change the 6.5% transaction fee — it applies to the discounted price, which helps slightly, but Offsite Ads fees (12–15%) on the same order can dwarf your discount margin.

eBay: promoted listings ad rates apply on top of a discounted price. A 20% discount plus a 6% ad rate is a 26% total hit.

Shopify: the cheapest place to discount, since you keep the 2.9% + $0.30 structure with no marketplace commission on top. Your own store is where promotions do the least damage.

BNPL (Klarna, Afterpay): 3–6% per transaction. Stacking BNPL on a discounted price can eliminate the remaining margin entirely.

Getting your inputs right

The calculation is only as good as your fee and cost figures:

• Use landed cost, not supplier cost, for imported goods — see the Landed Cost Calculator
• Use your real fee rate including payment processing — the eBay, Etsy, and Stripe calculators give exact figures
• Include the fixed per-order fee — it is what makes deep discounts on low-ticket items so damaging
• Account for return rate — discounted sales often return at similar or higher rates, and you eat the fees either way

A practical framework

Before approving any promotion:

1. Compute profit per unit at the discounted price. If it is negative, stop unless you are deliberately clearing stock.
2. Compute the volume lift needed to hold total profit flat.
3. Ask honestly whether that lift is realistic. Past promotions are the best evidence. Most businesses find their actual lift is 20–60%, which supports discounts around 10–20%, not 40%.
4. Check the maximum affordable discount so you know where the floor is before negotiating with anyone.
5. Consider a cheaper alternative from the tactics above.
6. Measure after the fact — actual units sold, actual profit, versus a normal period. Repeat what worked; retire what did not.

The bottom line

Discounts are a legitimate tool used deliberately and an expensive habit used casually. The difference is knowing, before you commit, that a 30% discount needs 87% more sales and that your floor is 64.5% off — not discovering it in the month-end numbers.

Related calculators

• Profit Margin Calculator — establish the margin a discount eats into
• Breakeven ROAS Calculator — the ad math on discounted products
• Landed Cost Calculator — get COGS right before discounting against it
• Break-Even Calculator — volume needed to cover fixed costs