Black Friday Discounts: How Much Extra Volume You Need to Break Even

A Black Friday discount feels like a volume decision, but it is a margin decision first. Cut your price and every unit earns less, so you need more units just to stand still. This guide gives you the formula and a worked example so you can say yes or no to a promotion before you agree to it.

The core idea

Your unit contribution is what a sale earns after all the per-unit costs: product cost, fees, fulfilment, shipping and so on. A discount comes straight out of that contribution because the costs do not fall when the price does. To break even on the promotion you need enough extra units to make up the lost contribution on every unit.

The formula: required volume = old contribution divided by new contribution. A figure of 2 means you need to double your units.

A worked example

These figures are illustrative. A product sells for £25 excluding VAT and earns £9.25 per unit after all costs, which is 37% of the price. At 10% off, the price falls by £2.50, so each unit earns £6.75. You need 9.25 divided by 6.75, or about 1.37 times your usual units, around 37% more, just to match your normal profit.

At 20% off, the price falls by £5, so each unit earns £4.25. You now need 9.25 divided by 4.25, or about 2.18 times your usual units, around 118% more. A promotion that doubles sales would still earn less than a normal week.

What this tells you

• The deeper the discount, the faster the required uplift grows. Going from 10% to 20% off roughly triples the extra volume you need.
• Thin-margin products cannot afford a discount at all. If a discount takes your contribution close to zero, no amount of volume will make it pay.
• Discounts only make sense with a clear aim: clearing stock, winning new customers, or raising ranking on a key product.

Things that make it worse

• Higher ad spend during the promotion, which cuts contribution further.
• Peak fulfilment fees on Amazon from 15 October 2026 to 14 January 2027.
• Returns, which reverse the sale after you have paid the discount, the fees and the shipping.
• Stock that runs out, because uplift you cannot fulfil is wasted.

How to decide

For each product you plan to discount: calculate contribution with the discount, work out the uplift needed, then ask whether you can realistically reach it with your stock, traffic and budget. If not, discount less, or leave the price alone. Doing this per SKU and per channel is easier when your profit data is already in one place, which is what WhatSales provides.

How do I calculate the sales uplift I need for a discount?

Divide your current profit per unit by your profit per unit after the discount. The result is the multiple of your usual units you need to match your normal profit.

Is it worth discounting on Black Friday?

Only where the product margin can afford it or where you have a specific goal such as clearance or customer acquisition. Judge it by profit, not by sales volume.

Should VAT be included in the calculation?

Work on the price excluding VAT, because VAT is not your income, and keep the same basis throughout.