A 10% discount can remove much more than 10% of contribution

Discounts reduce revenue immediately, while many costs do not move. The useful question is not only whether a promotion creates more orders. It is whether the extra orders replace the contribution lost on every discounted sale and still leave enough capacity and cash to fulfil them.

Start with the price customers normally pay and subtract only costs caused by making one more sale: materials, fulfilment, marketplace commission, card fees, sales commission and other genuinely variable costs. The result is contribution per sale. Fixed overhead belongs in the later break-even check, not inside the unit contribution calculation.

Work out the contribution before and after the discount

Suppose a product sells for £100 excluding VAT and has £60 of variable cost. Its contribution is £40 and its contribution margin is 40%. A 10% discount reduces the selling price to £90. If variable cost stays at £60, contribution falls to £30—a 25% reduction in contribution per sale even though the customer price fell by 10%.

Use the Contribution Margin Calculator for the normal price and again for the discounted price. Keep VAT treatment consistent and use the expected realised price after all discounts rather than an optimistic list price. If fees are a percentage of revenue, update those too instead of assuming every variable cost is unchanged.

Calculate the volume needed to stand still

Divide the original contribution per sale by the discounted contribution per sale. In the example, £40 divided by £30 equals 1.333. The business therefore needs about 33.3% more sales at the discounted price to produce the same total contribution as before.

The effect becomes sharper as the discount approaches the original contribution. A 20% discount takes the price to £80 and contribution to £20. The business then needs twice as many sales to stand still on contribution. If the discounted price is no higher than variable cost, extra volume cannot repair the unit economics because each additional sale contributes nothing or loses money.

  • Required volume multiplier = original contribution per sale ÷ discounted contribution per sale.
  • Required percentage increase = (volume multiplier − 1) × 100.
  • Contribution-neutral is only a floor; it does not cover extra campaign or capacity costs.

Add the costs created by the promotion

A promotion may require advertising, packaging changes, temporary labour, expedited delivery, additional returns handling or support. Add those costs to the campaign decision rather than allowing the discounted price to carry them invisibly. If £2,000 of campaign cost is expected and discounted contribution is £30 per sale, nearly 67 additional sales are needed just to recover that £2,000.

Check whether the business can supply the required volume without overtime, quality problems or delayed orders. A mathematically contribution-neutral promotion can still damage cash flow when stock and supplier payments occur before customer receipts, or when the extra demand displaces full-price sales that would have happened anyway.

Define the test before publishing the offer

Write down the eligible products, customers, dates, normal price, discounted price and stop condition. Compare the promotion with a credible baseline rather than last week's sales in isolation. Record units, realised price, contribution, new versus existing customers, returns, fulfilment cost and repeat purchases after the offer ends.

Run a small reversible test when practical. A discount that attracts existing customers who would have bought at full price is different from one that wins profitable new demand. Also compare alternatives such as a bundle, minimum order, limited service tier or added benefit; these may improve the offer without reducing contribution on every unit.

Present the customer price clearly

For consumer offers, review how the normal price, discount and total payable amount are shown. The CMA's current price-transparency guidance says businesses must give complete and accurate pricing information and include mandatory fees, taxes and charges in the total price where required. Do not use an attractive discounted headline and reveal unavoidable charges later.

Keep evidence for any reference price or saving claim and make the terms prominent. Then place the expected receipts, stock purchases and campaign spending into the Cash Flow Forecast Calculator and use the Break-even Calculator to test whether the wider business still covers fixed costs. This guide supports commercial planning and is not legal, tax or accounting advice.

Official sources

Tool IQ provides general educational information and calculation support. It is not financial, tax, legal or accounting advice.