What promotions cost
Your promotional revenue is what shoppers paid after the discount. Grossing it back up to full price shows what the discount actually cost you – promotional revenue × depth ÷ (1 – depth).
Promotions lift volume, and the lift is easy to see. What nobody sees is the share of that discount handed to shoppers who would have bought anyway. Put four numbers you already know into the calculator below and get an indicative annual margin-leak range – in your own numbers, in under a minute, no form required.
Promotional performance is usually judged on uplift – and uplift flatters. When a promotion sells 40% more units, the trading meeting celebrates. Whether half of those units would have sold at full price anyway rarely makes the slide. That difference is not a rounding error: across UK retail, a large share of promotional spend generates no incremental volume at all. The calculator makes that invisible cost visible – in your numbers, not an industry average.
Every step of the maths is shown – no proprietary scoring, no black box. The calculator is free to use without giving us anything; the emailed copy of your result is the only thing behind a form.
The model is deliberately simple enough to check on a napkin – because a number you can interrogate is worth more than a precise-looking one you can't.
Your promotional revenue is what shoppers paid after the discount. Grossing it back up to full price shows what the discount actually cost you – promotional revenue × depth ÷ (1 – depth).
Cannibalisation is the share of promoted volume that would have sold at full price anyway. The discount handed to those shoppers bought you nothing – that spend is the leak.
The model ignores basket effects and supplier funding (which offset the leak) and forward-buying (which worsens it), so we publish a ±25% band rather than false precision. The real number needs your transaction data.
Common questions we get from senior technology leaders evaluating this work. Direct answers, no hedging. Open one to read in full.
Numbers most commercial directors can quote from memory: annual revenue, the share of sales sold on promotion, your average discount depth, and an estimate of cannibalisation – the share of promoted volume that would have sold at full price anyway. If you have never measured cannibalisation, published grocery studies typically land between 30% and 70%; start in the middle and test the sensitivity with the slider.
It is an honest indicative range, not a measurement – that is why we publish a ±25% band rather than a single confident-looking number. The model deliberately ignores basket effects and supplier funding (which offset the leak) and forward-buying (which worsens it). Its job is to tell you whether the question is worth asking properly, in your own numbers rather than an industry average.
No. The calculator is free to use and the maths is shown in full – nothing is gated. The only thing behind a form is the emailed copy of your result, which is useful if you want the number in your inbox for the next trading meeting.
That is what the Retail Commercial Capability diagnostic is for: baseline modelling on your own transaction data, run by senior operators who have held commercial P&Ls in major retailers. Fixed scope, findings in weeks, and the engagement is designed to pay for itself out of the leak it finds. The calculator tells you whether that conversation is worth having; the diagnostic tells you the real number.
The slider values never leave your browser while you explore. If you ask for the emailed copy, we store that submission – your contact details and the inputs – to send the result and follow up if you ask us to. Reply to any email from us and we’ll delete it.
Our Retail Commercial Capability diagnostic runs the same question properly – baseline modelling on your own transaction data, run by senior retail operators, fixed scope, findings in weeks. If the calculator's range made you wince, the diagnostic tells you the real number and what to do about it.