Discount Stacking and Rebate Calculator

Twenty per cent off, then ten off what is left, then five off what is left of that.

Written down that looks like thirty-five per cent. It is thirty-one point six.

Discounts multiply rather than add, because each one comes off a smaller number than the one before. The second ten per cent is ten per cent of eighty, not of a hundred — eight pounds, not ten. The third is five per cent of seventy-two. The page shows each step against what it actually came off, which is the part that makes the arithmetic obvious rather than merely stated.

The gap always runs in the seller’s favour. The real discount is smaller than the sum of the parts, never larger, and it is worth knowing that when somebody tells you a stack has destroyed the margin — because on these figures it takes the margin from thirty-eight per cent to nine, and thirty-eight per cent off is where it would actually have run out.

That kill point is on the page too: the total discount, all in, at which the price meets the cost.

The order the discounts come in makes no difference to the total. People argue about this and there is nothing to argue about — multiplication commutes, so twenty then five leaves exactly the same price as five then twenty. It changes what each line on the paperwork looks like and not what you end up with.

A settlement discount or an end-of-year rebate goes in the same stack, because that is how it lands on the same invoice.

It will not tell you whether to agree to the deal. A stack that leaves a thin margin may be worth having if the volume is real, and a comfortable one may not be if it quietly becomes the price everybody expects next year. The page prices the arithmetic and leaves the negotiation to you.

Save as many versions as you like and put any two side by side, figure against figure, with the better one on each line marked and the difference in its own column. That screen is the one that decides things, and it is the part a free calculator cannot do at all. The same stack on a different cost base is a different answer entirely.

One HTML file. No account, no subscription, no internet needed. Opens on Windows, Mac and Android; the readme explains the extra step on iPhone and iPad, where Apple blocks JavaScript in downloaded files and every tool of this kind is affected.

Everything lives in your browser on your device. Nothing is uploaded, which is why it works offline and why nobody else can see your figures — and it also means clearing your browsing data deletes it. The tool does not pretend otherwise: the header shows how long since your last export at all times, turns amber after three days and red after seven, and export is one click that produces a plain file you keep.