What is a Discount Calculator?
A discount calculator answers one of two questions, and they are mirror images. Either you know the original price and the discount and want the new price, or you know both prices and want to know what discount was applied. The arithmetic is straightforward — a 25% discount leaves 75% of the price — but the order of operations with tax is where people routinely get it wrong.
Discounts apply before tax, not after. A £100 item at 20% off costs £80, and with 20% tax that is £96 — not £100 plus tax (£120) minus 20% (£96), which happens to match here only because the percentages are equal. With a 25% discount and 20% tax the two orders give £90 and £95 respectively. This tool applies the discount first and then the tax, which is how retail pricing works.
The reverse mode is the one worth having a tool for. Retailers quote a percentage and a struck-through price, so the temptation is to trust both; checking that £149 down from £199 really is a 25% saving takes a second here. It is also the fastest way to detect the classic pattern where a price was raised before being discounted.
How to calculate a discount
- Enter the original price.
- Either enter a discount percentage or a fixed amount off. Typing in one clears the other, since they are alternative inputs for the same thing.
- Add a sales tax rate if you want to see what you actually pay at the till. Leave it blank to work with the pre-tax figure.
- Read the price after the discount, the amount saved, and the effective discount percentage — the last of which matters when a discount is capped.
- Switch to What discount is this? and enter the price you paid to see the percentage that was applied.
Discount Calculator limits and rules
| Item | Limit |
|---|---|
| Discount input | A percentage or a fixed currency amount — one or the other |
| Discount cap | A fixed amount larger than the price is capped, so the result never goes negative |
| Order of operations | Discount first, then tax — the retail convention |
| Reverse mode | Derives the percentage from the original and final prices |
| Effective discount | Reported separately because a capped fixed reduction rarely equals the advertised percentage |
| Currency | A display symbol only; nothing is converted |
| Rounding | Shown to two decimals; real prices are often rounded to the nearest whole unit |
| Not included | Voucher stacking rules, member pricing, buy-one-get-one logic and shipping |
| Not covered | Compound discounts applied one after another, though the effective rate is shown for a single step |
When to use it
- Checking a sale price at the shelf, particularly where the tag shows a percentage of an old price that may since have changed.
- Working out what a purchase actually costs including tax, so the number at the till does not come as a surprise.
- Comparing two offers that are worded differently — 20% off versus £10 off — against the item price you have in mind.
- Detecting a fake discount by entering the original and current prices and seeing whether the advertised percentage matches the real one.
- Calculating a discount to apply to an invoice or a quote, where a fixed reduction is more useful than a percentage.
Is it private?
Everything runs inside your browser. Your text and files are never uploaded, never stored and never seen by anyone else — including me. You can use the tool with the network tab open and watch no request leave the page. Closing the tab erases the input.
Frequently asked questions
Before. Retail discounts reduce the taxable price, so the discount comes off first and tax is charged on what remains. Doing it the other way overstates the saving whenever the two percentages differ — with 25% off and 20% tax, the correct order gives 90 where the wrong order gives 95.
Subtract the final price from the original, divide the difference by the original, and multiply by 100. From 199 down to 149: the difference is 50, divided by 199 is 0.2513, so the discount is 25.1%. The reverse mode in this tool does exactly that arithmetic.
Because the reduction is capped at the price. An advertised £50 off a £40 item can only remove £40, making the effective discount 100% rather than whatever the tag claims. The same gap appears whenever a fixed amount is applied to a cheap item.
In everyday retail use they describe the same thing. Accounting distinguishes them: a markdown is usually a permanent reduction recorded against the item, while a discount is a reduction applied at the point of sale. For working out what you pay, the arithmetic is identical.
It handles one, but the trap with two is worth knowing: 20% then 10% is not 30% off. It leaves 0.8 × 0.9 = 72% of the price, so the effective discount is 28%. Apply the first discount, then feed the result back in as the original price for the second.
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Written and maintained by Md. Nur Hossain Repon.