Discount and sale price calculator
Three discount tools in one page, for shoppers comparing deals, retailers pricing promotions, and anyone checking whether a “stacked” offer is as good as it sounds. Pick a mode, enter your numbers, and the sale price, amount saved and effective percentage off appear instantly.
How it works
- Forward — given an original price and a discount, the sale price is
original × (1 − discount ÷ 100)and the amount saved isoriginal − sale. - Reverse — given the original and the sale price, the percentage off is
(original − sale) ÷ original × 100. - Stacked — each discount is applied to the already-reduced price in turn:
final = original × (1 − d1 ÷ 100) × (1 − d2 ÷ 100) × (1 − d3 ÷ 100). The effective percentage off is(original − final) ÷ original × 100. The tool compares this against the naive sum of the percentages so you can see why stacking gives less than the headline numbers suggest.
Example
A £200 coat is 20% off, then a member coupon takes a further 10% off the reduced
price. Stacking gives 200 × 0.80 × 0.90 = £144, a saving of £56 — an effective
28% off, not the 30% you would get by adding 20% and 10% together.
| Discounts applied | Effective % off | £200 becomes |
|---|---|---|
| 10% + 10% | 19% | £162.00 |
| 20% + 10% | 28% | £144.00 |
| 25% + 20% | 40% | £120.00 |
| 50% + 50% | 75% | £50.00 |
All calculations happen instantly in your browser. No data is uploaded or stored.
When to use each mode
Forward mode is the go-to for shopping. You see a price tag of £85 and a “30% off” sign — enter 85 and 30 and you instantly know the till price (£59.50) and how much you save (£25.50). There is no need to do mental arithmetic while standing in a queue.
Reverse mode answers the question retailers sometimes make deliberately opaque: “how big a discount am I really getting?” Enter the original price and the sale price and the true percentage pops out. It is also useful when you already bought something and want to know the deal you got.
Stacked mode is where the calculator earns its keep. Retail promotions regularly layer offers: a seasonal 25% sale plus a loyalty-card 10% extra, or a trade discount on top of a clearance price. The instinct to add percentages together is wrong because each successive discount applies to the already-reduced amount. The tool shows you both the naive sum and the true combined saving so you can see exactly how much less you are saving than the headline implies.
Common mistakes and edge cases
- Percentage off vs. percentage of: “90% off” and “10% of the price” mean the same thing. If you see an offer worded as “pay only X% of the price,” subtract that from 100 to get the discount percentage for forward mode.
- Tax applied after discount: In many jurisdictions sales tax or VAT is added after the discount is calculated, so the taxable base is the sale price, not the original. This calculator works on pre-tax prices; add the tax afterwards.
- Sequential vs. simultaneous discounts: This tool models sequential (multiplicative) stacking, which is how almost all real retail offers work. True simultaneous discounts applied to the same base would be additive, but that is rare in practice.
- Rounding: retailers often round individual steps to the nearest cent or pence, so the till receipt may differ from this tool by a penny or two. The tool shows the mathematically precise result before any rounding.