How to Calculate a Discount: Sale Price, Savings and Stacked Offers
The formulas for the price after a discount and the amount you save, why 20% plus an extra 10% is only 28% off, and how to find the original price, with worked examples.
Published 29 October 2026
The short answer
Price after discount = original price × (1 − discount ÷ 100), so ৳2,500 with 20% off is ৳2,000, a saving of ৳500. Two discounts multiply rather than add: 20% plus an extra 10% is 28% off, not 30%.
The formula for the price after a discount
A sign that says “20% off” means you pay ৳20 less for every ৳100 of the original price. There are two formulas:
Saving = original price × discount ÷ 100
Price after discount = original price × (1 − discount ÷ 100)
- A ৳2,500 shirt with 20% off: saving 2,500 × 20 ÷ 100 = ৳500, price 2,500 × 0.80 = ৳2,000.
- ৳1,000 with 20% off: 1,000 × 0.80 = ৳800, a saving of ৳200.
The second formula is the handy one: 20% off means you pay 80%, so multiply by 0.80 in one step. 15% off is × 0.85, 25% off is × 0.75 and 40% off is × 0.60. Enter a price and a percentage in the Discount Calculator and it shows the final price and your saving together.
How much you save, and the paisa
When the price doesn’t come out in whole Taka, you get paisa. ৳999 with 15% off: 999 × 0.85 = ৳849.15, and the saving is 999 − 849.15 = ৳149.85. The two add back up to exactly ৳999, which is the easiest way to check your sum.
Round to the nearest paisa (halves go up). Round the final price first and then subtract it from the original to get the saving; that way the receipt never ends up a paisa out.
Quick mental maths
- 10% means divide by 10: 10% of ৳2,500 = ৳250.
- 20% is double that: ৳500. 5% is half of it: ৳125.
- So 15% = 250 + 125 = ৳375, and 25% = 500 + 125 = ৳625.
The same trick for 30% off ৳1,800: 10% = ৳180, so 30% = ৳540 and the price is 1,800 − 540 = ৳1,260. Having a rough figure in your head before you check the till or a calculator makes a big mistake easy to spot.
Why 20% + 10% off is not 30% off
Many offers read “20% off, plus an extra 10% with your card”. The second discount is taken off the price that is already reduced, not off the original:
- ৳1,000 with 20% off: ৳800.
- Now 10% of ৳800 = ৳80, not ৳100.
- You pay 800 − 80 = ৳720 and save ৳280, a total discount of 28%.
Final price = original × (1 − first ÷ 100) × (1 − second ÷ 100)
Total discount % = 100 − (100 − first) × (100 − second) ÷ 100
For 20% and 10%: 100 − 80 × 90 ÷ 100 = 100 − 72 = 28%. A few offers side by side on ৳1,000:
| Offer | Looks like | You pay (৳) | Real discount |
|---|---|---|---|
| 20% + 10% | 30% | 720 | 28% |
| 10% + 20% | 30% | 720 | 28% |
| 30% + 20% | 50% | 560 | 44% |
| 50% + 50% | 100% | 250 | 75% |
The order doesn’t matter, because both steps are multiplications: 10% and then 20% also leaves ৳720. And two 50% discounts never make an item free: ৳400 with 50% + 50% off is ৳100, a 75% discount. Turn on “Add an extra discount” in the Discount Calculator to see the true combined percentage next to the simple sum.
So if one shop offers a straight 30% off and another offers 20% + 10%, at the same starting price the first is cheaper: ৳700 against ৳720 on ৳1,000.
Other offers as a percentage: buy one get one, cashback
Not every offer is written as “% off”. Turning each one into a percentage makes deals easy to compare:
- Buy one, get one free: you get two items for the price of one, so each is 50% off. It is only a saving if you actually need both.
- Buy two, get one free: you get three for the price of two. At ৳300 each, ৳900 worth of goods costs ৳600, a discount of 300 ÷ 900 × 100 = about 33.33%, not 50%.
- Cashback with a cap: with “10% cashback, up to ৳150”, spending ৳2,500 would earn ৳250 at 10%, but you get ৳150, which is really 150 ÷ 2,500 × 100 = 6%. Cashback usually arrives later in your account; it doesn’t lower the price at the till.
Discounts and VAT together
When a receipt has both a discount and VAT, treat them as two separate multiplications. A ৳2,500 shirt with 20% off is ৳2,000, and 15% VAT on that makes ৳2,300. Adding VAT first and then taking the discount gives the same ৳2,300. What doesn’t work is reading “20% off and 15% VAT” as a net 5% off: 2,500 × 0.95 = ৳2,375 is wrong. To see the VAT part on its own, use the VAT Calculator.
Finding the original price from a sale price
If a tag shows only the price after the discount, work backwards. A price after 20% off is 80% of the original, so:
Original price = sale price ÷ (1 − discount ÷ 100)
If ৳2,000 is the price after 20% off, the original was 2,000 ÷ 0.80 = ৳2,500. You can do this reverse sum with the “Find the whole” option in the Percentage Calculator: 2,000 is 80% of what number?
To check, multiply back: 2,500 × 0.80 = ৳2,000. If it matches, you have the right original price. When a tag shows an old price crossed out next to a new one, the same check tells you whether the advertised discount matches the real one.
Common discount mistakes
- Adding two discounts together. 20% + 10% is 28%, not 30%; the second one comes off the lower price.
- Adding the percentage back to find the original price. ৳2,000 plus 20% is ৳2,400, not ৳2,500, because the 20% was taken off the larger price. Divide by 0.80 instead.
- Letting a discount and VAT cancel out. 20% off and 15% VAT is not 5% off; multiply them separately.
- Choosing between “৳500 off” and “20% off” without checking. A flat amount doesn’t depend on the price. At ৳2,500 they are equal; above that 20% saves more, below it ৳500 does.
- Reading “up to 50% off” as 50% off everything. Usually only a few items get the top discount; check the rate on the tag of the item you want.
To check a deal while you’re still in the shop, enter the price and percentage in the Discount Calculator. Everything is worked out on your phone, and nothing you type is sent anywhere.