How to use this calculator
- Enter the percentage of extra product.
- Enter the percentage discount you want to compare.
- Check which offer produces the lower unit price.
20% extra vs. 20% off
With 20% extra, you pay the regular price for 1.2 times the usual amount. The unit price is therefore 100 ÷ 1.2 = 83.33% of the original, which is the same as an effective discount of about 16.67%.
With 20% off, you pay 80% of the regular price for the usual amount. For the same product, 20% off is better than 20% extra by about 3.33 percentage points.
Why the same percentage gives a different result
A discount reduces the price. An extra-product offer increases the amount. Because the extra amount becomes part of the new total, “20% extra” does not mean “20% off.”
The effective discount from extra product is 100 − 100 ÷ (1 + extra percentage ÷ 100). For example, 25% extra and 20% off produce the same unit price.
Frequently asked questions
Do I need the product's price or package size?
No. If both offers apply to the same product at the same regular price, the two percentages are enough. If the brands, sizes, or regular prices differ, compare the actual unit prices instead.
Is 25% extra really equal to 20% off?
Yes. A 25% increase makes the amount 1.25 times as large. Dividing the price by 1.25 makes the unit price 80% of the original, the same as 20% off.
Should I always choose the mathematically better deal?
Not necessarily. Extra product is only useful if you can use or store it. Product quality, expiration dates, and the amount you actually need can matter more than a small difference in unit price.