How Discounts Stack (and Why They Are Not Simply Additive)

By the Toolbench team · Updated September 2026

Seeing "20% off" followed by an additional "10% off" often leads people to assume the total discount is 30%. It is not, and the actual math consistently favors the seller slightly more than that intuition suggests.

Why stacked discounts are not additive

Each discount applies to whatever the current price is at that moment, not to the original price. A second discount applied after the first is calculated on an already-reduced number, which means it removes a smaller absolute amount than it would have on the original price.

A worked example

Take an item priced at Rs 1,000. A 20% discount brings it to Rs 800. A further 10% discount is then applied to that Rs 800, not the original Rs 1,000, removing Rs 80 and landing at Rs 720. The combined effect is a 28% discount off the original price, not the 30% that simple addition would suggest.

The general pattern

Stacked percentage discounts always produce a smaller combined discount than adding the percentages directly, and the gap grows larger as the individual discount percentages grow larger. This is a direct consequence of each step operating on a shrinking base.

Why this matters when comparing offers

Two stores advertising "20% + 10% off" and "28% off" respectively are, in this example, offering the exact same final price, even though one sounds more generous at first glance. Recognizing this prevents being misled by how a discount is framed rather than what it actually delivers.

Check the real price

The Discount Calculator shows the final price and total savings for a single discount instantly, and can be applied twice in sequence to check a stacked offer the same way this example did.