Why 20% + 10% Isn’t 30%

When you apply a 20% increase followed by a 10% increase, the second increase is calculated on the new value, not the original one. Starting at 100, a 20% increase gives 120. A further 10% increase on 120 gives 132. The overall change from 100 to 132 is 32%, not 30%.

How to Add Percentages Correctly

To combine two percentage changes, turn each change into a factor. For example, +20% becomes 1.20, and +10% becomes 1.10. Multiply the factors (1.20 × 1.10 = 1.32) and then convert the combined factor back into a percentage change by subtracting 1 and multiplying by 100.

Common Mistakes with Adding Percentages

A very common mistake is simply adding percentage figures together without accounting for compounding, especially with discounts, interest, and year‑on‑year price changes. Another mistake is forgetting that a 20% increase followed by a 20% decrease does not bring you back to the starting point; it leaves you below where you started.

Adding Percentages Calculator

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