Percentage change tells you how much a number grew or shrank relative to where it started. It is everywhere — prices, traffic, weight, salaries — and there is one counter-intuitive trap in it that catches almost everyone.
The formula
Percentage change is (new − old) ÷ old × 100. The key is that you always divide by the original value, not the new one. A jump from 40 to 50 is a 25% increase (10 ÷ 40), not 20% — dividing by the wrong number is the most common mistake.
How to calculate it
- Open the percentage change calculator.
- Enter the original value and the new value.
- Read whether it is an increase or a decrease, and by how much.
Why a drop and a rise don't cancel out
If a £100 stock falls 50% to £50, it then needs a 100% rise to get back to £100 — not 50%. That is because the percentages are measured against different starting points. This asymmetry is why losses hurt more than equal-sized percentage gains help.