Percentage change is the most common calculation in numerical tests, and the easiest to get slightly wrong.
- Percentage change = (new − old) ÷ old × 100. Always divide by the old value, the one you start from.
- Percentage points are for rates and shares. A share that moves from 12% to 15% has gone up 3 percentage points, which is a 25% increase.
- To reverse a percentage, divide. If a price after a 20% rise is $96, the original is 96 ÷ 1.2 = $80, not $76.80.
- Growth compounds. Two years of 8% growth is × 1.08 × 1.08 = × 1.1664, a rise of 16.64%, not 16%.
Before you calculate, decide whether the answer should be bigger or smaller than where you started, and by roughly how much. That one check catches most slips.