Percentage change

Percentage change tells you how much a number has changed. Percentage change is a concept strikingly similar to percentage increase and percentage decrease. In simple terms: percentage change is either of them. Let's consider this problem: yesterday I had 8 pimples on my face and today it's up to 14. What's the percentage change?

If you read our other articles, you already know what is percentage and that percentage formula goes like this:
percentage = 100 * numerator / denominator, where numerator is our difference (14 - 8 = 6) and denominator is our original value (8). So the formula for percentage change is:
percentage change = 100 * difference / original value
So in our example percentage change = 100 * 6 / 8 = 600 / 8 = 75. Pimples on my face increased by 75% since yesterday.

How about the other direction? Yesterday there were 30 kids in the classroom. There was a flu epidemic and today 12 kids stayed home (one of them were not really sick, he just really likes computer games and knows how to trick the thermometer to show a higher temperature). Anyways, only 18 kids are present today. Let's try the same formula:
Percentage change = 100 * 12 / 30 = 1200 / 30 = 120 / 3 = 40. Today's attendance is 30 percent lower (a 30 percent decrease) than yesterday's. Exactly the same calculation as before, only this time the new number is lower than the original value, so we say it's a decrease.

This might help you practice: