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Percentage Change
Why Percentage Change Matters
Whether you're tracking your investment portfolio, calculating a discount, or measuring weight loss, raw numbers don't tell the whole story. A $10 increase is huge if you started with $5, but tiny if you started with $10,000. Percentage change gives you the context.
The Simple Formula
💡 Pro Tip from Dr. Alex M., Ph.D.
"Be careful with 'Percentage Points' vs 'Percent'. If interest rates go from 2% to 4%, that's a 2 percentage point increase, but a 100% increase in the rate itself. News outlets confuse this all the time. Don't be like them."
⚠️ Common Mistake: The Reverse Calculation
If a stock drops 50%, it needs to gain
100% to get back to even.
Example: $100 → $50 (50% loss).
$50 → $100 (needs +$50, which is 100% of $50).
Losses hurt more than gains help!
Reviewed by Dr. Alex M., Ph.D.
Mathematics Professor
Last updated: November 2025
Frequently Asked Questions
How do I calculate percentage increase?
Subtract the original number from the new number. Then, divide that result by the original number. Finally, multiply by 100. Example: From 10 to 15. (15-10)/10 = 0.5. 0.5 * 100 = 50% increase.
Can percentage change be negative?
Yes! If the new number is smaller than the old one, you'll get a negative percentage. This represents a percentage decrease (or loss).
What if the original number is 0?
Mathematically, you can't divide by zero, so the percentage change is undefined. However, in business terms, going from $0 revenue to $100 is often called 'infinite growth' or just stated as absolute growth.
Is 100% increase the same as doubling?
Yes. If you have $50 and it increases by 100%, you add another $50, giving you $100 total. That's exactly double.
How do I calculate percentage decrease?
Same formula! (New - Old) / Old. If you go from 100 to 80: (80-100)/100 = -20/100 = -20%. That's a 20% decrease.