Mastering Percentages: Increase, Decrease and Percentage Change
Percentages trip up nearly everyone at some point. Master these few patterns and you will handle discounts, tips, taxes and statistics with confidence.
By ToolJolt Team · May 26, 2026
Percentage of a number
To find X% of a number, multiply by X/100. So 15% of 240 is 240 × 0.15 = 36. This single move covers tips, taxes, commissions and most everyday percentage questions.
Percentage change
Percentage change = (new − old) ÷ old × 100. If a price rises from 80 to 100, that is (100 − 80) ÷ 80 × 100 = 25% increase. A drop from 100 to 80 is (80 − 100) ÷ 100 × 100 = −20%. Note the two are not symmetric — which surprises many people.
The asymmetry trap
A 50% drop needs a 100% rise to get back to where you started, not another 50%. This matters for investments, sales figures and statistics. Always divide by the original (starting) value, not the new one.
Reverse percentages
If a price after 20% tax is 120, the original was 120 ÷ 1.20 = 100 — not 120 minus 20%. To undo a percentage you divide by (1 + rate), which is the step people most often get wrong.
Let a tool handle it
ToolJolt's percentage calculator does each of these — percentage of, percentage change, and reverse — with the working shown, so you can check your own arithmetic.
Free tools mentioned in this guide
Frequently asked questions
How do I calculate percentage change?
Subtract the old value from the new value, divide by the old value, then multiply by 100. A positive result is an increase, negative is a decrease.
Why isn't a 50% loss fixed by a 50% gain?
Because the gain is calculated on the smaller, reduced amount. After a 50% loss you need a 100% gain to return to the original value.
How do I remove tax from a price?
Divide the tax-inclusive price by (1 + tax rate). For 20% tax, divide by 1.20 — do not simply subtract 20%.