How to calculate percentages
A percentage is simply a fraction expressed as parts per hundred. The most common calculation is finding X% of Y, which equals (X ÷ 100) × Y. For example, 20% of 150 equals (20 ÷ 100) × 150 = 30. This is useful whenever you need to find a portion of a whole — like a tip, a discount, or a commission.
What percentage is X of Y?
To find what percentage one number is of another, divide X by Y and multiply by 100. For example, 30 out of 150 is (30 ÷ 150) × 100 = 20%. This mode is helpful for test scores, tax rates, budget allocations, and any situation where you want to express a ratio as a percent. Use the "X is what % of Y" tab above for this calculation.
Percentage change calculation
Percentage change measures how much a value has increased or decreased relative to its starting value. The formula is ((new − old) ÷ |old|) × 100. A positive result means an increase; a negative result means a decrease. This is widely used in finance (stock price changes, revenue growth), science (experimental results), and everyday comparisons like price changes at the grocery store.
Common percentage mistakes to avoid
A frequent error is reversing the percentage change formula — always divide by the original (starting) value, not the new value. Another common mistake is applying percentages in sequence: a 20% increase followed by a 20% decrease does not bring you back to the original number; it results in a 4% net decrease. Our calculator handles all three modes correctly and shows the result immediately as you type.