Percentage Calculator
Choose a formula type, enter values, and click Calculate.
Percentage Formula (Quick Reference)
- What is X% of Y? →
(X ÷ 100) × Y - X is what percent of Y? →
(X ÷ Y) × 100 - Percent change from old to new →
((new − old) ÷ old) × 100 - Original value before increase →
final ÷ (1 + p/100) - Original value before decrease →
final ÷ (1 − p/100)
How to Use This Calculator Formula for Percentage
This page gives you a practical calculator formula for percentage in four common situations: finding a percentage of a number, finding what percent one number is of another, calculating percentage increase/decrease, and reversing a percentage change to get the original value.
Pick the formula type in the dropdown, enter your numbers, and click Calculate. The tool returns both the answer and the exact formula used so you can verify the math.
Understanding the Core Percentage Formula
1) Find a percentage of a number
If you want to find 30% of 250, convert 30% to decimal first: 30% = 0.30. Then multiply:
0.30 × 250 = 75.
In formula form: (30 ÷ 100) × 250 = 75.
2) Find what percent one number is of another
Suppose 45 students passed out of 60. The percentage is:
(45 ÷ 60) × 100 = 75%.
This is the formula you use for scores, completion rates, and conversion rates.
3) Percentage increase or decrease
If a price goes from 80 to 100, the change is +20. Divide by the old value:
20 ÷ 80 = 0.25. Multiply by 100:
25%. So the price increased by 25%.
If the value drops from 100 to 80, the same formula gives -20%, which means a 20% decrease.
4) Reverse percentage to find original amount
If a final amount is 120 after a 20% increase, the original was:
120 ÷ 1.20 = 100.
If a final amount is 80 after a 20% decrease, the original was:
80 ÷ 0.80 = 100.
Where These Formulas Are Used
- Shopping discounts and sales tax
- Budget planning and monthly spending analysis
- Investment returns and interest growth
- Grade calculations and exam performance
- Business metrics like growth rate and margin change
Common Percentage Mistakes to Avoid
- Using the wrong base: In percent change, always divide by the old value.
- Forgetting decimal conversion: 7% is
0.07, not 7. - Ignoring sign: A negative result means decrease, positive means increase.
- Division by zero: You cannot calculate a percent of a whole value of 0 in formulas that divide by the whole.
FAQ: Calculator Formula for Percentage
How do I calculate 15% of 300 quickly?
Use (15 ÷ 100) × 300 = 45.
How do I know if it is increase or decrease?
In the percent change formula, a positive answer is increase and a negative answer is decrease.
Can this calculator handle decimals?
Yes. You can enter decimal percentages and decimal values in every mode.