How to Calculate Percentages — Without Mixing Up the Formula
Four common percentage questions, the formula behind each one, and a free calculator so you never have to remember which number goes where.
Percentage math is simple once you know which formula applies — the tricky part is that four fairly different everyday questions all get called "percentage calculations," and each one uses the numbers in a different order. Here's a quick reference for each, plus a free calculator that handles all four without needing to remember the formula.
Finding X% of a Number
This is the most common case — "what is 15% of 200?" The formula is:
For 15% of 200: (15 ÷ 100) × 200 = 30.
Finding What Percent One Number Is of Another
This is the reverse question — "30 is what percent of 200?" Common for test scores, survey results, or figuring out what share something represents.
For 30 out of 200: (30 ÷ 200) × 100 = 15%.
Percentage Increase and Decrease
This measures how much something changed relative to where it started — a salary, a stock price, a weight, a business metric.
Going from 80 to 100: ((100 − 80) ÷ 80) × 100 = 25% increase. Note the change is always measured against the starting value — going from 50 to 100 is a 100% increase, not 50%, because 100 is double the original 50.
Discounts, Tips, and Adding Tax
To find a final price after a discount, or a total after adding a tip or tax, calculate the percentage amount first, then add or subtract it from the original value:
Discount: value − (value × percent ÷ 100) = final price
Adding tax/tip: value + (value × percent ÷ 100) = final total
For a ₹500 item with a 20% discount: 500 − (500 × 20 ÷ 100) = ₹400.
Skip the manual math
Try the Percentage Calculator Free →Frequently Asked Questions
Why is going from 50 to 100 a 100% increase, not 50%?
Percentage change is measured relative to the starting value. Since 100 is exactly double 50, that's a 100% increase over the original amount, not a flat 50-point difference expressed as a percent.
What's the fastest way to calculate a discount?
Multiply the price by the discount percent, divide by 100, then subtract that from the original price — or use the Add/Subtract mode on the calculator to skip the manual steps.
How do I calculate a percentage decrease?
Same formula as an increase — ((new − old) ÷ old) × 100 — the result comes out negative when the new value is lower, which represents the decrease.
Can percentages be negative?
Yes — a negative percentage in a change calculation simply means a decrease rather than an increase.
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