By the Tooliday Team. Last updated 14 November 2026.
Percentages show up everywhere: discounts in a shop, GST on a bill, marks in an exam, a salary hike. Most of us can do the easy cases in our heads, but the moment a question is phrased slightly differently, it is easy to go wrong.
Almost every percentage question is one of three types. Learn these three and you can handle nearly all of them. You can always check your answer with our Percentage Calculator.
Type 1: What is X% of a number?
Answer = number × X ÷ 100
Example: What is 20% of ₹2,499? That is 2,499 × 20 ÷ 100 = ₹499.80.
If a shirt priced at ₹2,499 is 20% off, you save ₹499.80 and pay 2,499 − 499.80 = ₹1,999.20. A faster way to get the final price is to multiply by 0.80, because paying 80% of the price is the same thing.
Type 2: What percentage is one number of another?
Percentage = part ÷ whole × 100
Example: A student scores 432 out of 600. Then 432 ÷ 600 × 100 = 72%.
Another: 45 is what percentage of 180? 45 ÷ 180 × 100 = 25%.
The key is to put the right number on the bottom. The whole, meaning the thing you are comparing against, always goes in the denominator.
Type 3: Percentage change
Change % = (new value − old value) ÷ old value × 100
Example: A price rises from ₹850 to ₹1,020. The change is (1,020 − 850) ÷ 850 × 100 = 20% increase.
A positive answer is an increase and a negative answer is a decrease.
Why a rise and a fall are not symmetrical
If the price then falls from ₹1,020 back to ₹850, the change is (850 − 1,020) ÷ 1,020 × 100 = about −16.7%. The same ₹170 difference is 20% going up but only 16.7% going down, because each percentage is measured against a different starting value.
Everyday situations
Adding GST
To add 18% GST to ₹1,000, multiply by 1.18: ₹1,000 × 1.18 = ₹1,180.
Removing GST from a final price
This is where mistakes happen. If a bill is ₹1,180 including 18% GST, the original price is 1,180 ÷ 1.18 = ₹1,000. Taking 18% off ₹1,180 gives ₹967.60, which is wrong, because the 18% was charged on the smaller original amount, not on ₹1,180.
A salary increase
A salary of ₹50,000 with an 8% raise becomes 50,000 × 1.08 = ₹54,000.
Percentage points are not percentages
If an interest rate moves from 6% to 8%, it has risen by 2 percentage points, but it has increased by 33.3% in relative terms (2 ÷ 6). News and advertisements often use whichever sounds bigger, so read carefully.
Quick mental tricks
- 10%: move the decimal point one place left. 10% of 2,499 is about 249.9.
- 5%: half of 10%.
- 1%: move the decimal two places left.
- Build up: 15% is 10% plus 5%. 20% is double 10%.
- Multiplier method: for a 20% discount multiply by 0.8; for a 12% increase multiply by 1.12.
Common mistakes
- Dividing by the wrong number, usually the new value instead of the old one in a change calculation.
- Applying two successive percentages as if they add up. A 10% rise followed by a 10% fall does not return to the start; it ends at 99% of the original.
- Taking GST off the total as a simple percentage, as shown above.
- Forgetting to multiply by 100 at the end, so the answer shows as 0.72 instead of 72%.
Try it yourself
Use the Percentage Calculator for all three types: percentage of a number, percentage share and percentage change. Do the problem by hand first, then check yourself with the tool.