Percentages are the simplest math everyone gets wrong under pressure. Here are the four common operations, what they mean, and when to use each.
Open the Percentage Calculator and pick the mode.
1. "X% of Y"
result = X / 100 × Y
15% of 2,400 = 360.
This is the textbook one. Use for tip calculation, "what's 20% of my salary as savings", commission cuts.
2. "X is what % of Y"
result = (X / Y) × 100
300 is what % of 2,400? = 12.5%.
Use for: "I spent ₹3,000 on groceries this month, what's that as a % of my income?" Or "Sales of product A were ₹X, total revenue was ₹Y — what's A's share?"
3. "Percentage change from A to B"
result = ((B − A) / A) × 100
From 80 → 96 = +20%. From 100 → 75 = −25%.
Use for: portfolio returns, sales growth quarter over quarter, weight change, price hikes.
The trap: percentage up and percentage down aren't symmetric. Going 100 → 150 is +50%; going back 150 → 100 is −33% (not −50%).
4. "Add X% to Y" / "Subtract X% from Y"
add = Y × (1 + X/100)
subtract = Y × (1 − X/100)
Add 12% GST to ₹500 = ₹560. Subtract 25% discount from ₹2,000 = ₹1,500.
For repeated operations, multiply the factors. Adding 18% then subtracting 18% does not return you to the start:
100 × 1.18 × 0.82 = 96.76
(You end up 3.24% below the start.) Same logic for stacked discounts — covered in detail in the discount calculator post.
The percentage trap most people fall into
"Sales fell 50% this year, then rose 50% next year — back to where we started!"
No. Started at 100. Fell 50% → 50. Rose 50% on the new base → 75. Still 25% below the original.
To get back to 100 from 50, you need a +100% rise. The percentage that gets you back from a fall of x% is x / (1 − x/100)%. Down 20% needs +25% recovery. Down 50% needs +100%. Down 80% needs +400%.
This is why losses are mathematically asymmetric with gains, and why "don't lose money" is rule #1 of investing.
Run yours
Open the Percentage Calculator. Pick mode. Get answer. Move on with your life.