Percentages get taught early in school and then, for a lot of people, never really click into place as something they can do confidently on the spot. Ask an adult to work out an 18% GST amount, a 15% restaurant tip, or how much a salary went up by, and you'll often see them reach for a phone rather than trust their own arithmetic. The good news is that almost every percentage question you'll actually run into in daily life is one of just three underlying calculations.

1. Finding a percentage of a number

This is the most common one: "what is X% of Y?" The formula is simply:

Result = (Percentage ÷ 100) × Number

So 18% of ₹2,000 is (18 ÷ 100) × 2,000 = ₹360. This same formula covers tips, taxes, discounts and interest amounts — anywhere you're taking a slice of a whole number.

2. Finding what percentage one number is of another

This one runs the other direction: "what percentage is X of Y?" Useful when you know two raw numbers and want the relationship between them expressed as a percentage — like a test score, or how much of a budget has been spent.

Percentage = (Part ÷ Whole) × 100

If you scored 42 out of 50 on a test, that's (42 ÷ 50) × 100 = 84%. Same formula whether it's a test score, a completion rate, or a market share.

3. Percentage change (increase or decrease)

This is the one people trip over most, because it's easy to confuse with the first formula. Percentage change measures how much something has moved relative to where it started:

Percentage change = ((New value − Old value) ÷ Old value) × 100

If a product's price went from ₹500 to ₹575, the change is ((575 − 500) ÷ 500) × 100 = 15%. Notice the denominator is the old value, not the new one — using the new value by mistake is probably the single most common percentage error people make, and it quietly produces a different (wrong) answer.

Why "reversing" a percentage isn't just subtracting it back

A related trap: if a price increased by 20% to reach ₹1,200, the original price isn't ₹1,200 minus 20% of ₹1,200. Because the 20% increase was calculated on the original smaller number, not the new larger one, you have to divide instead:

Original value = New value ÷ (1 + Percentage ÷ 100)

Here that's ₹1,200 ÷ 1.20 = ₹1,000. This exact logic is also what sits underneath reversing GST out of a tax-inclusive price, which is covered in more detail in the GST guide linked below.

A quick way to sanity-check yourself

If a percentage answer ever feels off, plug it back the other way: take your answer, apply the original percentage in reverse, and see if you land back on your starting number. It takes ten seconds and catches the majority of everyday percentage mistakes before they turn into an under- or over-payment.

Percentage points are not the same as percentages

This distinction causes genuine confusion, including in news reporting: if an interest rate moves from 5% to 7%, that's an increase of 2 percentage points, but expressed as a percentage change, it's actually a 40% increase — because ((7 − 5) ÷ 5) × 100 = 40%. Both descriptions are technically correct, but they sound very different, and it's easy to see how "rates went up 2%" versus "rates went up 40%" could leave two very different impressions of the same underlying move. When you see a percentage figure describing a change in something that was already a percentage (an interest rate, an inflation rate, a tax rate), it's worth pausing to check which of the two the source actually means.

A combined worked example

These formulas rarely show up in isolation — real situations often stack two or three together. Say an item is priced at ₹2,000, a store is running a 20% discount, and 18% GST applies on top of the discounted price. First, the discount: 20% of ₹2,000 is ₹400, bringing the price to ₹1,600. Then GST on that discounted price: 18% of ₹1,600 is ₹288, for a final price of ₹1,888. Notice that GST is calculated on the post-discount price, not the original one — applying it to the original ₹2,000 instead would overstate the tax by charging it on money the buyer never actually paid, which is a mistake worth watching for whenever a discount and a tax are combined on the same bill.

Try it yourself

Our Percentage Calculator covers all three calculations above in one place, so you don't have to remember which formula applies to which question.

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