How to Calculate Percentages Without a Calculator

Percentages turn up everywhere: a 20% discount board outside a shop, an 18% GST line on a bill, a 7% salary hike letter. Most of these can be worked out in your head in a few seconds once you know a couple of tricks, and understanding the method means you spot an error when a calculator gives you something odd.

The word percent simply means 'per hundred'. So 35% of something is 35 parts out of every 100 parts. Every technique below is a shortcut around that single idea.

Finding 10% of any number just means moving the decimal point one place to the left. 10% of 2,480 is 248. From there, 20% is double that (496), 5% is half of it (124), and 1% is 24.8.

Combine those blocks to reach almost any percentage. For 35% of 2,480: take 10% three times (744), add 5% (124), and you get 868. No calculator involved.

For a 30% discount, many people calculate 30% and then subtract. It is usually faster to calculate the 70% you will actually pay. On a ₹1,600 item, 70% is 10% (160) times seven, which is ₹1,120.

Watch out for stacked discounts. An extra 10% off an already 30%-off price is not 40% off — it is 70% × 90% = 63% of the original, so 37% off in total.

To find how much something changed, subtract the old value from the new one, divide by the old value, and multiply by 100. A salary moving from ₹42,000 to ₹47,000 is a rise of 5,000 ÷ 42,000 × 100, which is about 11.9%.

The order matters. A 50% rise followed by a 50% fall does not return you to the start: 100 becomes 150, then 75. Always divide by the value you started that step with.

x% of y is always the same as y% of x. That sounds like trivia until you need 4% of 75 — flip it to 75% of 4, which is obviously 3.

This one shortcut handles a surprising number of awkward-looking questions.

Mental maths is perfect for a quick sanity check, but for anything you are going to write down — an invoice, a tax figure, an exam answer — use an exact calculation. Our percentage calculator handles all three common cases and shows the result as you type.