How to Calculate Percentage: Formulas, Examples and Mental Math Shortcuts

Guides · Jun 5, 2026 · 3 views

Percentages show up in marks, discounts, taxes, loans and news — and most mistakes come from applying the right number to the wrong formula. There are only three patterns to know.

Pattern 1: What is X% of Y?

Multiply: Y × X ÷ 100. What is 18% of 2,500? → 2,500 × 18 ÷ 100 = 450. This is the GST-on-a-bill, tip-on-a-meal calculation.

Pattern 2: X is what percent of Y?

Divide: X ÷ Y × 100. Scored 432 out of 600? → 432 ÷ 600 × 100 = 72%. The order matters — the "out of" number always goes on the bottom.

Pattern 3: Percentage change

(New − Old) ÷ Old × 100. Salary went from 40,000 to 52,000? → 12,000 ÷ 40,000 × 100 = 30% hike. The base is always the OLD value — this is the one news headlines get wrong.

The reverse percentage trap

A jacket costs ₹2,550 after a 15% discount. The original is NOT 2,550 + 15%. It's 2,550 ÷ 0.85 = ₹3,000 — because the 15% was taken off the larger original, not the smaller final price. Same logic for extracting GST from an inclusive bill: divide by 1.18, don't subtract 18%. The percentage calculator handles all of these directions instantly.

Mental math shortcuts

10% = shift the decimal (10% of 730 = 73). 5% = half of 10%. 1% = shift twice. 15% = 10% + 5%. 18% GST ≈ 20% minus a tenth of that. And the elegant one: X% of Y equals Y% of X — 4% of 75 is easier as 75% of 4 = 3.