How to Calculate Percentages — Formulas, Examples & Mental Math

Percent just means "per hundred." Master three formulas and a couple of tricks and you'll never be stuck at a shop counter or exam result again.

Formula 1: What is X% of Y?

Answer = Y × X ÷ 100. A restaurant bill of 4,800 with a 10% service charge adds 4,800 × 10 ÷ 100 = 480. An 18% tax on 450 is 81. This is the formula for tips, taxes, commissions and zakat alike.

Formula 2: X is what percent of Y?

Answer = X ÷ Y × 100. You scored 684 out of 850: 684 ÷ 850 × 100 = 80.5%. Your team won 13 of 20 matches: 65%. This is the formula for exam results, attendance and market share.

Formula 3: Percentage change

Change % = (new − old) ÷ old × 100. Salary went from 80,000 to 95,000: (95,000 − 80,000) ÷ 80,000 × 100 = +18.75%. Note the divisor is always the old value — that's why a 50% drop needs a 100% rise to recover: 100 → 50 is −50%, but 50 → 100 is +100%.

Mental math tricks worth knowing

• The 10% anchor: move the decimal one place (10% of 640 = 64), then build: 5% = half of that (32), 20% = double (128), 1% = a tenth (6.4). Almost any percentage can be assembled this way.
• The reversal trick: x% of y = y% of x. 8% of 50 feels hard; 50% of 8 = 4 is instant. Same number.
• Discount shortcut: 30% off means you pay 70% — one multiplication (price × 0.7) instead of two steps.
• Percentage points ≠ percent: a rate going from 10% to 15% rose 5 points but 50 percent. News headlines mix these up constantly.

Where you'll use this weekly

Exam and assignment scores, sale prices and taxes, tips, loan interest, salary raises, battery and storage indicators, statistics in the news, and profit margins if you sell anything. Percentages are the single most reused piece of school math — which is exactly why getting the three formulas straight pays off daily.

Frequently asked questions