Percentage Calculations You Keep Having to Google (With Actual Examples)

Multiply the number by the percentage expressed as a decimal. 20% of 150 = 150 × 0.20 = 30. The key step people mess up is converting percentage to decimal — divide the percentage by 100. 15% = 0.15. 7.5% = 0.075. 100% = 1.0. Alternatively, use the 'of equals times' rule: 'x% of y = x × y ÷ 100.' Both give the same result. A mental math shortcut: 10% of any number is that number with the decimal moved one place left. 10% of 340 = 34. 20% = double the 10% = 68. 5% = half the 10% = 17.

Divide the part by the whole, then multiply by 100. If 45 students out of 180 passed a test, the percentage is 45 ÷ 180 × 100 = 25%. If you spent $340 on food out of a $2,200 monthly budget, that's 340 ÷ 2,200 × 100 = 15.5%. The formula is: Percentage = (Part ÷ Whole) × 100. This is the same calculation used for grades, market share, survey results, and almost any 'what fraction is this?' question.

Percentage change = ((New Value – Old Value) ÷ Old Value) × 100. If your salary went from $65,000 to $72,000: (72,000 – 65,000) ÷ 65,000 × 100 = 10.8% increase. If a stock went from $120 to $95: (95 – 120) ÷ 120 × 100 = –20.8% decrease. The sign tells you direction: positive is increase, negative is decrease. Note that a 20% decrease followed by a 20% increase does not get you back to the starting point — the base changes. $100 → $80 (–20%) → $96 (+20%). You need a 25% increase to recover from a 20% decrease.