Percentage: Easy rules you can use right away

Percentages show parts of a whole using 100 as the base. You see them in discounts, interest rates, exam scores, and polls. Once you know a few simple steps and formulas, you can handle most percentage problems quickly—no calculator panic needed.

Quick rules and formulas

Convert percent to decimal: divide by 100. So 25% = 25 ÷ 100 = 0.25. Convert decimal to percent: multiply by 100. So 0.07 = 7%.

Find percent of a number: multiply the number by the decimal form of the percent. Example: 15% of 250 = 250 × 0.15 = 37.5.

Find the whole from a part: whole = part ÷ (percent ÷ 100). Example: If 30 is 20% of a number, the number = 30 ÷ 0.20 = 150.

Percent change (increase or decrease): (new − old) ÷ old × 100. If a price goes from 50 to 65, change = (65 − 50) ÷ 50 × 100 = 30% increase.

Percentage points vs percent change: if support moves from 40% to 45%, that’s a 5 percentage point rise but a 12.5% relative increase because (45 − 40) ÷ 40 × 100 = 12.5%.

Practical examples and mental tricks

Discounts: For 10% off, move the decimal one place left. A $80 item at 10% off saves $8, so you pay $72. For 20% off, double the 10% result: $80 × 0.20 = $16, pay $64.

Half of 10% is 5%. To get 5% of a number: first get 10%, then halve it. Example: 10% of 200 is 20, so 5% is 10.

Quick compound vs simple interest note: simple interest = principal × rate × time. Compound interest uses growth each period: A = P × (1 + r)n. Use this when interest gets added to the balance regularly.

Convert fractions: turn the fraction into a decimal, then into a percent. Example: 3/8 = 0.375 → 37.5%.

Rounding: keep one extra decimal during calculation, then round at the end. For money, round to two decimals. For rough checks, use easy benchmarks: 25% is a quarter, 50% is half, 75% is three quarters.

When comparing values, pick the right base. If you want to know how much more X is than Y, divide the difference by Y, not by the average. That gives the correct percent change relative to the starting value.

Last tip: practice with real items—calculate sale prices, calorie reductions, or monthly interest on a small loan. Doing a few quick examples makes percent work feel natural and saves mistakes when it matters.