Percentage Calculator
Calculate percentages, percentage increase/decrease, and what percentage one number is of another.
Disclaimer
This calculator provides estimates for general information purposes only. Results are based on standard formulas and may not reflect your individual circumstances. Always consult a qualified professional for advice specific to your situation.
Frequently Asked Questions
How do you calculate a percentage of a number?
How do you calculate percentage change?
What is Percentage?
A percentage calculator is a multipurpose maths tool for working with percentages. It can find a percentage of a number, calculate percentage change between two values, or determine what percentage one number is of another.
How this calculator works
Three modes cover the most common percentage calculations: 'X% of Y' finds a specific percentage of any number (e.g., 15% of 200 = 30). '% Change' calculates the percentage increase or decrease between two values — useful for comparing prices, salaries, or investment returns. 'X is what % of Y' determines the ratio as a percentage — useful for working out tax rates, discounts, or proportions. The quick reference table shows mental shortcuts for common percentages (10% = divide by 10, 25% = divide by 4, GST 10% = divide by 11).
Common Australian Percentage Calculations
GST (10%): divide GST-inclusive price by 11 for the GST component. Tip/service (10-15%): multiply meal cost by 1.10 or 1.15. Sale discount (25%): multiply by 0.75. Salary increase (3%): multiply current salary by 1.03. Super (12%): multiply salary by 0.12. Medicare levy (2%): multiply income by 0.02. The 'divide by' tricks are mental shortcuts: 10% = ÷10, 20% = ÷5, 25% = ÷4.
Percentage Change Formula
% change = (new value - old value) ÷ old value × 100. Negative result = decrease. Example: salary $80k → $84k = 5% increase. House price $500k → $600k = 20% increase. Stock dropped $50 → $40 = -20% change. CRITICAL: a 50% gain followed by a 50% loss doesn't return to start! $100 × 1.5 = $150, then ×0.5 = $75 (lost 25%).
Compounding Percentages
Repeated percentage changes compound multiplicatively, not additively. 5% growth for 10 years: 1.05^10 = 1.629, so $100 grows to $163. Not $150 (which would be simple interest). Inflation, investment returns, mortgage interest, super growth — all compound. Rule of 72: divide 72 by annual % rate for years to double (e.g., 7% → 10 years to double).
Common Mistakes
(1) Adding percentages from different bases — 10% off + 10% off ≠ 20% off (it's 19% off). (2) Confusing percentage point with percent — RBA raising rate from 3% to 4% is a 1 percentage POINT change but a 33% increase. (3) Reversing percentages — if price went up 10%, you reduce by ~9.09% to return to original (1/1.10 = 0.909).
All calculations are performed in your browser — your data never leaves your device. Results are for general guidance only and should not be considered professional financial advice.
Built and maintained by Konstantin Iakovlev. Data sourced from the ATO and official Australian government sources.