How the Compound Interest Calculator works

This free calculator shows how your savings or investments grow over time when interest is earned on both your original deposit and the interest already accumulated — the power of compounding. Enter an initial deposit, set up regular contributions, choose a compounding frequency, and see projected results across up to 50 years.

What makes this calculator different

• Tax on interest — Australian savings interest is taxable income. Toggle on tax and pick your marginal rate (19%, 32.5%, 37% or 45%) to see after-tax growth, or use 15% for super fund earnings.
• Inflation-adjusted real returns — see what your final balance is worth in today's purchasing power. Toggle inflation on and set your expected rate (default 2.5% p.a.).
• S&P 500 comparison — overlay the historical S&P 500 average (~10.5% p.a. nominal) against your savings rate to see the cost/benefit of staying in cash vs investing.
• Crossover year — see exactly which year the interest you earn exceeds what you're depositing. That's when compounding truly takes over.
• 3-rate comparison — model three different rates side by side to compare term deposits, HISA accounts, or investment returns.

The compound interest formula

Compound interest is calculated as A = P(1 + r/n)^(nt), where P is the principal, r is the annual rate, n is the number of compounding periods per year, and t is time in years. Daily compounding (n=365) produces slightly more than monthly (n=12) because interest is applied more frequently. This calculator simulates compounding period by period to correctly account for ongoing deposits and annual tax deductions.

Rule of 72

A quick way to estimate how long it takes to double your money: divide 72 by the annual interest rate. At 6%, your money doubles in roughly 12 years. At 10.5% (S&P 500 historical average), it doubles in about 6.9 years. The calculator shows this hint next to the rate field.

FUP Analytics builds free tools like this alongside our stock screener, portfolio tracker and dividend tools. Explore the dashboard →