Compound Interest Calculator
See how investments grow with compounded returns. Project savings, contributions, real value vs inflation, and yearly breakdown.
Growth over time
Final mix
Compounding advantage
Compounding adds $0 over simple interest.
Yearly breakdown
| Year | Start | Contributions | Interest | End balance | Real value |
|---|
Educational projection only. Real-world returns vary; this calculator assumes a constant rate and does not account for fees, market volatility, currency risk, or specific tax rules in your jurisdiction.
How to use
- Enter your starting balance and any regular monthly contribution.
- Set the expected annual interest rate and how often it compounds.
- Choose the investment duration in years.
- Optionally enable inflation adjustment to see today's-money values.
- Review the chart and table to see how the balance grows year by year.
Frequently asked questions
What is compound interest?
Interest calculated on both the original principal and previously accrued interest. Over long periods this snowball effect produces dramatically larger balances than simple interest.
How often does interest typically compound?
Savings accounts often compound daily or monthly, bonds twice a year, and many investments compound continuously for modelling purposes. The more often it compounds, the slightly higher the effective yield.
Does the calculator account for taxes?
Not by default, because tax rules vary widely. To approximate after-tax returns, reduce the input interest rate by your marginal tax rate on investment income.
What is the rule of 72?
A shortcut: divide 72 by your annual return percentage to estimate the number of years it takes for an investment to double. A 6% return doubles in roughly 12 years.
The formula and the intuition behind it
Compound growth follows A = P(1 + r/n)^(nt): principal P, annual rate r, n compounding periods per year, t years. The exponent is what separates it from simple interest. At 7% annual growth, 10,000 EUR becomes 19,672 in 10 years, 38,697 in 20 and 76,123 in 30... the final decade alone adds more than the first two combined, because growth applies to all prior growth. This back-loading is why starting age dominates contribution size: 200 EUR monthly from 25 beats 400 EUR monthly from 40 by retirement, despite half the money invested.
The Rule of 72
Divide 72 by the annual percentage rate to estimate doubling time: 8% doubles money in about 9 years, 6% in 12, 3% in 24. It works in reverse for inflation... at 4% inflation, purchasing power halves every 18 years... which is the quiet argument against leaving long-term savings in cash.
Details that change real outcomes
- Compounding frequency matters less than people expect: 5% compounded monthly yields 5.116% effective annual rate vs 5.095% quarterly. Rate and time dwarf frequency.
- Fees compound identically against you. A 1.5% annual fund fee on a 7% return consumes roughly a quarter of the final balance over 30 years.
- Use real (inflation-adjusted) rates for purchasing-power projections: 7% nominal minus 2.5% inflation is about 4.4% real.
- Regular contributions transform the math: the calculator's monthly-deposit mode models the accumulation pattern most people actually follow.
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