How compound interest works
Compound interest is interest earned on both your original money and the interest it has already earned. Each compounding period the balance grows, and the next period's interest is calculated on that larger balance — so growth accelerates over time. The formula for a lump sum is A = P(1 + r/m)^(mt), where P is the principal, r is the annual rate, m is how many times a year interest compounds and t is the number of years. The more frequently interest compounds, the faster the balance grows, which is why this calculator lets you choose annual, quarterly, monthly or daily compounding and adds your regular contributions on top.
Why time is the most powerful input
Compounding rewards patience more than any other factor. Because growth builds on itself, the final years of a long investment add far more than the early ones — the curve bends upward. Starting ten years earlier often beats contributing twice as much later. A modest monthly deposit left to compound for thirty years can outgrow a much larger sum invested for ten. Use the time-horizon field to see this for yourself: extending the period usually moves the future value more dramatically than nudging the interest rate, which is the mathematical case for starting to save as early as possible.
Regular contributions supercharge growth
A lump sum compounds on its own, but adding money every month layers a second engine on top. Each contribution starts its own compounding journey, so early deposits have decades to grow while recent ones have only just begun. This is exactly how retirement accounts and index-fund investing build wealth — steady contributions plus compounding over a working lifetime. The 'total contributions' line shows how much of your future value you put in, and the 'interest earned' line shows how much the compounding generated for free. Over long horizons, the interest commonly dwarfs the contributions.
Real returns, inflation and tax
This calculator shows nominal growth at a fixed rate. Real-world returns fluctuate year to year, inflation erodes purchasing power, and gains may be taxed depending on the account and jurisdiction. As a rough guide, subtract your expected inflation rate from the interest rate to estimate growth in today's money. For tax-advantaged accounts the headline figure is closer to reality; for taxable accounts, apply your capital-gains or income-tax rate to the interest earned. These calculators use standard financial formulas and the figures you enter. Real interest rates, lender fees and tax treatment vary by bank and country, so treat the results as planning estimates and confirm exact numbers with your lender or a qualified adviser before committing.
Frequently asked questions
What is compound interest?
It is interest earned on both your original principal and the interest already accumulated. Because each period's interest is added to the balance, growth accelerates over time.
How does compounding frequency change the result?
More frequent compounding produces slightly more growth, because interest is added to the balance sooner and starts earning sooner. Daily compounding beats annual at the same rate, though the gap is small.
Do contributions need to be monthly?
This calculator assumes a monthly contribution grown at the equivalent rate of your chosen compounding frequency. Set the contribution to zero to model a lump sum with no additions.
Does this account for inflation or tax?
No — it shows nominal growth. Subtract expected inflation from the rate to see growth in today's money, and apply your tax rate to the interest earned for taxable accounts.