Compound interest calculator with monthly contributions

Single deposit: FV = P × (1 + r/m)m × n, where P is the principal, r the annual rate, m the number of compounding periods per year and n the number of years.

Regular contributions (paid at the start of each month, monthly compounding): FV = C × ((1 + i)N − 1) / i × (1 + i), where C is the contribution, i = r/12 and N the number of months.

Worked examples

• 10,000 at 5% for 10 years: yearly compounding gives 16,288.95, monthly compounding 16,470.09.
• 10,000 plus 500 a month at 6% for 20 years, compounded monthly: about 265,278. Contributions total 130,000, so roughly 135,278 is interest.
• The same plan with 19% tax on the total gain at the end: about 239,575. Adjusted for 3% inflation, that is about 146,878 in today’s money.

With compounding, interest earns further interest, so the balance grows exponentially rather than in a straight line. 10,000 at 7% a year becomes about 19,672 after 10 years but about 76,123 after 30 years - three times the period produces almost seven times the profit (9,672 vs 66,123).

Rule of 72: divide 72 by the annual rate to estimate how many years it takes to double your money. At 6% it takes about 12 years, at 8% about 9 years, at 3% about 24 years. The rule works best for rates between roughly 4% and 12%.

Compounding frequency matters far less than rate and time: moving from yearly to monthly compounding on 10,000 at 5% over 10 years adds only about 181.

• Tax at each compounding - how savings accounts and term deposits usually work: 19% of the interest is withheld every time interest is credited, so less money keeps compounding.
• Tax on the total gain at the end - 19% of the whole profit when you cash out, as with many investment funds. Deferring tax leaves more capital working, so the end result is higher.
• The tax rate is fixed at 19% (the Polish capital gains rate). If your country uses a different rate or your account is tax-free, choose “No tax” and adjust the result yourself.
• Inflation - real value = nominal value / (1 + inflation)^years. It shows what the final amount would buy today.

The calculator assumes a constant rate for the whole period. Real deposit rates change, and market investments can return more, less or even lose money, so treat the result as a scenario, not a forecast. Fees are not included: a 1% annual fee on a 6% return cuts the 20-year result above by tens of thousands. Run several scenarios (e.g. 4%, 6% and 8%) to see a realistic range, and compare a fixed-term deposit with a long-term plan before committing money.