About the compound interest calculator
Compound interest means earning interest on your interest. This calculator shows how a starting amount, regular monthly contributions, an interest rate and time combine into a future balance, and how much of that balance is your own money versus interest earned.
It is useful for savings accounts, bonds, tax-wrapped savings and long-term investment projections, as long as you remember that the rate you enter is an assumption, not a promise.
Who should use the compound interest calculator
- Savers who want to see the effect of starting early or adding a little more each month.
- People comparing accounts that compound at different frequencies.
- Students learning why compounding is described as exponential growth.
How to use the compound interest calculator: what each input means
- Starting amount
- The balance at the beginning.
- Monthly contribution
- What you add each month. Use 0 for a one-off lump sum.
- Contribution timing
- Whether each contribution is made at the start or the end of the month.
- Annual interest rate
- The nominal yearly rate you expect to earn.
- Time
- How long the money stays invested, in years (decimals allowed).
- Compounding frequency
- How often interest is added: annually, semi-annually, quarterly, monthly or daily.
How the compound interest calculator works: method and formula
Without contributions the balance follows the standard compound-interest formula. With contributions, the calculator steps month by month: the balance grows by the effective monthly growth factor, then the contribution is added (or added first, if you choose start-of-month).
Because the growth factor is derived from the compounding frequency, the lump-sum part is exact. For contributions made between compounding dates (for example monthly deposits into a quarterly-compounding account), the month-by-month method is a close approximation rather than a bank's exact day-by-day accrual.
- Lump sum
- A = P × (1 + r ÷ n)^(n × t)
- Monthly growth factor used
- g = (1 + r ÷ n)^(n ÷ 12)
- Where
- P = starting amount, r = yearly rate, n = compounding periods per year, t = years
How to read your compound interest calculator result
Compare the final balance with total contributions to see how much is growth. Early on, contributions dominate; in the later years interest can overtake them. The effective annual rate shows what the compounding frequency adds to the nominal rate.
Remember the result is before taxes, fees and inflation. A real-world return is also not constant from year to year.
Compound Interest Calculator worked example
Start with $10,000.00, add $200.00 at the end of every month, earn 5% a year compounded monthly, for 10 years.
- Contributions: $10,000.00 + 120 × $200.00 = $34,000.00.
- The lump sum alone grows to $16,470.09 (that is $10,000.00 × (1 + 0.05 ÷ 12)^120).
- Adding the monthly deposits and their interest gives a final balance of $47,526.55.
Final balance $47,526.55, of which $13,526.55 is interest earned. The effective annual rate is 5.116%.
Practical uses of the compound interest calculator
- Projecting a savings pot for a deposit, a wedding or retirement.
- Comparing the same rate with annual and monthly compounding.
- Showing a young person why starting five years earlier matters.
Assumptions and limitations of this calculator
- Assumes a constant rate. Real investments rise and fall, so use the result as a scenario, not a forecast.
- No taxes, fees or inflation are deducted.
- With contributions, interest between compounding dates is approximated using the effective monthly factor, which can differ slightly from an institution's own method.
- Not financial advice.