Compound Interest Calculator
Estimate what your savings or investment could be worth over time. Enter what you start with, what you add each month, the rate you expect and how often interest compounds — and see how much of the final figure is your own money and how much is growth.
How we calculate this
Your starting balance grows by the standard compound interest formula, FV = P × (1 + r/m)^(m×t), where r is the annual rate as a decimal, m is the number of compounding periods per year and t is the number of years. Monthly contributions are handled separately with the future value of an ordinary annuity, FV = C × [((1 + j)^N − 1) ÷ j], where N is the number of monthly payments and j is the monthly rate that is equivalent to your chosen compounding frequency: j = (1 + r/m)^(m/12) − 1. Converting the rate this way keeps each monthly deposit growing at exactly the same effective rate as the starting balance, instead of pretending a year of deposits arrives as one lump sum. The two results are added together for the estimated future value. Total contributed is your starting amount plus every monthly payment, and total interest is simply the future value minus that.
Tip: Time matters more than the amount. Adding a few years to the end of the projection usually moves the final figure more than raising your monthly contribution does.
Assumptions & things to watch
- This is an estimate for illustration, not a guarantee — real investment returns vary year to year and can be negative.
- The rate you enter is treated as a constant for the whole projection.
- No allowance is made for taxes, platform or fund fees, or inflation, so the figure is in today's dollars before any of those are deducted.
- Contributions are assumed to be made at the end of each month and never missed or increased.
Estimates only — always double-check quantities with your supplier before ordering.
Quick tips: Compound Interest
What is compound interest?
Interest earned on your original money and on the interest it has already earned. Each compounding period the balance grows slightly larger, so the next period's interest is calculated on a bigger number — which is why the curve steepens over long periods.
Does compounding frequency make much difference?
Less than most people expect. At the same nominal rate, monthly compounding beats annual compounding by a fraction of a percent a year. The rate itself, the amount you contribute and the number of years matter far more.
How are my monthly contributions treated?
Each contribution is compounded from the month it is paid in until the end of the period, using a monthly rate converted from your chosen compounding frequency. Money you pay in later has less time to grow, which the annuity formula handles correctly.
What rate of return should I use?
That's your own judgement call, and this calculator can't advise you. People often run the numbers two or three times with a cautious rate and an optimistic one to see the range of outcomes rather than relying on a single figure.
Does this account for inflation or tax?
No. The result is a nominal figure. If you want a rough real-terms view, run it again with your expected return minus your expected inflation rate.