Assumptions. Interest compounds monthly and contributions are added at the end of each month. Figures are before tax and inflation: real returns are lower once both are counted. This is an estimate for planning, not a guarantee of any return.
This is one number. Your real plan has many.
Manticore Finance runs this projection on your actual portfolio (with real market history, sequence-of-returns risk, fees and tax) not a single flat rate.
How compound interest works
Compound interest is interest earned on your interest. Each period, the return is calculated on everything you hold (your original money, your contributions, and every bit of interest added so far) so the balance grows faster the longer it runs. Time is the biggest lever: the same rate over 30 years produces far more than over 10, because the later years compound on a much larger base.
The formula behind the number above is the future value of a starting sum plus a stream of regular deposits: FV = P(1 + i)N + PMT · ((1 + i)N − 1) / i, where i is the monthly rate (annual ÷ 12) and N is the number of months.
How to use it
Enter what you have today, what you'll add each month, a rate you think is realistic, and how long you'll leave it. A broad global equity index has historically returned around 5-8% a year over long periods, but any single decade can be very different: try a few rates to see how much the outcome depends on it.