💹 Investment Calculator
See what a lump sum plus monthly deposits could grow into, year by year.
How it works
Your pot grows in two parts, and the calculator works them out separately. First, the starting amount compounds on its own: it is multiplied by (1 + r) every month, where r is the annual return divided by 12. Second, every monthly deposit compounds for whatever time it has left in the pot — the first deposit gets the full run, the last one barely gets started. Added together:
Future value = P x (1 + r)n + PMT x [((1 + r)n - 1) / r], where P is the starting amount, PMT the monthly deposit, r the monthly rate and n the number of months.
The growth line is simply the final value minus everything you paid in, so you can see how much of the ending pot is your money and how much is money your money made. One honest warning: real markets do not grow in a smooth line like this. Some years fall. What you are looking at is the path an average return would take if it arrived evenly, which is good enough for planning how much to save and for how long, but it is not a prediction of any particular year.
An example
Start with $10,000, add $300 a month, earn an average 8% a year and leave it alone for 25 years. You pay in $100,000 of your own money — the $10,000 lump sum plus 300 deposits of $300. The pot ends at $358,709.68, which means $258,709.68 of it is growth: more than two and a half dollars earned for every dollar you put in.
The early years feel painfully slow — after year 1 the balance is only $14,565, of which just $965 is growth. By year 10 it is $77,080. Most of the magic is squeezed into the final years, because by then the pot itself is large and each year's 8% is a big number. That back-loaded shape is exactly why starting early beats saving more later.
Common questions
Is an 8% annual return realistic?
It is a common planning figure. US shares have averaged roughly 10% a year over very long periods before inflation, and 7-8% is what many people use once inflation and fees are allowed for. Nothing is guaranteed — some decades have done much worse — so try the calculator at 6% and 10% too and plan around the cautious end.
What happens if I stop the monthly deposits?
Set the monthly contribution to 0 and compare. With the figures in the example above, the deposits and the growth on them make up the large majority of the final $358,709.68 — the starting lump sum does the least of the work. Regular deposits are the engine here, not the opening balance.
Are taxes and fund fees included?
No. The result is before tax and before any fund or platform charges. A fee of 1% a year quietly takes a real bite over 25 years; to model it roughly, knock 1 off the return rate — run the same numbers at 7% instead of 8% and look at the difference.
Related calculators
This calculator gives estimates for learning. Rates, taxes and rules change, and your lender or the IRS has the final numbers — check big decisions against the real documents.