How to use this calculator
What this investment calculator shows
Enter a starting balance, what you plan to add each month, how many years you will invest and the average annual return you expect. The calculator projects what the portfolio could be worth, how much of that is money you put in and how much is compound growth.
Alongside your base case it draws a cautious line 3 percentage points lower and an optimistic line 3 points higher, so you can see a range rather than a single number. Switch to Breakdown to see contributions and gains year by year.
The inflation rate converts the result into today's money, which is the figure to compare with prices and salaries now. The historical presets fill in rounded long-run averages for the S&P 500, FTSE 100, MSCI World and government bonds. The year-by-year data behind them is on the historical stock market returns page.
How the maths works
The calculator compounds monthly. Each month the balance grows by one twelfth of the annual return, then that month's contribution is added:
balance = balance × (1 + r ÷ 12) + monthly contribution
Repeated over every month, that is the standard future-value formula for a lump sum plus regular payments. The value in today's money is the final balance divided by (1 + inflation)years. For the full explanation see how compound interest works.
Worked example
The default settings are $25,000 to start, $750 a month, 25 years at 7% with 2.5% inflation. You put in $250,000. The projection is $750,689, so about two thirds of the final figure is growth. In today's money that is roughly $404,915. The cautious case (4%) ends near $453,441 and the optimistic case (10%) near $1.30 million. That gap is the difference a few points of return make over 25 years. More examples by age and strategy are in investment scenarios.
Common questions
How much will $500 a month be worth in 20 years?
At a 7% average annual return, $500 a month for 20 years grows to about $260,463. You contribute $120,000 and roughly $140,463 is growth. Leave it for 30 years and it reaches about $609,985. Real returns vary year to year, so treat this as a projection, not a promise.
What annual return should I use?
Use a rate that matches what you actually hold. Broad stock indexes have averaged high single to low double digits over long periods, before inflation. Bonds have returned much less. Many people plan with 5–7% for a diversified portfolio and check the cautious line as well.
Does it include taxes and fees?
No. Results are before tax and fees. To allow for fees, subtract them from the return. A 1% annual fee on a 7% return is roughly 6%. On the default example that cuts the result from $750,689 to about $631,370.
What does "in today's money" mean?
It is the projected value adjusted for inflation, so you can compare it with what things cost now. At 2.5% inflation, $750,689 in 25 years buys about what $404,915 buys today.
When are contributions added?
At the end of each month, after that month's growth. Adding them at the start of the month would give a slightly higher result.
More answers are in the investment FAQ.