Investment Calculator
Project how your investments could grow over time, combining a starting amount with regular monthly contributions at an assumed rate of return. This uses a higher, variable return assumption, so it's built for market investments rather than a bank savings account.
Off means contributions land at the end of the month.
Your projected balance will appear here
Enter your starting amount, contribution, and expected return, then click calculate.
Quick Answer
FV = PV(1 + r/n)^(nt) + PMT x [((1 + r/n)^(nt) - 1) / (r/n)], where your starting amount grows like compound interest and your monthly contributions grow like a future value annuity. On $10,000 to start, $500 a month, a 7% return, and 10 years, that ends up around $106,639, with roughly $70,000 of that from your own contributions.
How It Works: Formula & Variables
FV = PV(1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) − 1) / (r/n)]
- PV
- Your starting amount, which grows on its own like compound interest.
- PMT
- Your monthly contribution, added at the end of each month by default.
- r, n, t
- The annual rate, the compounding frequency, and the number of years.
For contributions made at the start of each period instead, multiply the contribution portion of the formula by an extra (1 + r/n).
Source: SEC, Investor.gov Compound Interest Calculator, which applies the same lump-sum plus monthly-contribution math. Projections are estimates, not guarantees.
Worked Examples
Example: $10,000 start, $500 a month, 7% return, 10 years
The starting amount grows to $20,096.61 on its own. The contributions grow to $86,542.40. Added together, the ending balance is about $106,639.02, against a total of $70,000 actually contributed, so roughly $36,639.02 of that is growth.
Key Concepts
Consistency usually beats timing: A steady monthly contribution, kept up over years, tends to build a larger balance than trying to time when to add money.
Returns aren't guaranteed: Investment products can lose value and aren't FDIC-insured, unlike a bank deposit, so treat any assumed return as a planning estimate rather than a promise.
Small timing choices add up over decades: Contributing at the start rather than the end of each month is a small change on its own, but it compounds the same way every other dollar in the account does.
Common Mistakes
Treating an assumed return as guaranteed: A 7% or 10% projection is an illustration based on historical averages, not a locked-in outcome, and real returns vary year to year.
Mixing up ordinary annuity and annuity due: Forgetting the extra (1 + r/n) factor for start-of-period contributions slightly understates a balance that's actually funded at the start of each month.
Forgetting the starting amount grows too: Some people only account for the growth on their contributions and forget that the initial lump sum is compounding the entire time as well.
Frequently Asked Questions
This tool adds regular monthly contributions on top of a one-time starting amount. A basic compound interest calculator only grows a single lump sum.
The math is identical, but the rate assumption is not. An investment calculator uses a higher, variable expected return, like the stock market, that isn't guaranteed. A savings calculator uses a bank APY, which is usually guaranteed and FDIC-insured.
There's no guaranteed number to plug in. Historical averages are often used as a rough guide, and the S&P 500 has averaged around 10% a year before inflation over long periods, though any given year or decade can land well above or below that.
A little. Contributing at the start of the month gives each deposit one extra month to grow, which adds up slightly over a long time horizon.
Growth is your ending balance minus your starting amount and all your contributions added together. The results panel breaks this down so you can see the split at a glance.
The rate needs to be converted to match whatever period you're contributing on. Most calculators, including this one, simplify by assuming your contributions and your compounding happen on the same monthly schedule.
Related Calculators
Savings Calculator
Use the same math with a bank APY instead of a market return assumption.
Savings Goal Calculator
Work backward from a target amount to find your required monthly deposit.
401(k) Calculator
Project retirement savings with salary growth, employer match, and IRS limits.
Compound Interest Calculator
Grow a single lump sum with no ongoing contributions, and compare frequencies.
ROI Calculator
Calculate total and annualized return on one investment you already made.
Interest Rate Calculator
Work backward from a starting and ending amount to find the rate.
Simple Interest Calculator
Calculate straight-line interest with I = Prt, without any compounding.