Investment ROI Engine
Project your long-term wealth accumulation by modeling recurring contributions against historical market benchmarks.
Growth Inputs
Define your investment goals to generate a visual performance chart.
How the Investment Growth Calculator Works
This tool projects how a starting balance plus regular monthly contributions could grow over time at an assumed average annual return, compounding monthly — a standard model for long-term investing projections like retirement or brokerage accounts.
Formula
Balance(month) = Balance(month − 1) × (1 + r/12) + Contribution- Balance — account value, recalculated one month at a time
- r — assumed annual rate of return, as a decimal
- Contribution — fixed amount added at the end of each month
Rather than using a single-step formula, the calculator simulates the account balance month by month for the full time horizon: each month it applies one month's worth of growth to the existing balance, then adds that month's contribution. This mirrors how a real brokerage or retirement account actually accrues value.
Because contributions compound for less time than the original principal, the earliest dollars — whether from the initial deposit or the first few monthly contributions — end up doing more work than dollars contributed near the end of the term.
Worked Example: $5,000 start, $300/month, 6% return, over 15 years
- Principal grows for 180 months at a monthly rate of 6%/12 = 0.5%
- Each of the 180 monthly $300 contributions compounds for its own remaining time
- Summing the compounded principal and all compounded contributions
Projected balance ≈ $99,500 — of which $5,000 was the original deposit and $54,000 was contributed, with the remainder from investment growth.
Frequently Asked Questions
Is a 6% return realistic?
Long-run average annual returns for a diversified stock portfolio have historically been in the high single digits before inflation, though any specific year can vary widely and past performance does not guarantee future results. Adjust the rate to stress-test more conservative or optimistic scenarios.
Why does contributing early matter more than contributing later?
Every contribution only compounds for the months remaining until the end of the projection. A contribution made in month 1 compounds for the full term; a contribution made in the final month compounds for essentially zero additional time.