Saving money is only half the battle; the other half is letting that money work for you. This investment calculator projects what a starting lump sum, combined with regular monthly contributions, could grow into over time. It separates the dollars you actually put in from the dollars your investments earn, so you can see the difference between saving and compounding.
Built by Smekys, this tool is for anyone who wants a clear, uncluttered estimate: a first-time investor testing a 401(k) plan, a saver deciding whether to raise a monthly contribution, or a near-retiree checking whether their pace is on track. Adjust the four inputs and the result card updates instantly.
How to Use
- Enter your Initial Investment — the lump sum you already have invested today, or the amount you are opening the account with.
- Set your Monthly Contribution — the amount you plan to add at the end of every month. Consistency matters more than size.
- Type an Expected Annual Return. A common long-run assumption for a diversified stock portfolio is about 7% after inflation; test a range from 4% to 10% to see how sensitive the outcome is.
- Choose your Investment Horizon in years. The longer the horizon, the more time compounding has to accelerate.
The result card shows four numbers: your starting balance, the total you personally contributed, the growth earned from investment returns, and the projected future value at the end of the period. A final multiple tells you how many times your contributions grew.
The Formula Behind the Number
The calculator combines two standard finance formulas. First, the future value of a single lump sum left to grow:
FV(lump) = P₀ × (1 + r)ⁿ
Then the future value of an ordinary annuity — a series of equal monthly contributions made at the end of each period:
FV(annuity) = PMT × ((1 + r)ⁿ − 1) / r
Where:
- P₀ is the initial investment.
- PMT is the monthly contribution.
- r is the monthly rate — the annual return divided by 12.
- n is the total number of months — years times 12.
The projected future value is the sum of the two. If the expected return is 0%, the division in the annuity formula is undefined, so the engine simply adds up the contributions with no growth: starting balance plus monthly contribution times the number of months.
Why Contributions Beat the Starting Balance
On a 30-year plan starting with $10,000 and adding $500 per month at 7%, the math is revealing. Your own contributions total only $190,000, yet the projected future value is roughly $691,000. The extra $501,000 comes almost entirely from compounding — returns earning returns. Notice that the initial $10,000 lump sum is a small slice of the total; the recurring monthly deposits are what build the bulk of the nest egg. This is why financial advisors emphasize starting early and automating contributions, even when the monthly amount feels modest.
Time is the single biggest lever. Move the horizon from 20 years to 30 years and the final balance does not rise by half — it can more than double — because the last decade compounds on top of a much larger base.
Examples
Example 1 — The default long-term plan. With a $10,000 starting balance, $500 per month, a 7% annual return, and a 30-year horizon, the calculator projects a future value near $691,150, on $190,000 of personal contributions. That is roughly 3.6 times what you put in.
Example 2 — A zero-return stress test. If returns are 0% (a worst-case flat market), the same $10,000 and $500 per month over 20 years simply accumulates to $130,000, with zero growth. This example shows the floor: you still walk away with your contributions, which is why automated saving is valuable even before strong returns arrive.
Example 3 — A large starting balance. Suppose you invest a $1,000,000 windfall and add $10,000 per month for 10 years at 6%. The projected future value reaches about $3,458,190, with roughly $1,258,190 of growth on top of $2,200,000 contributed. The engine handles balances above one million dollars without rounding errors, so high-value savers can trust the result.
Example 4 — Raising the contribution. Increasing the monthly contribution from $500 to $750 on the same 30-year, 7% plan lifts the final balance by well over $300,000. Small, permanent increases compound dramatically over decades.
FAQ
How does this investment calculator work?
It adds the future value of your starting lump sum to the future value of your monthly contributions. Each month, the balance earns the monthly rate, and your contribution is added — exactly how a brokerage or retirement account grows over time.
Is the 7% annual return realistic?
Seven percent is the long-run average annual real return of a diversified US stock market, but any single year can swing from -30% to +30%. Treat the 7% default as a planning average, and run the calculator with 4% and 10% to see the range of outcomes.
What is the difference between this and a compound interest calculator?
A compound interest calculator usually projects a single lump sum growing with no new deposits. This investment calculator is built for recurring savings: it models regular monthly contributions on top of a starting balance, which is how most people actually build wealth over time.
Are contributions made at the start or end of each month?
This calculator assumes end-of-month contributions (an ordinary annuity), which is the standard, slightly more conservative convention. If you contribute at the very start of each month, your balance would be marginally higher.
Does this account for taxes, fees, and inflation?
No. The result is a gross projection before investment fees, taxes, and inflation. Subtract roughly 0.1%–1.0% per year for fund fees, and remember that the 7% default is already an after-inflation (real) figure; if you enter a nominal return, the future dollars will be worth less in today's purchasing power.
How much should I invest each month?
Aim to invest at least 10%–15% of your gross income for retirement. Use this calculator to test what a comfortable percentage feels like, then automate the transfer so the decision is made once rather than every month.
What happens if the expected return is 0%?
The calculator falls back to simple addition: your starting balance plus all your monthly contributions, with no growth. This shows the guaranteed floor of your savings even in a flat market.
Disclaimer
This investment calculator from Smekys is provided for informational purposes only and does not constitute financial, investment, tax, or legal advice. Projected values are estimates based on the inputs you enter and the standard future-value formula; actual investment returns are variable, not guaranteed, and can result in losses. Fees, taxes, inflation, and account rules are not modeled. Always consult a qualified financial advisor before making investment decisions.