Investment Calculator – Compound Interest Growth

Future Value of Savings & Investment Tool

Savings / Investment Growth Formula

Formula

\[ FV = P \times \left( \frac{(1 + r)^n - 1}{r} \right) \]

Where:

$FV$ — future value of your savings
$P$ — monthly contribution
$r$ — monthly interest rate (annual rate divided by 12)
$n$ — total number of months (years $\times$ 12)

This is the future value of an annuity formula. It calculates the accumulated balance after making equal contributions at the end of every month, with interest compounding monthly. It assumes you start from a zero balance and make no withdrawals.

Savings / Investment Growth – Calculation Example

Example: you save $200 every month in an account that pays 6% annual interest, compounded monthly, for 10 years.

\[ r = \frac{6\%}{12} = 0.005, \quad n = 10 \times 12 = 120 \text{ months} \]

\[ FV = 200 \times \frac{(1 + 0.005)^{120} - 1}{0.005} \approx 200 \times 163.879 \approx 32{,}775.87 \]

Result: the future value is $32,775.87. Your total contributions are $200 $\times$ 120 = $24,000.00, so the interest earned is $8,775.87.

How to Use the Calculator

Enter the Monthly contribution — the amount you plan to deposit every month.
Enter the Annual interest rate — the yearly return you expect, as a percentage (e.g. 6 for 6%). You can use decimals such as 5.25.
Enter the Investment period in years — how long you will keep contributing.
Read the results: Future Value of Savings (the final balance), Total Contributions (the money you put in), and Interest Earned (the difference, i.e. the growth your money generated).

What Is Compound Savings Growth?

When you save a fixed amount every month and the account earns interest, two forces work together. First, your own contributions steadily add to the balance. Second, the interest you earn starts earning interest itself — this is compounding. Over time, compounding contributes an increasingly large share of the total growth, which is why even modest monthly amounts can grow into substantial sums over decades.

The results show three numbers, and each tells you something different. Total Contributions is the money you could have spent instead — your own effort. Interest Earned is what the account gave you for free, purely for letting the money sit and compound. Future Value of Savings is the sum of both: the amount you would have at the end of the period.

Notice how sensitive the result is to the interest rate and the time horizon. In the example above, 6% for 10 years turns $24,000 of contributions into roughly $32,776. Stretch the same plan to 20 years and the interest portion grows disproportionately, because the later years compound on a much larger balance. This is why financial planners emphasize starting early: time in the market is often more powerful than the size of individual deposits.

This calculator is useful for retirement planning, building an emergency fund, saving for a house deposit, a child's education, or comparing savings accounts and recurring-deposit products with different rates. If you are evaluating whether an investment's returns justify its risk, you may also find our Sharpe Ratio calculator and the Return on Capital Employed calculator helpful for assessing performance.

Assumptions and Limitations

Contributions are made at the end of each month (an ordinary annuity). If you deposit at the start of the month, the actual balance will be slightly higher.
Interest compounds monthly at one-twelfth of the annual rate. Products that compound daily or annually will give slightly different results.
The rate is assumed constant. Real investments (stocks, funds) fluctuate year to year; treat the result as a projection, not a guarantee. A 6% average is a common long-term assumption for diversified portfolios, but returns are never smooth.
No taxes, fees, or inflation are included. Management fees, account fees, and taxes on interest reduce the final balance. To compare contributions in today's purchasing power, discount the result by expected inflation.
No initial balance. The calculator starts from zero. If you already have savings, compute their growth separately (a lump-sum compound interest calculation) and add it to the result.
No withdrawals or skipped months. Every contribution is assumed to be made on schedule for the whole period.

Reference: Growth of $200/Month at Different Rates and Horizons

Annual rate	10 years	20 years	30 years
3%	$27,925	$65,645	$116,547
6%	$32,776	$92,408	$200,903
9%	$38,713	$133,277	$366,148

Total contributions for comparison: $24,000 over 10 years, $48,000 over 20 years, and $72,000 over 30 years. The table shows how a higher rate and a longer horizon multiply the interest portion — at 6% over 30 years, interest ($128,903) far exceeds the money you deposited yourself.

Frequently Asked Questions

How do I calculate savings growth by hand?

Use the future value of an annuity formula: $FV = P \times \frac{(1 + r)^n - 1}{r}$. First convert the annual rate to a monthly rate by dividing by 12 (e.g. 6% becomes 0.005), and multiply the years by 12 to get $n$. Raise $(1 + r)$ to the power of $n$, subtract 1, divide by $r$, then multiply by your monthly contribution. A spreadsheet or scientific calculator handles the exponent easily.

What is a good annual return rate to use?

It depends on the product. High-yield savings accounts and certificates of deposit often pay around 3–5%. A diversified stock portfolio is commonly projected at 6–8% per year before inflation over long horizons, though some years can be negative. Bonds typically sit between these ranges. Use a conservative estimate for planning, and remember that real market returns fluctuate — no rate is guaranteed.

What happens if I contribute at the beginning of the month instead of the end?

Deposits at the start of the month form an annuity due, which earns one extra month of interest on every contribution. Multiply the calculator's result by $(1 + r)$ to adjust. For example, at 6% annual over 10 years, the factor is 1.005, so the balance is about 0.5% higher — roughly $164 more on a $32,776 result. The difference grows with the rate but stays modest.

How do I account for inflation in the results?

The calculator shows nominal future dollars. To estimate real purchasing power, use an inflation-adjusted rate: subtract expected inflation from the return (e.g. 6% return minus 2% inflation ≈ 4% real rate) and run the calculation again. This approximation works well for typical rates. Over 20–30 years, inflation can substantially erode nominal gains, so real-value planning matters for long-term goals like retirement.

Can I include money I've already saved?

Yes. The calculator assumes a zero starting balance, so calculate your existing savings separately as a lump sum: $PV \times (1 + r)^n$, where $PV$ is the current balance. Then add that result to the future value from this calculator. For instance, $10,000 already saved at 6% for 10 years grows to about $18,194, which you'd add to your monthly-contribution total.

What's the difference between monthly and annual compounding?

Monthly compounding applies one-twelfth of the annual rate each month, which this calculator assumes. Annual compounding applies the full rate once a year, giving slightly less growth for the same stated rate. Daily compounding gives slightly more. The differences are small — typically a fraction of a percent per year — but over decades they add up, so check which method your bank or product actually uses.

Why is my real account balance different from the calculator's result?

Common reasons include fees (management or account fees deducted from the balance), taxes on interest or dividends, deposits made on different dates, variable interest rates that change over time, and skipped or irregular contributions. Market-based investments also fluctuate, so a steady 6% projection never matches reality exactly. Treat the result as a planning estimate, and revisit your assumptions yearly to stay on track.

Investment Calculator – Compound Growth