Investment Calculator with Monthly Contributions
This investment calculator shows how much your money can grow over time when you start with an initial capital and add a fixed contribution every month. Enter your starting amount, your monthly contribution, the expected annual return and the number of years, and it instantly returns the projected final value, the total you will have contributed, and the gain generated by compound interest. It is ideal for planning retirement, a down payment, or any long-term savings goal where regular investing turns small amounts into a much larger sum.
Final value
Total contributed
Gain
Assumes a constant rate of return and fixed contributions over the entire period. It does not adjust for inflation or taxes. This is an estimate for informational purposes.
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How it works
The future value has two parts that are added together. First, the initial capital grows on its own through monthly compound interest: it is multiplied by (1 + r)^n, where r is the annual return divided by twelve and n is the total number of months. Second, each monthly contribution is invested and compounds from the month it is made until the end, which is the future value of an ordinary annuity: contribution × ((1 + r)^n − 1) / r. Adding the compounded initial capital and the compounded stream of contributions gives the final value. Because interest is earned on interest month after month, the longer you stay invested the larger the share of the result that comes from compounding rather than from your own contributions.
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Preguntas frecuentes
- What does this investment calculator do?
- It projects the future value of an investment that starts with an initial capital and receives a fixed monthly contribution, assuming a constant annual return compounded monthly. It shows three figures: the final value, the total amount you contributed, and the gain, which is the difference between the two and represents the compound interest earned.
- How is the future value calculated?
- The initial capital is compounded monthly over the whole period, and each monthly contribution is compounded from the month it is added until the end. In formula terms, final value = P·(1 + r)^n + c·((1 + r)^n − 1) / r, where P is the initial capital, c is the monthly contribution, r is the annual return divided by twelve, and n is the number of months.
- Is this suitable for planning retirement?
- Yes. Regular monthly investing over many years is exactly the scenario this calculator is built for, and it makes the power of compound interest very visible. It is a useful tool for retirement, education funds, or any long-term savings goal, though it should be treated as an estimate rather than a guarantee.
- Does it account for inflation and taxes?
- No. The calculator uses a constant nominal return and does not subtract taxes or adjust for inflation. Real purchasing power will be lower than the nominal final value, so for long horizons you may want to use a more conservative return that reflects returns after inflation.
- What return rate should I use?
- That depends on where you invest. Diversified stock portfolios have historically returned around 6–8% per year on average over the long run, while bonds and savings accounts tend to be lower. Because markets fluctuate, it is wise to try several rates to see a range of possible outcomes rather than relying on a single optimistic figure.