Compound Interest Calculator
Project how your savings or investments grow over time with compound interest and regular contributions.
Last updated
How it works
Enter a starting amount, an annual interest rate, and a time period in years, plus an optional monthly contribution. The calculator converts your annual rate to a monthly rate by dividing by 12, then works month by month: each month it multiplies the current balance by (1 + monthly rate) to apply that month's interest, then adds your monthly contribution on top before moving to the next month. Because the contribution is added after interest for the current month, it starts earning its own interest beginning the very next month.
For example, starting with $5,000, contributing $200 a month, at a 7% annual rate, over 10 years, the balance grows to $44,665.27. Of that, $29,000 came from your own contributions (the $5,000 starting amount plus $200 x 120 months) and the remaining $15,665.27 came purely from compounding interest. Compare that to the same $5,000 with no monthly contribution at all: it only grows to $10,048.31 over the same 10 years, which shows how much of the total growth in the first example is driven by consistent contributions rather than the starting amount alone. Time also matters enormously: the same $5,000 start and $200 monthly contribution at 7% grows to $124,379.03 over 20 years instead of 10, more than double, since a much larger balance is earning interest for a much longer stretch.
This calculator assumes a constant annual interest rate applied every single month with no interruptions and monthly compounding throughout. Real investment returns, especially in the stock market, fluctuate year to year and aren't guaranteed the way this fixed-rate projection implies, so treat the output as an illustration of how compounding works rather than a promise of what any specific account or investment will actually return. It also doesn't subtract taxes, fees, or account for withdrawals during the period.
Frequently asked questions
What compounding frequency does this use?
Monthly compounding, which is a common assumption for savings accounts and many investment projections. It calculates interest and adds it to the balance 12 times a year rather than once annually, which slightly speeds up growth compared to annual compounding at the same stated rate.
Can I include regular contributions?
Yes, enter an optional monthly contribution and it's added at the end of each month, after that month's interest is applied to the existing balance and before interest is calculated for the following month. This means each contribution starts earning interest the month after you make it.
Is the growth this calculator shows guaranteed?
No. This tool assumes one constant interest rate applied consistently over the entire period, which is realistic for something like a fixed-rate savings account but not for stock market investments, where returns vary significantly from year to year and can be negative in some years. Use it to understand how compounding works mathematically, not as a guarantee of future returns.
What's a realistic interest rate to enter?
For a savings account or CD, use the actual advertised APY. For a diversified stock market investment, many long-term projections use a historical average in the range of 7-10% before inflation, though this varies by source and time period and isn't a guaranteed future rate. Generally, higher assumed returns come with higher risk and more year-to-year variability.
Does this account for taxes on investment gains?
No, the final balance and interest earned figures are shown before any taxes. Depending on the account type, taxable brokerage accounts, tax-deferred retirement accounts, and tax-free accounts like Roth IRAs are all taxed differently, so your actual after-tax growth will differ from what's shown here.
What if I withdraw money partway through the time period?
This calculator only models a balance that grows through contributions and interest, with no withdrawals. Withdrawing funds partway through reduces the balance that future interest is calculated on, so your actual ending balance would be lower than this projection if you take money out along the way.
Why does starting earlier matter so much for compound interest?
Because interest earns interest on itself over time, so a longer time horizon lets a smaller amount of money do more work. In one example, the same $5,000 starting amount and $200 monthly contribution at 7% grows to $44,665.27 over 10 years but $124,379.03 over 20 years, more than double, purely from having twice as long for the balance to compound.
Tracking contributions alongside your investment growth? The Clever Fox Budget Planner is a well-reviewed way to keep it all on paper.
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