How the Compound Interest Calculator Works
This calculator uses the standard compound interest formula to project how your money grows over time. It combines two components: the future value of your initial investment compounding at the chosen frequency, and the future value of your regular monthly contributions, also compounded. The results update instantly as you change any input.
What Each Input Does
- Initial Investment: the lump sum you are starting with, such as current savings or an initial deposit.
- Monthly Contribution: a fixed amount added every month. Even small regular contributions have an outsized effect over long periods because each deposit also starts compounding immediately. Set this to zero to see pure lump-sum growth.
- Annual Interest Rate: the expected annual return. For broad stock market index funds, historical long-run nominal returns have averaged roughly 7 to 10 percent before inflation. For high-yield savings accounts or CDs, typical rates are lower.
- Time Period: the number of years you plan to let the money grow. Time is the most powerful variable in compound growth; doubling the time period has a much larger effect than doubling the rate.
- Compounding Frequency: how often interest is calculated and added to the balance. Most savings accounts compound daily or monthly. The difference between daily and monthly is usually small, but both outperform annual compounding over long periods.
Reading the Growth Table
The year-by-year table below shows your projected balance, total contributions to that point, and cumulative interest earned at the end of each year. The last row shows the final totals. Notice how the interest earned column accelerates in later years, which is the compounding effect in action.
Year-by-Year Growth Table
Projected balance, total contributions, and cumulative interest at the end of each year.
| Year | Balance | Total Contributions | Interest Earned |
|---|
Results are projections assuming a fixed annual rate and constant monthly contributions. Actual returns will vary.
Frequently Asked Questions
Compound interest is interest calculated on both your initial principal and the accumulated interest from previous periods. Unlike simple interest, which only earns on the original amount, compound interest grows exponentially over time because each period's interest is added to the balance before the next period's interest is calculated. This is why it is often described as earning interest on your interest.
The more frequently interest compounds, the more you earn. Daily compounding produces slightly more than monthly compounding, which produces more than quarterly, which produces more than annual compounding, all at the same annual rate. The difference between daily and monthly compounding is usually small, but the difference between monthly and annual compounding can be meaningful over long periods at higher rates.
The standard formula is A = P(1 + r/n)^(nt), where A is the final amount, P is the principal, r is the annual interest rate as a decimal, n is the number of compounding periods per year, and t is the time in years. When regular contributions are added, their future value is calculated separately using the annuity formula and added to the principal's future value.
The Rule of 72 is a quick mental math shortcut for estimating how long it takes money to double at a fixed annual rate. Divide 72 by the annual interest rate, and the result is approximately the number of years to double. For example, at 6 percent per year, your money doubles in roughly 72 divided by 6 equals 12 years. The rule is an approximation; the calculator gives exact results.
No. This calculator shows nominal growth before taxes and inflation. In practice, investment returns in taxable accounts are reduced by capital gains or income taxes, and the real purchasing power of the final amount is reduced by inflation over time. Tax-advantaged accounts such as 401(k) plans and IRAs defer or eliminate taxes on growth, which is one reason they are recommended for long-term investing.
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