Calculate how your money grows over time with compound interest. Add monthly contributions and see a full year-by-year breakdown — instantly, for free.
Compound interest is the process of earning interest not only on your original principal, but also on the interest you have already accumulated. In other words, your interest earns interest — and this snowball effect is one of the most powerful forces in personal finance.
When you deposit money in a savings account or invest in the stock market, your returns are typically compounded over time. Each compounding period, the bank or broker calculates interest on your current balance (not just your original deposit), adds it to your account, and then calculates the next period's interest on the new, larger balance.
A = P × (1 + r/n)^(n×t)Suppose you invest $10,000 at an annual interest rate of 5%, compounded monthly, for 10 years. Using our calculator:
Now compare: with simple interest at the same 5%, you would earn exactly $500/year × 10 years = $5,000 in interest — giving you $15,000. Compound interest adds an extra ~$1,470 for doing nothing differently, simply by letting interest compound each month.
The core difference is what the interest is calculated on:
| Feature | Simple Interest | Compound Interest |
|---|---|---|
| Calculated on | Principal only | Principal + accumulated interest |
| Growth curve | Linear | Exponential |
| $1,000 at 5% for 10 years | $1,500 | $1,629 (monthly) |
| $1,000 at 5% for 30 years | $2,500 | $4,467 (monthly) |
| Common in | Short-term loans | Savings, investments, mortgages |
For short time periods the difference is modest. Over 20–40 years, compound interest produces dramatically larger results — which is why financial advisors consistently emphasize starting early.
At 6% annually, money doubles in roughly 12 years. At 9%, it doubles in just 8 years.
The Rule of 72 is a mental math shortcut that gives a surprisingly accurate estimate. Divide 72 by the annual interest rate and you get the approximate number of years for your investment to double. For example:
The more often interest is compounded within a year, the higher the effective annual yield. Here is how $10,000 at 5% grows over 10 years depending on frequency:
| Compounding | Final Balance | Total Interest |
|---|---|---|
| Annually | $16,288.95 | $6,288.95 |
| Semi-annually | $16,386.16 | $6,386.16 |
| Quarterly | $16,436.19 | $6,436.19 |
| Monthly | $16,470.09 | $6,470.09 |
| Daily | $16,486.65 | $6,486.65 |
The difference between annual and daily compounding is roughly $200 over 10 years — meaningful but not dramatic. The real power of compound interest comes from the length of time, not the compounding frequency alone.
Time is the most critical variable in compound interest. Consider two investors, both aiming for retirement at age 65:
Investor A invested for only 10 years yet ends up with more money than Investor B who contributed for 30 years — all because of the extra decade of compounding. This is why starting early, even with small amounts, is one of the most impactful financial decisions you can make.
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest, it grows exponentially over time.
The formula is A = P(1 + r/n)^(nt), where P is principal, r is annual rate (decimal), n is compounding frequency per year, and t is time in years.
The more frequently interest is compounded, the faster the balance grows. Daily compounding yields slightly more than annual compounding for the same rate.
The Rule of 72 is a quick mental math shortcut: divide 72 by the annual interest rate to estimate how many years it takes to double your money. At 6%, money doubles in about 12 years.
Simple interest is calculated only on the principal. Compound interest is calculated on principal plus accumulated interest, leading to faster growth over time.
Yes. Enter an optional monthly contribution amount and the calculator includes it in the final balance and year-by-year breakdown using the future value of annuity formula.
The calculator supports annually (1×/year), semi-annually (2×/year), quarterly (4×/year), monthly (12×/year), and daily (365×/year).
Compound interest works in your favor when you are saving or investing (your money grows faster), but against you when you carry debt (your balance grows faster). Understanding it is key to financial health.
At 7% compounded annually, $10,000 grows to approximately $38,697 — nearly 4× the original amount — purely from compound interest.
For long-term stock market estimates, 7–10% annually is commonly used (historical S&P 500 average). For savings accounts, 4–5% is typical in higher-rate environments.
Yes. Enter your current savings as principal, your expected annual return, the number of years to retirement, and any monthly contributions to project your retirement balance.
The longer the time horizon, the more compounding cycles occur and the more interest earns interest on itself. This exponential effect is why starting to invest early makes such a dramatic difference.