Discover the power of compound interest. See how your savings snowball over time with regular contributions and different compounding frequencies.
Same inputs, different compounding frequencies
| Frequency | Times/Year | Final Balance | Total Interest | Difference vs Annual |
|---|
| Year | Starting Balance | Contributions | Interest Earned | Ending Balance |
|---|
Enter your starting principal — the initial amount you're investing or saving. Then add your monthly contribution — the amount you plan to add each month going forward. Set your expected annual interest rate, choose how often interest compounds, and select your time horizon in years.
The calculator instantly shows your final balance, the total amount you contributed, the total interest earned (the power of compounding), and your overall return on investment. The stacked area chart clearly shows how the interest component accelerates over time — this is the famous "hockey stick" of compound growth.
For a lump sum with no additional contributions:
Of that $343,778, your total contributions were $10,000 + ($500 × 240) = $130,000. The remaining $213,778 is pure compound interest.
The power of compound interest becomes dramatic over long time horizons. Here are real examples using 8% annual return, monthly compounding:
This is why starting as early as possible is the single most important thing you can do for long-term savings. Time, not the rate of return, is the primary driver of wealth.
More frequent compounding yields slightly more, but the difference between daily and monthly compounding is small. What matters far more is the interest rate and your time horizon. That said, compounding frequency does have a real impact over decades:
Use the comparison table above to see the exact difference for your inputs. The "effective annual rate" (EAR) accounts for compounding: EAR = (1 + r/n)^n − 1.
The compound interest formula is A = P(1 + r/n)^(nt), where A is the final amount, P is the starting principal, r is the annual rate as a decimal, n is compounding periods per year, and t is years. When you add regular contributions (PMT), the full formula is: A = P(1+r/n)^(nt) + PMT × [(1+r/n)^(nt) − 1] / (r/n).
More frequent compounding (daily vs. monthly vs. annual) yields slightly more, but the differences are small compared to the impact of your interest rate and time horizon. Daily compounding at 8% has an effective annual rate of 8.328%, versus monthly at 8.300% and annual at exactly 8%. Use the comparison table to see the exact difference for your inputs.
The Rule of 72 estimates how long it takes money to double at a given interest rate. Simply divide 72 by the annual rate: at 8%, money doubles in roughly 9 years (72 ÷ 8 = 9). At 6%, about 12 years. At 10%, about 7.2 years. It's a useful mental shortcut for gauging the power of compound growth.
Starting earlier is the single most powerful factor. A 25-year-old who saves $500/month for just 10 years ($60K total) and then stops will — at 8% return — end up with more at age 65 than a 35-year-old who saves $500/month for 30 years ($180K total). The extra 10 years of compounding on that early money is more powerful than three times as many contributions made later.