See how your money grows with the power of compounding. Calculate compound interest, compare strategies, and visualize your wealth over time.
The Rule of 72 estimates how long it takes for an investment to double. Divide 72 by the interest rate to find the number of years — or divide 72 by the years to find the required rate.
See the dramatic difference between earning interest on interest (compound) versus only on the original principal (simple). Based on $10,000 at 10% annual rate.
How $10,000 with $500/mo contributions grows at different annual return rates over 30 years. Even small rate differences create massive gaps.
How does compounding frequency affect your returns? Here's $10,000 at 10% over various periods:
| Frequency | 10 Years | 20 Years | 30 Years |
|---|---|---|---|
| Annually (1×/yr) | $25,937 | $67,275 | $174,494 |
| Quarterly (4×/yr) | $26,851 | $72,096 | $193,581 |
| Monthly (12×/yr) | $27,070 | $73,281 | $198,374 |
| Daily (365×/yr) | $27,183 | $73,891 | $200,855 |
* The biggest jump is from annual to quarterly. Monthly to daily makes less than 1% difference in most cases.
Compound interest is often called the "eighth wonder of the world" — a quote widely attributed to Albert Einstein, who supposedly added: "He who understands it, earns it; he who doesn't, pays it." Whether Einstein actually said it or not, the principle is undeniably powerful.
Unlike simple interest, which only earns returns on your original principal, compound interest earns returns on both your principal and all previously accumulated interest. This creates a snowball effect: each year, your interest earns its own interest, and that interest earns interest, and so on. The longer you let it run, the more explosive the growth becomes.
The compound interest formula is: A = P(1 + r/n)nt, where P is your principal, r is the annual interest rate, n is the number of times interest compounds per year, and t is the number of years. With regular contributions, the future value of an annuity formula adds: FV = PMT × [((1 + r/n)nt - 1) / (r/n)].
The key insight is that compound growth is exponential, not linear. This means the first few years feel slow, but given enough time, the curve turns dramatically upward. This is why starting early is the single most important factor in building wealth — and why compound interest is the foundation of every long-term investment strategy.
Compound interest is interest calculated on both the initial principal and all previously accumulated interest. Unlike simple interest which only earns on the original amount, compound interest creates a snowball effect where your money earns returns on its returns. The formula is A = P(1 + r/n)^(nt), where P is principal, r is the annual rate, n is compounding frequency, and t is time in years.
The Rule of 72 is a simple formula to estimate how long it takes for an investment to double. Divide 72 by your annual interest rate to get the approximate number of years. For example, at 8% interest, your money doubles in roughly 72 ÷ 8 = 9 years. It works best for rates between 4% and 15%.
More frequent compounding produces slightly higher returns because interest begins earning interest sooner. For a $10,000 investment at 10% over 20 years: annual compounding yields $67,275, monthly yields $73,281, and daily yields $73,891. The difference is most noticeable at higher rates and longer time periods, though monthly vs daily compounding has minimal practical impact.
Simple interest is calculated only on the original principal amount, using the formula I = P × r × t. Compound interest is calculated on principal plus accumulated interest, using A = P(1 + r/n)^(nt). Over long periods, the difference is dramatic: $10,000 at 10% for 30 years grows to $40,000 with simple interest but $174,494 with compound interest — more than 4× as much.
Investing $500 per month at a 10% average annual return for 30 years would grow to approximately $1,130,244. Your total contributions would be $180,000, meaning compound interest generated over $950,000 in returns — more than 5× your invested amount. Starting just 10 years earlier could nearly triple your final balance due to the exponential nature of compounding.