Math
Compound Interest Calculator
Calculate the final balance and total interest by entering principal, annual rate, period, and monthly deposit. Use it for long-term investment simulations.
Result
| Final balance | ¥ {{ fmt(result.total) }} |
| Total invested | ¥ {{ fmt(result.invested) }} |
| Total interest | ¥ {{ fmt(result.interest) }} |
Tips
- Rule of 72: divide 72 by the annual rate to estimate the years needed to double your money. At 6%, that's about 12 years.
- Monthly deposits use monthly compounding; the principal uses annual compounding.
- In real-world investing, account for taxes (≈20% on gains), fees, and inflation.
- Try the long-term average return of a broad stock index (historically around 7–8% annually) as a reference rate.
Note — Is Compound Interest "Humanity's Greatest Invention"?
The phrase "Compound interest is the eighth wonder of the world" is widely attributed to Albert Einstein. However, it has never been found in any of his writings or recorded lectures, and many researchers consider it a misattribution. That said, the power of compound interest is real.
Invest ¥1,000,000 at 5% annually: after 10 years it grows to ≈¥1,630,000; after 20 years ≈¥2,650,000; after 30 years ≈¥4,320,000. Time is the real asset — that's the essence of compounding. The same logic works against you with high-interest debt: a revolving credit card balance at 15–18% nearly doubles in 4–5 years.
One reason Warren Buffett built extraordinary wealth is that he started investing in his teens. Because compound growth is exponential with time, starting just 10 years earlier can multiply the final outcome several times over.