A daily coffee costs about $5, or roughly $150 a month. That doesn’t sound like much. Invested instead, it can become one of the largest numbers in your financial life, thanks to compound interest.
What compounding means
Simple interest pays you only on the money you put in. Compound interest pays you on your money and on the interest it has already earned. In the first few years the difference is small. After a few decades, the interest on your interest can be larger than everything you contributed.
The ten-year head start
Compare three savers who each invest monthly and earn 7% a year until age 65:
- Starts at 25, saves $150/mo (40 years)
- $393,722
- Starts at 35, saves $150/mo (30 years)
- $182,996
- Starts at 35, saves $300/mo (30 years)
- $365,991
The early saver puts in $72,000. The late saver who doubles their contribution puts in $108,000, more money in total, and still ends up behind. Time does the heavy lifting.
The Rule of 72
For a quick estimate, divide 72 by your annual return to see how many years it takes your money to double. At 7%, money doubles about every 10 years. At 4%, it takes about 18. At a 24% credit card rate, your debt doubles in just 3 years, which is compounding working against you.
How to put it to work
- Start with any amount. A small automatic transfer beats a big plan you never begin.
- Keep costs low. A 1% yearly fee compounds too, quietly eating a large share of long-term growth.
- Use tax-advantaged accounts. A 401(k) or IRA lets more of your growth stay invested.
- Don’t interrupt it. Selling in a downturn stops compounding right when prices are lowest.