What is compound interest?
Compound interest is the famous “interest on interest”: each month, earnings join your balance and start earning too. It's the snowball effect that turns small amounts into large wealth over time.
Unlike simple interest, which always pays on the initial amount, compound interest accelerates growth. That's why starting early and keeping regular contributions matters more than finding the perfect investment.
How to calculate compound interest: the formula
The compound interest formula with monthly contributions is:
M = C × (1 + i)n + A × [((1 + i)n − 1) / i]
Where M is the final balance, C is the initial capital, i is the monthly rate (decimal), n is the number of months and A is the monthly contribution. Our calculator runs this month by month for you, no spreadsheet needed.
How to use the calculator in 4 steps
Enter the initial amount
How much you have to start today, even if it's $100.
Set the monthly contribution
How much you can invest every month without fail.
Adjust rate and term
Use your investment's real monthly rate and a long horizon (years, not months).
Hit calculate and compare scenarios
Change the contribution or term and instantly see the impact on the final balance.
Practical example: $500/month at 0.8% monthly
Investing $500/month for 10 years (120 months) at 0.8% per month, you contribute $60,000 and reach about $102,000. Almost half came from interest - without you working for that money. Extend to 20 years and it passes $340,000. Time is compounding's greatest ally.
5 tips to multiply your compound interest
- Start today, even with little: each delayed month costs a lot long-term.
- Automate the monthly contribution on payday.
- Reinvest 100% of earnings, with no withdrawals midway.
- Look for better rates with safety (Treasury bonds, CDs, low-cost funds).
- Track everything in the Finplan app to stay on course.
Frequently asked questions about compound interest
How to calculate compound interest with monthly contributions?
Multiply the initial amount by (1 + rate) to the power of months and add each contribution compounded month by month. The easiest way is to use the calculator above: enter initial value, contribution, monthly rate and term to see the final balance instantly.
What is the difference between simple and compound interest?
Simple interest always pays on the initial amount. Compound interest pays on the accumulated balance, creating exponential growth. Long-term, compounding earns far more.
How much does $1,000 grow with compound interest?
It depends on rate and term. At 0.8% monthly, $1,000 becomes about $1,100 in 1 year and $2,600 in 10 years, with no new contributions. With monthly contributions, the result is much larger.
Which monthly rate should I use?
Use your investment's net monthly rate. To convert an annual rate to monthly, use (1 + annual rate)^(1/12) − 1.
Is the calculator free? Do I need to sign up?
Yes, it's 100% free and needs no sign-up to simulate. If you want, you can leave your email after simulating to receive an exclusive Finplan gift.
Are the simulated values guaranteed?
No. It's an educational projection with a fixed rate, ignoring inflation, taxes, fees or volatility. Use it for planning, not as a return promise.
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