Free tool
Compound interest calculator
See how much your investment can grow with compound interest: enter your starting capital, what you contribute each month, the number of years and an estimated annual return. You will see your final balance and, most revealing of all, how much of it comes from interest rather than from what you put in.
Final balance
$82,178
Total contributed
$37,000
Interest earned
$45,178
Over 20 years, contributing $150/mo at 7% a year, 55% of your final balance comes from interest, not from what you put in. That is compound interest at work.
See year-by-year breakdown
| Year | Contributed | Value |
|---|---|---|
| 1 | $2,800 | $2,931 |
| 2 | $4,600 | $5,002 |
| 3 | $6,400 | $7,222 |
| 4 | $8,200 | $9,603 |
| 5 | $10,000 | $12,157 |
| 6 | $11,800 | $14,894 |
| 7 | $13,600 | $17,830 |
| 8 | $15,400 | $20,978 |
| 9 | $17,200 | $24,353 |
| 10 | $19,000 | $27,972 |
| 11 | $20,800 | $31,853 |
| 12 | $22,600 | $36,015 |
| 13 | $24,400 | $40,477 |
| 14 | $26,200 | $45,262 |
| 15 | $28,000 | $50,393 |
| 16 | $29,800 | $55,895 |
| 17 | $31,600 | $61,795 |
| 18 | $33,400 | $68,121 |
| 19 | $35,200 | $74,904 |
| 20 | $37,000 | $82,178 |
An estimate assuming a constant return and full reinvestment, for educational purposes only. Actual returns vary every year and are not guaranteed. This is not financial advice.
What compound interest is and why it is so powerful
Compound interest means earning returns on the returns you have already accumulated, not just on your starting capital. It is the difference between linear growth (simple interest) and exponential growth. Einstein supposedly called it the eighth wonder of the world—probably apocryphal—but the idea is real: the longer you let your money work, the faster it accelerates.
The key is to reinvest. If you take the interest out and spend it, you only get simple interest. If you leave it in, the following year it earns interest of its own, and so on. That is why a portfolio compounding at 8% a year does not take 12.5 years to double in a straight line—it roughly doubles every 9 years (the rule of 72: 72 ÷ 8 ≈ 9).
The big enemy is short-term thinking: breaking the compounding chain (selling and buying back in, paying high fees, or triggering needless taxes) stalls the snowball. The big ally is time: that is why starting early, even with a little, usually beats starting late with a lot.
Compound interest is only half the equation—the other half is where you invest. If you want to take this plan to specific companies, explore the stock screener, the best stocks by quality, or the investing guide to learn how to choose with criteria. And browse all the investing calculators to plan the rest of your financial-independence target.
The compound interest formula
Final balance = C₀ · (1 + r)ⁿ + A · [((1 + r)ⁿ − 1) ÷ r]
- · C₀ = starting capital
- · A = periodic contribution (monthly)
- · r = return per period (annual return ÷ 12 when you compound monthly)
- · n = number of periods (years × 12)
The calculator above solves this formula by iterating month by month, so the result includes both the growth of your starting capital and that of every reinvested monthly contribution.
Example: $150/month for 30 years
Say you start with $1,000 and contribute $150 a month for 30 years at an average return of 7% a year. You will have put in around $55,000 out of your own pocket… yet your final balance would land near $183,000.
In other words: more than $125,000 (about 70% of the total) comes from interest, not from your saving effort. That is what a straight-line spreadsheet never shows, and what makes the long run so different from the short run. Change the numbers above and see for yourself.
How to use this calculator
- 1.Enter your starting capital (what you already have invested or will invest today).
- 2.Set how much you can contribute each month on a consistent basis.
- 3.Choose your time horizon in years: compound interest rewards long holding periods.
- 4.Estimate a realistic annual return for your type of investment (and test pessimistic, base and optimistic scenarios).
- 5.Open the year-by-year breakdown to see when the snowball really starts to accelerate.
Common compound interest mistakes
- ✕Using an unrealistic return (a steady 15–20%): almost no one sustains that over the long run. Stay conservative.
- ✕Ignoring inflation: a 7% nominal return with 2.5% inflation is roughly 4.5% real in purchasing power.
- ✕Breaking the compounding chain by jumping in and out: every interruption and fee slows the snowball.
- ✕Underestimating time: the final years contribute the most, so quitting early forfeits the best part.
Compound interest only works if your investments are high quality and you do not overpay for them. With DeepTicker you can see whether a stock is good and whether it is cheap or expensive, made simple.
Start for free →Frequently asked questions
How is compound interest calculated with monthly contributions?
The balance is compounded every month at the monthly rate (the annual return divided by 12), and that month’s contribution is added on top. This calculator iterates month by month for the full number of years, so the result reflects both your starting capital and the reinvested monthly contributions.
What annual return should I use?
It depends on where you invest. As an educational benchmark, the broad U.S. stock market has returned roughly 7–8% a year on average over the long run (before inflation), but it swings a lot from year to year and is never guaranteed. Try a few scenarios to see how sensitive the outcome is.
Does compound interest account for inflation?
Not directly. The calculator shows a nominal figure. To see real purchasing power, subtract your expected inflation rate from your return (for example, 7% − 2.5% ≈ 4.5% real) and use that number instead.
Why does starting early matter so much?
Because compound interest grows exponentially: your returns earn returns of their own. Each extra year at the beginning carries far more weight than one at the end. That is why time is the long-term investor’s biggest ally.
Is this financial advice?
No. This is an educational tool that assumes a constant return. Real returns fluctuate and are not guaranteed, and every investment decision is your own responsibility.
This is not financial advice.