Watch your money compound, not just accumulate.
Enter what you're starting with, what you'll add each month, and how long you'll let it ride. We'll show you exactly how much is your money — and how much is the interest doing the work.
Run the numbers
All fields update your results live as you type.
Interest makes up 0% of your final balance — that's money you didn't have to save, earned purely by time in the market.
The shape of compounding
Compound growth looks linear for years, then bends upward fast. This is that curve, built from your numbers.
Year-by-year balance schedule
See exactly how much of each year's growth came from your contributions versus interest.
| Year | Starting balance | Contributions | Interest earned | Ending balance |
|---|
The formula behind the numbers
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1
Your principal starts earning
Interest is calculated on your initial deposit for the first compounding period.
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2
Interest gets added to the balance
That interest is deposited into your balance — it doesn't go anywhere else.
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3
The next period compounds on more money
Now the following period's interest is calculated on principal + prior interest, not just the original amount.
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4
Contributions accelerate the curve
Every monthly deposit becomes new principal that starts compounding from the moment it lands.
What this looks like in practice
Starting at 30, retiring at 55
Maria opens an account at 30 with $10,000, adds $300 a month, and earns an average 8% a year, compounded monthly. By the time she's 55 — 25 years later — here's where she lands.
Advantages and limitations
Advantages
- Growth accelerates the longer money is left untouched.
- Small, consistent contributions matter more than timing the market.
- Works passively — no active management required.
Limitations
- This calculator assumes a constant rate — real markets fluctuate year to year.
- Doesn't account for taxes, fees, or inflation eroding real returns.
- Early withdrawals or interruptions to contributions significantly change outcomes.
Frequently asked questions
Compound interest is interest calculated on both the initial principal and the interest that has already accumulated. Unlike simple interest, it lets your earnings generate their own earnings over time.
The core formula is A = P(1 + r/n)^(nt), where P is principal, r is the annual interest rate, n is the number of compounding periods per year, and t is the number of years. Regular contributions are added on top of this base calculation for each period.
More frequent compounding produces slightly higher returns. Daily compounding will outperform annual compounding on the same nominal rate, though the difference is usually small at typical savings rates.
Yes. Regular contributions are typically the single biggest driver of your final balance over long time horizons, often outweighing the interest rate itself in the first decade.
For a high-yield savings account, 4-5% is typical. For a diversified stock portfolio over the long run, many planners use a historical average between 7-10% before inflation.