🔗 Use full-page Compound Interest Calculator here for best experience
What Is Compound Interest?
Compound interest is interest earned on both your original principal and the interest already accumulated. Unlike simple interest (which only applies to your original deposit), compound interest grows exponentially — the longer you wait, the faster your money grows.
Albert Einstein reportedly called it the “eighth wonder of the world.” Whether or not that’s true, the mathematics are undeniable.
How to Use the Calculator
- Enter your initial principal — the starting amount you’re investing or saving
- Enter the annual interest rate — historical stock market average is ~7% after inflation
- Set the time period in years
- Add a monthly contribution — even small amounts make a huge difference
- Choose compounding frequency — how often interest is applied
The Compound Interest Formula
For a lump sum without contributions:
A = P × (1 + r/n)^(n×t)
Where:
- A = final amount
- P = principal
- r = annual interest rate (as decimal)
- n = compounds per year
- t = time in years
Compounding Frequency Matters
| Frequency | Effect | Best For |
|---|---|---|
| Annually | Baseline | Bonds, some savings accounts |
| Quarterly | Slightly better | Some investment accounts |
| Monthly | More growth | Most savings accounts, CDs |
| Daily | Maximum growth | High-yield savings accounts |
The difference between monthly and daily compounding is small, but over decades it adds up.
The Power of Starting Early: Examples
Example 1: One-time $10,000 investment at 7%
| Years | Balance |
|---|---|
| 10 | $19,672 |
| 20 | $38,697 |
| 30 | $76,123 |
| 40 | $149,745 |
Example 2: $200/month at 7% (no initial deposit)
| Years | Total Contributed | Balance |
|---|---|---|
| 10 | $24,000 | $34,615 |
| 20 | $48,000 | $104,074 |
| 30 | $72,000 | $242,946 |
| 40 | $96,000 | $528,006 |
The $10/day rule
Saving just $10 a day ($300/month) at 7% for 30 years grows to $364,419 — even though you only contributed $108,000.
Tips for Maximising Compound Growth
- Start as early as possible — time in the market is more powerful than the amount invested
- Reinvest dividends — this is how you actually achieve compound growth in stock markets
- Increase contributions over time — even 1% more per year has a dramatic long-term impact
- Avoid withdrawing early — breaking the compounding chain is the biggest mistake investors make
- Use tax-advantaged accounts — 401(k), IRA, ISA — these let you compound without annual tax drag
Common Questions
What interest rate should I use?
For long-term stock market projections, 7% is a commonly used real (inflation-adjusted) return for diversified index funds. For savings accounts, use your actual APY (often 4–5% in 2024–25 high-yield accounts).
Does compound interest work against me with debt?
Yes — this is why credit card debt (often 20%+ APR, compounded daily) is so dangerous. The same mathematics that grows your savings destroys your finances when working against you.
Calculate Compound GrowthRelated Tools:
- Retirement Calculator — project your retirement savings
- Savings Goal Calculator — calculate how long to reach a target
- Loan Calculator — see how compound interest affects your debt