Simple vs compound interest
Simple interest is calculated only on the original amount. If $10,000 earns 5% simple interest for one year, the interest is $500. In year two, the interest is still based on the original $10,000.
Compound interest is calculated on the original amount plus previous interest or growth. If $10,000 grows by 5%, it becomes $10,500. The next 5% is calculated on $10,500, not just the original $10,000.
That difference looks small at first. Over long periods, it becomes the whole point.
| Year | Simple interest | Compound interest |
|---|---|---|
| 0 | $10,000 | $10,000 |
| 1 | $10,500 | $10,500 |
| 5 | $12,500 | $12,763 |
| 10 | $15,000 | $16,289 |
| 20 | $20,000 | $26,533 |
| 30 | $25,000 | $43,219 |
Why time matters
Compounding needs time because the base gets larger. Early on, most growth may come from your contributions. Later, growth on previous growth can become a bigger part of the result.
This is why small consistent contributions can matter. The early dollars have more years to work. Missing the first decade means later contributions have to work harder.
Frequency matters, but not as much as people think
Compounding frequency means how often interest or growth is added to the balance. Daily compounding grows slightly faster than monthly compounding, which grows slightly faster than annual compounding, all else equal.
The difference from frequency is usually smaller than the difference from starting earlier, contributing more, reducing fees, or earning a higher long-term return.
Frequency matters more for savings accounts and debt interest. For investments, market returns do not arrive in neat monthly or daily chunks, so the useful focus is long-term total return after fees and taxes.
| Frequency | Approximate ending balance |
|---|---|
| Annual | $16,289 |
| Monthly | $16,470 |
| Daily | $16,487 |
Example growth with contributions
Real investing usually includes recurring contributions. In the early years, contributions often do more work than returns. Over time, the existing balance can start doing more of the lifting.
| Years | Contributed | Approximate ending balance | Growth |
|---|---|---|---|
| 5 | $15,000 | $17,442 | $2,442 |
| 10 | $30,000 | $41,163 | $11,163 |
| 20 | $60,000 | $115,510 | $55,510 |
| 30 | $90,000 | $251,129 | $161,129 |
| 40 | $120,000 | $497,622 | $377,622 |
Starting early vs late
Starting early is powerful because time does part of the work. Starting late is not hopeless, but the monthly contribution usually has to be higher.
The lesson is not to panic. It is to start with the numbers today instead of waiting for the perfect plan.
| Investor | Contribution | Years invested | Approximate balance at 6% |
|---|---|---|---|
| Starts at 25 | $250/month | 40 | $497,622 |
| Starts at 35 | $250/month | 30 | $251,129 |
| Starts at 45 | $250/month | 20 | $115,510 |
| Starts at 35 but contributes more | $500/month | 30 | $502,258 |
Inflation changes the meaning of growth
Nominal growth is the dollar amount before inflation. Real growth adjusts for inflation and better reflects buying power.
If your investment grows 6% and inflation is 3%, your real return is much closer to 3% than 6%. The account balance may rise while the future purchasing power rises much less.
For long-term goals like retirement, always check both nominal and inflation-adjusted results.