Compound Interest Calculator

How Compound Interest Builds Wealth

Compound interest is the single most powerful concept in personal finance. When your investment earns returns, those returns are reinvested and begin earning their own returns. Over time, this compounding effect causes exponential growth that far exceeds what simple interest would produce.

Consider a simple example: $10,000 invested at 7% annually becomes $19,672 after 10 years with compound interest, but only $17,000 with simple interest. After 30 years, the gap is dramatic: $76,123 with compounding versus $31,000 with simple interest. The longer money compounds, the more powerful the effect becomes.

This calculator lets you model exactly how your savings and investments will grow based on your specific numbers. Enter your starting amount, how much you contribute each month, the expected return rate, and the time horizon to see a detailed year-by-year projection.

The Impact of Regular Contributions

Starting with a lump sum matters, but consistent monthly contributions often have a larger impact on your final balance than the initial principal. Someone who starts with $5,000 and contributes $500 per month at 7% for 20 years will accumulate over $265,000 — with $125,000 of that coming from contributions and over $135,000 from compound interest.

The key insight is that each contribution starts compounding from the moment it is deposited. Earlier contributions compound for more years and therefore generate more interest than later ones. This is why starting early, even with small amounts, is consistently recommended by financial advisors.

Choosing the Right Compounding Frequency

Banks and investment accounts compound at different frequencies. Savings accounts typically compound daily, while bonds may compound semi-annually. The difference between daily and annual compounding on a 5% return over 10 years is relatively small — about 0.13% — but it becomes more meaningful at higher rates and longer time horizons.

For most practical purposes, monthly compounding is a reasonable assumption when modeling investment growth in diversified stock portfolios.

Comparing Investment Scenarios

The most valuable use of this calculator is comparing different scenarios. What happens if you increase your monthly contribution by $200? How much sooner could you reach your goal at 8% versus 6%? What if you start five years earlier with less money?

Use the Compare mode to place two scenarios side by side and see exactly how changes in any variable affect the final outcome. These comparisons turn abstract financial planning into concrete numbers you can act on.