Investment Calculator

Compound interest is often called the eighth wonder of the world — and for good reason. Unlike simple interest (which earns returns only on your original principal), compound interest earns returns on your returns, creating exponential growth over time.

The key variables are: principal (starting amount), contribution (regular additions), annual interest rate (expected return), compounding frequency, and time. Of these, time has the most dramatic effect. An investment held for 40 years at 8% will grow to roughly 20× the principal — compared to just 10× over 30 years. That extra decade nearly doubles your final balance.

This calculator handles three scenarios: a lump sum investment with no contributions, regular contributions with no initial lump sum, and a combination of both (lump sum plus ongoing contributions). The last scenario — investing an initial amount and continuing to add to it regularly — is how most retirement accounts and savings plans work in practice.

Compounding frequency matters slightly. Daily compounding produces a tiny bit more growth than annual compounding at the same stated rate. At 5% annually, daily compounding produces 5.127% effective yield vs 5.000% for annual compounding. In practice, most online savings accounts compound daily; most bonds and mortgages compound monthly.

For long-term investors, the most important practical insights from compound interest: (1) Start as early as possible — even small contributions made consistently in your 20s dramatically outperform larger contributions started in your 40s. (2) Never interrupt compounding unnecessarily — pulling money out resets the growth curve. (3) Keep fees low — a 1% annual fee sounds small but reduces a 7% return to 6%, which costs roughly 15–20% of your total wealth over 30 years.

Use this calculator to compare investment scenarios, model retirement savings, or visualize how an emergency fund would grow in a high-yield savings account.