Albert Einstein may never have said that compound interest is the eighth wonder of the world — but the idea behind the quote is sound. Small, regular deposits can grow into surprisingly large amounts when the interest you earn also earns interest.
The formula
A = P × (1 + r/n)^(n × t). P is the starting amount, r the annual interest rate, n how many times interest compounds per year, and t the number of years. Our compound interest calculator handles this, including regular monthly contributions.
Why frequency matters
Daily, monthly, quarterly and yearly compounding all yield different results because interest begins earning its own interest earlier. For a given nominal rate, more frequent compounding is better for savers — but the difference is modest at typical rates.
The real lever is time
Run the numbers: two people saving the same amount every month end up with very different totals if one starts ten years earlier, because those extra years compound on top of every deposit that follows. Time, not cleverness, does most of the work.
Use it in both directions
Compound interest is a tailwind for savings and a headwind for loans. Credit-card debt compounds monthly at rates above 20%, which is why minimum payments barely move the balance. Understanding the formula helps you see why paying high-interest debt first is usually the smartest 'investment' available.