PlayInfoBox

Home /Investing /Compound Interest Basics

Compound Interest Basics

"Interest on interest" sounds simple enough, but really appreciating the power of compounding takes understanding how a handful of key variables interact.

What compound interest actually is

Compound interest means interest is earned not just on the original principal, but also on the interest that’s already accumulated, which then gets folded back into the principal for the next period — "interest on interest." Simple interest, by contrast, is always calculated on the original principal alone, without ever including previously earned interest in the base.

The basic compound interest formula

The formula for the future value of a compound-interest investment is A = P × (1 + r/n)^(n×t), where P is the principal, r is the annual interest rate, n is the number of times interest compounds per year, and t is the number of years invested. A higher compounding frequency n — say, monthly instead of annually — produces a slightly higher final return, all else being equal.

Time is one of the most important variables in compounding

At a given interest rate, the longer the investment horizon, the more pronounced the growth effect of compounding becomes — especially in later years, when growth accelerates noticeably faster than in the early years. That’s exactly why "start investing as early as possible" gets repeated so often.

The "Rule of 72": a quick way to estimate doubling time

Dividing 72 by an annual return rate (as a percentage) gives a rough estimate of how many years it takes for principal to double — at a 6% annual return, for example, it takes roughly 72 ÷ 6 = 12 years. It’s just a handy mental-math approximation; the actual result will vary somewhat depending on compounding frequency and other factors.

Compounding works against you too, on debt

Compound interest isn’t just about investment returns — credit card balances and certain loans that go unpaid for a long time also accrue interest on a compounding basis, which is part of why this kind of debt tends to snowball and is worth addressing sooner rather than later.

Nominal rate versus effective annual rate

When interest compounds more often than once a year, the effective annual return ends up slightly higher than the nominal annual rate on paper. A more accurate way to compare financial products is to convert everything to effective annual rate first, rather than comparing nominal rate figures alone.

The gap between compound and simple interest widens over time

Early on, the difference between compound and simple interest returns isn’t very noticeable, but as time passes, the "interest on interest" from compounding keeps stacking up, and the gap between the two keeps widening. This is exactly why so much financial education content stresses that compounding needs enough time to really show its effect.

The "Rule of 72" is an estimate, not a precise formula

The Rule of 72 tends to be most accurate when the annual return rate falls roughly between 6% and 10%; at rates much higher or lower than that range, the gap between the estimate and the exact calculation grows a bit wider. If you need a precise number, it’s still best to use the full compound interest formula or a dedicated calculator.

Frequently Asked Questions

Is bank deposit interest usually simple or compound?

It depends on the specific product and how interest is calculated. Demand deposits, and some fixed-term deposits that aren’t automatically rolled over at maturity, are typically calculated using simple interest. Many fixed-term deposits and investment products, though, compound within their agreed compounding periods — it’s best to check the product’s terms or confirm with your bank for the specific calculation method.

Is a higher compounding frequency always better?

At the same nominal annual rate, a higher compounding frequency does produce a slightly higher actual return, but the difference usually isn’t huge. Compared to compounding frequency, the size of the principal, the level of the nominal rate, and the length of the investment horizon typically matter much more for your final return.