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13 Jan 2026 • 16:10
4 min read
Compound interest is interest earned on both the original amount you invest and on interest that has already been added. Over time this creates exponential growth rather than the straight-line growth you get with simple interest.
That distinction matters because the longer you leave money to compound, the more dominant the interest-on-interest effect becomes. Small differences in rate or time produce large differences in outcomes.
Suppose you put money into an account that pays a fixed annual rate and you leave all interest in the account. Each year the balance increases, and the next year interest is calculated on the new, larger balance.
Mathematically the future value after t years is A = P (1 + r/n)^(n t), where P is the starting principal, r is the annual rate, n is how often interest compounds each year, and t is time in years. More frequent compounding slightly raises the end amount for the same nominal rate.
Time is the multiplier in compounding. Early contributions have many more periods to grow, so they contribute disproportionately to the final value. This is why steady saving from an early age is practical and effective.
Trying to pick the best short-term investments is less important for most long-term goals than maintaining consistent savings and staying invested. Patience and regular contributions allow the mechanics of compounding to work for you.
A quick way to estimate how long money will take to double is the Rule of 72: divide 72 by the annual interest rate. For example, at 6% a balance roughly doubles in 12 years.
The Rule of 72 is an approximation that is most accurate for rates in the mid single digits to low double digits and is handy for mental math. For precise planning, use exact formulas or a calculator. Investopedia explains the Rule of 72 and its limits.
Compound returns are reduced by fees and taxes. A seemingly small annual fee or tax drag compounds too, cutting deeply into long-term results. Always compare net returns — the yield after costs — when evaluating accounts or funds.
For consumer accounts and loans, look for the effective annual rate or annual percentage yield. These measures account for compounding frequency and let you compare offers on the same basis. Clear disclosure helps you see the real compounding effect.
Savings accounts, certificates of deposit, and many investment funds compound returns over time. Retirement accounts that reinvest dividends and interest are typical examples where compounding builds wealth across decades.
Compound interest also works against you for loans and credit card balances. Interest that capitalizes on unpaid balances causes debt to grow faster, which is why paying down high-rate debt quickly is often the best financial move.
Start with regular contributions, even modest ones. The habit of consistent saving increases the base that compounds and removes the need to time markets.
Keep costs low by preferring low-fee funds and tax-advantaged accounts where appropriate. Over long horizons, lower expenses can add substantially to your final balance. For clear comparisons of simple versus compound interest mechanics, see the overview at Investopedia.
Compounding increases both upside and downside depending on the asset. Use conservative, realistic return assumptions when planning for retirement or other long-term goals. Avoid promises of unusually high short-term gains.
A steady plan that balances savings rate, reasonable return assumptions, low costs, and time produces reliable outcomes. Compounding is a tool; disciplined use of that tool is what builds durable wealth.
