Definition
Interest calculated on the initial principal, which also includes all of the accumulated interest from previous periods on a deposit or loan.
In depth
Compound interest is the process by which interest is calculated on both the original principal and all previously accumulated interest. Unlike simple interest (which grows linearly by a fixed dollar amount each period), compound interest grows exponentially — the more time passes, the faster the total value increases. This is the fundamental mathematical mechanism behind long-term wealth building in savings accounts, investment portfolios, and retirement funds.
The formula is: A = P(1 + r/n)^(nt), where A is the final amount, P is the principal, r is the annual interest rate, n is compounding periods per year, and t is years. The frequency of compounding matters: daily compounding yields slightly more than monthly, which yields more than annual. Most savings accounts compound daily; investment returns compound continuously through reinvested dividends and capital appreciation.
The Rule of 72 provides a practical shortcut: divide 72 by your annual return rate to estimate the years needed to double your money. At 6% annual return, doubling takes approximately 12 years. At 9%, approximately 8 years. At 12%, approximately 6 years.
Time is the single most powerful variable in compounding. An investor who contributes $5,000/year from age 25 to 35 (10 years, then stops) will typically accumulate more by retirement than someone who contributes the same $5,000/year from age 35 to 65 (30 years continuously) — because the first investor's early contributions had decades more to compound. Starting early, even with small amounts, is the most important actionable insight in personal finance.
Frequently asked questions
What is the difference between compound and simple interest?
Simple interest calculates returns only on original principal: $1,000 at 5% earns exactly $50/year, every year — $1,500 after 10 years. Compound interest earns returns on principal plus accumulated interest: $1,000 at 5% compounded annually becomes $1,629 after 10 years and $4,322 after 30 years. Over long periods, the difference is enormous.
Does compound interest ever work against me?
Yes — dramatically. Credit card debt compounds against you at 20–30% APR. A $5,000 balance at 25% APR making only minimum payments can take 15+ years to repay and cost 2–3x the original balance in interest. The same mathematical force that builds wealth in investments can destroy it in high-interest debt, which is why eliminating credit card debt is typically the highest-return financial move available.
Which accounts or investments benefit most from compounding?
Any investment that automatically reinvests returns: index funds and ETFs with dividend reinvestment, target-date retirement funds, and interest-bearing savings accounts. Tax-advantaged accounts (401k, Roth IRA) amplify compounding by eliminating annual tax drag on growth. A 1% annual management fee — seemingly small — can reduce your final compounded portfolio value by 20–30% over 30 years.
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