Compound Interest

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Calculate future value with regular contributions and compounding.

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Common Questions
Compound Interest Calculator FAQ

How does compound interest differ from simple interest? ▾
Simple interest calculates interest only on the original principal. Compound interest calculates interest on the principal plus accumulated interest — so your earnings grow exponentially over time. At 7% annual return, $10,000 grows to about $19,670 in 10 years with simple interest, but $19,672 with annual compounding — and the gap widens dramatically the longer you invest.
How often does compounding frequency matter? ▾
Daily compounding produces slightly more than monthly, which produces slightly more than annual. On a $10,000 investment at 7% over 30 years: annual compounding gives $76,123; monthly compounding gives $81,165; daily compounding gives $81,645. For long-term investments, compounding frequency matters less than time in the market and rate of return.
What is the Rule of 72? ▾
The Rule of 72 is a quick mental math shortcut: divide 72 by your annual interest rate to estimate how many years it takes to double your money. At 6%, your money doubles in approximately 12 years. At 9%, about 8 years. It's not exact, but it's accurate enough for quick comparisons.
Why does starting early matter so much? ▾
Starting 10 years earlier can nearly double your final balance due to compounding. Contributing $300/month from age 25 to 65 at 7% grows to about $786,000. Starting at 35 grows to only about $365,000 — less than half — despite only missing 10 years of contributions. The first decade of compounding does enormous work.
Does compound interest work against me too? ▾
Yes — debt compounds exactly the same way but in reverse. A $5,000 credit card balance at 22% APR, paid only at the minimum, can take over 20 years to pay off and cost more than $8,000 in interest. High-interest debt is destructive precisely because compounding accelerates the balance faster than minimum payments can reduce it.

How Compound Interest Works

Compound interest means earning interest on your interest — not just on your original principal. In simple interest, $1,000 at 10% earns $100 every year regardless. With compound interest, the $100 earned in year one becomes part of the principal in year two, so you earn 10% on $1,100, then $1,210, then $1,331. The difference seems small early on but becomes enormous over time. At 7% annual return, money doubles roughly every 10 years — a $10,000 investment becomes $20,000 in a decade, $40,000 in two, $80,000 in three.

Compounding frequency matters: daily compounding produces slightly more than monthly, which produces slightly more than annual. For most long-term investments like stocks and index funds, the compounding is effectively continuous — dividends reinvested, gains added to principal — which maximizes the compounding effect over decades.

The Time Value of Money

The time value of money is the foundational principle behind compound interest: a dollar today is worth more than a dollar tomorrow because today's dollar can be invested and grow. This is why financial planners place such emphasis on starting early. Contributing $200/month to a retirement account from age 25 to 65 at 7% produces roughly $525,000. The same $200/month starting at 35 produces only about $243,000 — less than half — despite being only ten years shorter. The first ten years of compounding are just as powerful as the last twenty.

Compounding Against You: Debt

Compound interest works exactly as powerfully on debt as on investments — but in reverse. A $5,000 credit card balance at 22% APR, paid only at the minimum, will take over 20 years to pay off and cost more than $8,000 in interest — nearly twice the original balance. This is why high-interest debt is so destructive: the compounding accelerates the balance faster than minimum payments can keep up. Use the Debt Payoff Calculator to see exactly how much your debt costs over time and how extra payments change the outcome.

Compound Interest and Retirement

The 401(k) and IRA are powerful specifically because they allow compound interest to work in a tax-deferred or tax-free environment. Without the annual tax drag on dividends and capital gains, compounding is uninterrupted. A portfolio that compounds at 7% in a taxable account might only compound at 5.5% after tax — a difference that grows dramatically over 30–40 years. Maximizing contributions to tax-advantaged accounts is effectively a way to turbocharge the compounding effect. See the 401(k) Calculator for retirement projections.

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