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Compound Interest Calculator

See how money grows exponentially with compound interest. Model any starting amount, contribution schedule, and compounding frequency — with optional inflation adjustment.

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Compound interest is interest calculated on both the initial principal and the accumulated interest from prior periods — earning "interest on interest." Formula: A = P(1 + r/n)^(nt), where P is principal, r is annual rate, n is compounding frequency, and t is time. At 7% compounded monthly, $10,000 grows to $19,672 in 10 years, $38,697 in 20 years, and $76,123 in 30 years — without adding a single additional dollar.

Last updated: 2026-05-04
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Compound Growth Results
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Compound Interest — FAQs

Understanding how compound interest works and how to maximize it.

Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest (calculated on principal only), compound interest earns "interest on interest" — which accelerates growth dramatically over time. Albert Einstein reportedly called it the eighth wonder of the world. The formula is: A = P(1 + r/n)^(nt), where P is principal, r is annual rate, n is compounding frequency, and t is time in years.

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 about 12 years (72/6 = 12). At 9%, it doubles in 8 years. At 12%, in 6 years. It's a useful tool for quickly comparing investment options without a calculator. This calculator shows your exact Rule of 72 doubling time in the results.

At 7% annual compound interest, $10,000 grows to approximately: $19,672 after 10 years, $38,697 after 20 years, and $76,123 after 30 years — without adding a single dollar. Add $200/month in contributions at the same rate and the 30-year total exceeds $290,000. This illustrates why starting early is so powerful — the first decade of compounding matters as much as the last two combined. Use this calculator to model your specific scenario with our Retirement Calculator for long-term planning.

More frequent compounding produces more growth, but the practical difference is smaller than most people expect. At 7% over 20 years on $10,000: daily compounding produces $40,084 vs annual compounding's $38,697 — a difference of about 3.6%. What matters far more is your interest rate and time horizon. Focus on maximizing your rate of return and starting early rather than optimizing compounding frequency. Most savings accounts compound daily; most bonds compound semi-annually.

APR (Annual Percentage Rate) is the stated interest rate without accounting for compounding. APY (Annual Percentage Yield) reflects the actual return after compounding — it's always higher than APR when compounding more than once per year. A 6% APR compounded monthly has an APY of about 6.17%. When comparing savings accounts or investments, always compare APY — it's the true annual return. This calculator displays the APY for your selected compounding frequency.

Inflation erodes purchasing power, so your real return is your nominal rate minus inflation. If your investment earns 7% and inflation runs at 3%, your real return is approximately 4%. Over 20 years, this makes a significant difference: $10,000 at 7% nominal grows to $38,697, but in today's dollars (at 3% inflation) that's only about $21,490. Use the "Real (Inflation-Adjusted)" toggle in this calculator to see your true wealth growth in purchasing power terms.

Simple interest is calculated only on the principal: $10,000 at 7% simple interest earns a flat $700/year forever, producing $31,000 after 30 years. Compound interest earns interest on the growing balance: the same $10,000 earns $700 in year 1, $749 in year 2, $801 in year 3, and $14,974 in year 30 — producing $76,123 total. The gap widens dramatically over time. Nearly all savings accounts, investments, and loans use compound interest — simple interest is rare in practice.

Four levers drive compound growth: (1) Start early — time is the most powerful variable. $5,000 invested at 25 grows to more than $10,000 invested at 35 at the same rate. (2) Maximize rate — use tax-advantaged accounts (401k, Roth IRA) and diversified equity portfolios for long-term goals. (3) Contribute regularly — even small monthly contributions multiply dramatically. (4) Never interrupt — withdrawing early resets the compounding clock. Plan your long-term contributions with our Retirement Calculator.

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Disclaimer: This calculator provides estimates for educational and planning purposes only. Actual investment returns vary and are not guaranteed. Past performance does not predict future results. Compound interest calculations assume a fixed rate — real investments fluctuate. This does not constitute financial advice. Consult a qualified financial advisor before making investment decisions.