Compound Interest Calculator
The Compound Interest Calculator shows exactly how your money grows over time when interest is added to your balance and then earns more interest on top. It's the fundamental principle behind all long-term investing — and why starting early with even small amounts makes such a dramatic difference to final wealth.
Educational estimate. Calculator results are for planning and information only, not financial, tax, medical, legal, or engineering advice. Verify important decisions with official sources or a qualified professional.
Compound Interest Calculator
See How Your Money Grows Exponentially
📐 Formula & Method
Compound Interest Formula
A = final amount, P = principal, r = annual interest rate (decimal), n = compounding periods per year, t = years. Compound Interest earned = A − P.
Rule of 72
A quick mental math shortcut: at 7% annual return, money doubles every ~10.3 years (72÷7). At 10%, every ~7.2 years. At 4.5%, every ~16 years.
📋 How to Use
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Enter your starting amount (principal).
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Enter the annual interest rate or expected return.
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Enter the time period in years.
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Select the compounding frequency — daily is most accurate for savings accounts.
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Click Calculate to see total amount, interest earned, CAGR, and years to double.
The Power of Compound Interest — Real Examples
"Compound interest is the eighth wonder of the world. He who understands it, earns it; he who doesn't, pays it." — Often attributed to Albert Einstein. Whether or not Einstein said it, the math is extraordinary. $10,000 invested at 7% annually grows to $38,697 after 20 years — without adding a single penny. After 30 years it reaches $76,123. After 40 years: $149,745. The growth accelerates every decade.
The single most powerful lever in compound interest is time. Consider two investors: Alice starts at 25, invests $5,000/year at 8% for 10 years, then stops. Bob starts at 35, invests $5,000/year at 8% for 30 years. Alice contributes $50,000 total; Bob contributes $150,000. Yet at age 65, Alice has approximately $787,000 and Bob has $611,000. Alice wins by $176,000 — despite contributing $100,000 less — purely because she started 10 years earlier.
For savings accounts (HYSA, Cash ISA), daily compounding is standard. The effective yield from daily vs. monthly compounding on a 5% rate is minimal — $10,000 grows to $16,487 (daily) vs. $16,470 (monthly) after 10 years, a difference of just $17. The rate matters far more than compounding frequency. Focus on getting the best rate first.
High-yield savings accounts in the US (FDIC insured) are offering 4.5–5.5% APY in 2024 — the highest since 2007. In the UK, easy access savings accounts are offering 4.5–5.0% AER. This makes compound interest on savings genuinely meaningful again — $10,000 at 5% APY earns approximately $629 in interest in the first year alone (compounding daily).
🔬 Methodology & Accuracy
Formula: Uses the standard mathematical formula shown in the Formula & Method section above. All computations run client-side in your browser — no data is sent to our servers.
Data sources: Tax bands, contribution limits and regulatory rates are taken from official US (IRS, SSA) and UK (HMRC, gov.uk) publications for the current tax year, and updated when bands change.
Last reviewed: June 2026 · Accuracy: Results are precise to two decimal places using IEEE-754 double-precision arithmetic. Intended for educational and planning use only.
For informational purposes only. Results are estimates based on the inputs and formulas provided. For financial, tax, medical, or legal decisions, consult a qualified professional. Rates and regulations change — always verify current figures with official sources.