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Educational Tool — Estimates for educational purposes only. Not financial advice.Disclaimer

Compound interest is the most powerful force in personal finance. This calculator shows how your initial investment and regular contributions can grow exponentially over time, and how inflation may affect your real returns.

Compound Interest Calculator

See how your money grows over time with compound interest, recurring contributions, and optional inflation adjustment.

February 2026 (market data)

Optional: adjust for purchasing power

How the Compound Interest Calculator Works

The compound interest calculator shows how your money grows over time when interest is earned not only on the principal but also on previously accumulated interest. This exponential growth effect is often called the 'eighth wonder of the world' and is the foundation of long-term wealth building.

Enter your initial investment (principal), monthly contribution, expected annual return rate, investment period, and compounding frequency. The calculator computes the future value of your investment including all contributions and reinvested earnings. Optionally, enter an inflation rate to see the purchasing-power-adjusted value.

The Formula

FV = P(1+r/n)^(nt) + PMT × [((1+r/n)^(nt) − 1) / (r/n)]

Where FV = future value, P = principal, r = annual interest rate, n = compounds per year, t = years, PMT = periodic contribution.

Source: SEC Investor.gov ↗

Example Calculation

You invest $10,000 today and add $500/month for 20 years at 7% annual return, compounded monthly:

  1. Initial investment: $10,000
  2. Total contributions over 20 years: $10,000 + ($500 × 240) = $130,000
  3. Monthly rate: 7% ÷ 12 = 0.5833%
  4. Future value of principal: $10,000 × (1.005833)^240 = $40,387
  5. Future value of contributions: $500 × [((1.005833)^240 − 1) / 0.005833] = $260,464

Total future value: approximately $300,851. That's $170,851 in earnings on $130,000 invested — your money multiplied 2.3×.

Tips for Better Results

✓

Start early — time beats amount

Due to exponential growth, starting 10 years earlier often matters more than doubling your monthly contribution. A 25-year-old investing $300/month will likely outpace a 35-year-old investing $600/month by retirement.

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Account for inflation

A 7% nominal return with 3% inflation gives approximately 4% real growth. Use the inflation adjustment to see what your future dollars will actually buy.

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Increase contributions over time

As your income grows, increase monthly contributions. Even 3-5% annual increases compound dramatically over decades.

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Understand compounding frequency

Monthly compounding produces slightly more than annual compounding at the same rate. The difference grows with higher rates and longer periods.

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Compare with the Rule of 72

To estimate doubling time, divide 72 by your annual return. At 7%, your money doubles roughly every 10.3 years.

Frequently Asked Questions

What annual return should I assume?

The S&P 500 has historically returned approximately 10% nominal (7% real after inflation) over long periods, per Federal Reserve data. For conservative estimates, use 5-6%. For aggressive growth assumptions, 8-10%. Always test multiple scenarios.

Does this account for taxes?

No. This calculator shows pre-tax growth. In taxable accounts, capital gains and dividend taxes reduce your effective return. Tax-advantaged accounts (401k, IRA, Roth IRA) can grow tax-free or tax-deferred.

What's the difference between compound and simple interest?

Simple interest is calculated only on the principal. Compound interest is calculated on the principal plus all accumulated interest. Over 20+ years, compounding can more than double your returns compared to simple interest.

How does compounding frequency affect results?

More frequent compounding (monthly vs. annually) produces slightly higher returns because interest starts earning interest sooner. The difference is small but grows over long periods.

Related Calculators

Investment Return →Retirement Projection →Savings Goal →ROI Calculator →

Disclaimer: This calculator provides estimates for educational and informational purposes only. It does not constitute financial, tax, or legal advice. Results depend on user-provided inputs and the stated assumptions. Investment returns are not guaranteed. Past performance does not predict future results. This calculator does not model market volatility, fees, or taxes. Always consult qualified professionals before making significant financial decisions.

Last updated: February 2026

Your Results

Future Value

$300,850.72

Total Contributions

$130,000.00

Interest Earned

$170,850.72

Inflation-Adjusted Value

$166,573.75

Money Multiplier

2.31x

Visualization

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AI Disclosure

Wally's Analysis uses GPT-4o-mini for educational interpretations. AI responses are not personalized financial advice and may be inaccurate. All core calculations are deterministic — AI only interprets results.AI Transparency →

Accuracy Commitment

Industry-standard formulas vetted against IRS, SSA, BLS, CFPB. Client-side calculations. Regularly updated.

This calculator provides estimates for informational purposes only and does not constitute financial, tax, or legal advice. Consult a licensed professional before making financial decisions.

Updated: February 2026 (market data) · Methodology

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