Compound Interest Calculator
Runs in your browser · nothing is uploadedFinal Value
$302,370
Total Interest
$172,370
Contributed $130,000 · Growth $172,370 · 20y @ 7.0%
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Educational estimates only — not financial advice.
🔥 Top 3 Most Copied Scenarios
$500/month for 30 years at 8% = $745,000
Three decades of monthly investing turns $180,000 of deposits into roughly four times that.
Extra $200/month mortgage payment saves 6 years
Every extra dollar hits principal directly, so the remaining interest shrinks with it.
$10k initial at 7% for 20 years = $38k
A single lump sum left alone nearly quadruples without another deposit.
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Compound Interest
Project growth from a starting balance and monthly contributions, with 37 copyable scenarios showing how rate and time change your future value.
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Estimate your FIRE number and the age you can safely retire, with 52 scenario combinations across savings rates, returns, and withdrawal rates.
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Each calculator includes 37–52 pre-computed, copyable scenarios so you can compare realistic inputs without starting from scratch.
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Formulas and assumptions are cross-checked against reputable sources such as Investopedia and the CFPB, with a clear disclaimer that the tool is for education only, not financial advice.
Guides behind the math
All 28 guidesHow the Time Value of Money Quietly Builds Real Wealth
A dollar today is worth more than a dollar tomorrow. Understanding exactly how much more is the difference between saving hopefully and investing deliberately.
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One method is mathematically optimal, the other is behaviourally optimal. The cost of choosing the 'wrong' one is smaller than most people assume.
9 min read · January 6, 2026FIRE RetirementThe 4 Percent Rule for Retirement: How It Works and Where It Breaks
The most quoted number in retirement planning is a useful starting point wrapped in assumptions that rarely get mentioned. Here is the full picture.
10 min read · January 12, 2026Subscription AuditThe Real Cost of Subscription Fatigue
The average household now carries a dozen recurring charges and can name maybe seven. The gap between what you pay for and what you use is the most recoverable money in your budget.
8 min read · February 20, 2026Compound InterestCompound Interest With Monthly Contributions, Explained Step by Step
The formula that combines a starting balance with regular deposits, worked through line by line with realistic numbers.
8 min read · September 15, 2026Frequently asked questions
Quick answers to the money, accuracy, privacy, and calculator questions people search for most.
Empowering Your Journey to Financial Independence
FirePlanIO is a suite of free, privacy-first financial calculators built for smart wealth management. Whether you are calculating compound interest growth, planning early retirement via the FIRE movement, auditing monthly subscription leaks, or building an accelerated mortgage payoff strategy, our tools run 100% client-side in your browser with zero data tracking.
About the author
Y.W. Fung — Founder & Editor
Y.W. Fung built FirePlanIO to make financial planning transparent, private, and free. Every calculator, guide, and formula on the site is reviewed for accuracy against reputable sources and updated regularly. You can learn more on the About page or get in touch at contact@fireplanio.com.