Use a balance you can verify and a contribution you can sustain through ordinary months.
Compound Interest Calculator
Adjust any input and watch your projection update live.
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FIRE Calculators & Simulators
A growing toolkit. Each one is built to be deep, fast, and free.
Flagship · Real Data 1928–2025
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S&P 500 Backtest Simulator
"What if I'd invested back then?" Test any window with real total returns, dividends, inflation, CAGR and drawdowns.
Can I Retire On…?
→What $300k to $5M actually pays per month — and how long it lasts. 15 amounts answered.
Live Off Dividends Calculator
→Find your dividend crossover year, and compare dividends-only vs the 4% rule side by side.
Early Mortgage Payoff Simulator
→See the interest and years you save with extra monthly payments.
FIRE Number Calculator
→Find the portfolio size you need to retire early — and your years to freedom.
4% Rule Calculator
→How much you can safely withdraw, and how long your money lasts.
Debt Payoff Calculator
→Avalanche vs snowball — which clears your debt faster and cheaper.
Coast FIRE Calculator
→See if you can stop saving and let compounding finish the job.
Lean FIRE vs Fat FIRE
→Compare both targets and the years to reach each.
Barista FIRE Calculator
→Semi-retire with part-time income — the gap math done right.
Coast FIRE by Age
→Your coast number at every age, from 20 to retirement.
Savings Rate to FI
→How your savings rate sets your retirement date.
Rule of 72
→How fast your money doubles at any return.
Real Hourly Wage
→What your time is really worth per hour.
How Long Will My Money Last?
→See how long savings last — and how sequence-of-returns risk moves the date.
Retire on $X / Month
→The nest egg a target monthly income needs, with or without Social Security.
Net Worth Percentile
→See where your net worth ranks and what share of Americans have $1M+.
My Compound Growth Projection — Financial Depth
financialdepth.com
Quick Summary (TL;DR)
- Compound interest pays you on your principal and on prior interest — growth accelerates over time.
- Example: $10k + $500/mo at 7% for 30 yrs → ~$870k, from just $190k contributed.
- Time matters more than timing — starting earlier beats saving more later.
- Formula:
FV = P(1+i)n + PMT·[((1+i)n−1)÷i]
How Compound Interest Builds Wealth
Compound interest is the quiet engine behind every FIRE journey. Unlike simple interest, which pays only on your original principal, compounding pays you on your principal and on the interest you've already earned. Each period's gains join the base that earns the next period's return, so growth curves upward instead of climbing in a straight line — and the effect gets dramatic over decades.
This calculator projects future value with the formula FV = P(1+i)n + PMT · [ ((1+i)n − 1) / i ], where P is your initial investment, PMT is the monthly contribution, i is the monthly return (annual rate ÷ 12), and n is the number of months. Contributions are added monthly and compounded from the moment they land.
Worked example: Start with $10,000, add $500/month, and assume a 7% annual return for 30 years. You'd contribute $190,000 of your own money, but end with roughly $870,000 — meaning compounding did more of the heavy lifting than your deposits did. Stretch the timeline to 40 years and the gap widens even further. Time, not timing, is the FIRE investor's biggest advantage.
Frequently Asked Questions
What is compound interest?+
It's the interest you earn on both your original principal and on the interest already accumulated. Because each period's gains are added to the base that earns the next period's return, growth accelerates over time — the engine behind long-term investing and FIRE.
How is growth with monthly contributions calculated?+
Future value equals the initial amount compounded over the period, plus the future value of your stream of monthly contributions. We compound monthly using FV = P(1+i)ⁿ + PMT[((1+i)ⁿ − 1) / i], where i is the monthly rate and n is the number of months.
What return rate should I assume for FIRE planning?+
Many in the FIRE community model a long-run nominal return near 7% for a diversified stock portfolio, or roughly 4–5% after inflation. Use a conservative figure for planning, since actual returns vary year to year.
Decision guide
How to read a compound-growth projection
The large future-value number is only the starting point. Separate money you contributed from growth, check whether the return is nominal or inflation-adjusted, and compare more than one scenario before treating the result as a plan.
Run a lower-return case. Long horizons amplify even a one-percentage-point difference.
Compare future value with future spending needs and purchasing power after inflation.
What this model includes
- Monthly contributions and compounding
- A split between invested principal and growth
What to add outside the model
- Taxes, fund fees, and changing contribution levels
- Market volatility and the order of annual returns
Calculation standard: Formula details, source policy, tests, and limitations are documented in the FinancialDepth methodology. Reviewed July 18, 2026.