Compound Interest Calculator
See how your money grows through two phases — the years you contribute, and the years you let compound interest do the work.
Final Value
$391.2K
After 35 total years
After Contributing
$144.2K
End of year 15
Total Invested
$95.0K
Your cash in
Compound Gains
$296.2K
76% of final value
You invested $95.0K and compound interest added $296.2K — your money did 312% of the work for you.
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Compound Growth Over Time
Amber = your contributions · Teal = compound growth
Investing $500/mo for 15 years grows your portfolio to $144.2K.
Zero contributions for 20 years. Compound interest grows $144.2K to $391.2K.
For educational purposes only. Past market performance does not guarantee future results. Full disclaimer →
How This Calculator Works
① Two phases
The calculator models a contribution phase where you invest monthly, then a hands-off phase where contributions stop and compound growth takes over. Most calculators only model one phase.
② Real returns
Setting an inflation rate subtracts it from your nominal return to give a real return. At 8% return and 3% inflation, the calculator uses 5% — so all results are in today's purchasing power, not inflated future dollars.
③ All inputs today's dollars
Your contributions, initial investment, and final value are all expressed in today's dollars. No need to mentally deflate the results — $800k in the calculator means $800k in today's purchasing power.
The maths
Contribution phase: each month, your balance grows by the monthly real return, then your contribution is added. Annual contribution increase compounds at the start of each year after year 1.
Hands-off phase: contributions stop. Balance compounds at the monthly real return for each remaining year.
Real return = nominal return − inflation rate. Leave inflation at 0% for nominal projections that match most other compound interest calculators.
Frequently Asked Questions
How does compound interest work? +
Compound interest means you earn returns not just on your original investment, but on all the accumulated gains too. Each year your gains are added to your balance, and next year you earn returns on that larger amount. Over time this creates exponential growth — the longer your money compounds, the faster it grows.
What is the difference between contribution years and coast years? +
Contribution years are the years you actively invest money each month. Coast years are the years after you stop contributing — your money keeps growing through compound interest alone. This two-phase model shows you the real impact of starting early: even after you stop investing, your portfolio continues to compound.
What annual return should I use? +
The S&P 500 has historically returned approximately 10% annually before inflation, or roughly 7% adjusted for inflation. For conservative projections use 6–7%. For nominal projections 8–9% is common. The default 8% is a reasonable middle ground for long-term stock market investing.
What does "annual contribution increase" mean? +
This models salary growth — if you expect to earn more over time and increase your monthly investments accordingly. For example, at 3% annual increase, a $500/month contribution becomes $652/month by year 10. This more accurately reflects real-world investing where contributions tend to grow with income.
How is this different from the Coast FIRE calculator? +
The compound interest calculator answers "what will I end up with?" — showing your final portfolio value based on your contributions and timeline. The Coast FIRE calculator answers "when can I stop contributing?" — it works backwards from your retirement target to find the exact moment you can stop investing and still reach your goal.
What is compound interest? +
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest (which only earns on the principal), compound interest earns on a growing base — so your returns accelerate over time. Einstein reportedly called it the eighth wonder of the world. A $10,000 investment at 7% grows to $76,123 in 30 years through compounding alone.
How much does $500 a month grow to over 30 years? +
At 7% annual return, $500 per month invested for 30 years grows to approximately $567,000. Of that, $180,000 is your contributions and $387,000 is compound growth. At 8% it reaches $679,000. At 10% it reaches $1,031,000. The exact figure depends on your return rate and whether contributions increase over time — use the calculator above to model your specific scenario.
What is the Rule of 72? +
The Rule of 72 is a quick mental math shortcut: divide 72 by your annual return rate to find how many years it takes to double your money. At 6% return, money doubles in 72÷6 = 12 years. At 8% it doubles in 9 years. At 10% in 7.2 years. It is a rough approximation but accurate enough for quick planning.
How does starting early affect compound interest? +
Starting early has a disproportionate impact. A dollar invested at 25 at 8% grows to $21.72 by 65. The same dollar invested at 35 grows to only $10.06 — less than half. This is why 10 years of contributions starting at 25 can outperform 30 years of contributions starting at 35. The first years of a long investment horizon do the heaviest compounding lifting.