With monthly contributions · Visual growth chart · Daily to annual compounding · Updated 2025
How Compound Interest Works
Compound interest means you earn interest on your interest — not just your original principal. This creates exponential rather than linear growth. The longer the time horizon, the more dramatic the effect. $10,000 at 10% for 30 years without contributions grows to over $174,000 — that's $164,000 in pure interest on a $10,000 deposit.
The Rule of 72
Divide 72 by your annual interest rate to estimate how many years it takes to double your money. At 8% return: 72 ÷ 8 = 9 years to double. At 10%: 7.2 years. At 6%: 12 years. This simple mental model reveals why starting early is so powerful — each doubling period adds more dollars than all previous periods combined.
Why Monthly Contributions Dominate
Adding $500/month to a $10,000 initial investment at 10% over 20 years produces $408,000. Without the monthly contributions, the same $10,000 grows to only $67,000. The $500/month habit — totaling $120,000 contributed — generated $281,000 more in the final balance. Consistency beats lump-sum size.
Choosing a Realistic Return Rate
- S&P 500 index fund — historical ~10% average annual return (before inflation)
- Diversified stock portfolio — 7–9% is a conservative long-term estimate
- High-yield savings account — currently 4–5% (2025 rates)
- Bonds / CDs — 3–5% typically
- Real estate (appreciation only) — 3–6% average
Frequently Asked Questions
What is the difference between APY and APR?
+
APR (Annual Percentage Rate) is the stated rate before compounding. APY (Annual Percentage Yield) accounts for compounding and is always higher than APR when compounding occurs more than once per year. A savings account with 5% APR compounded monthly has an APY of approximately 5.12%. This calculator uses APR — select monthly compounding to closely match most savings accounts and investment accounts.
How much do I need to invest to become a millionaire?
+
At 10% annual return: contributing $500/month for 30 years reaches approximately $1.13 million. Starting at 25 and contributing $300/month until 65 (40 years) produces over $1.9 million. The key variable is time — each extra year of compounding is worth more than the last. Someone who starts at 25 vs 35 ends up with roughly twice the final balance even with identical monthly contributions.
Does this calculator account for inflation?
+
No — this shows nominal growth before inflation. To estimate real (inflation-adjusted) returns, subtract the expected inflation rate from your return rate. If you expect 10% nominal returns and 3% inflation, use 7% in this calculator for an inflation-adjusted projection. For retirement planning, always use real return estimates to avoid overestimating purchasing power.