💰 Finance 📈 Wealth-building Last tested2026-05-28
Compound Interest + contributions.
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Quick answer
Investing $10,000 + $500/month for 20 years at 8% grows to roughly $345,742. You contributed $130,000; compounding turned the rest ($216K) into pure interest.
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Compound Interest
$
$
%
Final Balance
$345,742
after 20 years
Total Contributed
$130,000
your money in
Interest Earned
$215,742
62% of total
Year-by-year growth
Year 1Year 20
Contributions
Interest earned
✨ Live · Compound interest = the 8th wonder of the world (Einstein, allegedly)
🧮 The math
The compounding formula.
Without contributions
A = P(1 + r/n)nt
- A = final amount
- P = initial principal
- r = annual rate (as decimal)
- n = compounding periods per year
- t = number of years
With monthly contributions, the formula gets more complex. This calculator simulates period-by-period growth, which is what real compounding looks like.
Sources: SEC Investor.gov Compound Interest · Federal Reserve Historical Interest Rates
💡 Insights
3 truths about compounding.
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Time > amount
Starting 10 years earlier often beats doubling your contribution. The exponent of time is brutal.
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Rate matters more than you think
A 2% difference (5% vs 7%) doubles your money over 35 years. Fees + low yields silently kill compounding.
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Frequency = small effect
Daily vs monthly compounding adds <1% over 20 years. Focus on rate, time, and contributions instead.
❓ FAQ
Common questions.
What is compound interest?
Compound interest is interest earned on both your original principal AND on interest previously earned. Unlike simple interest, it accelerates over time. Einstein allegedly called it the 8th wonder of the world.
How is it calculated?
The classic formula is A = P(1 + r/n)^(nt), where A is final amount, P is principal, r is annual rate, n is compounding periods per year, t is years. This calculator extends it to include regular contributions.
Why does compounding frequency matter?
More frequent compounding = more growth, but the difference between annual and monthly is small. Over 30 years at 7%, daily compounding only beats annual by ~3%. Time and contribution amount matter way more.
Whats a realistic return rate to use?
Long-term US stock market average: ~10% nominal, ~7% after inflation. Bond yields: 3–5%. Savings accounts: 0.5–5%. Use 7% for stock-heavy portfolios, 4% for balanced retirement estimates.
How much does starting early matter?
Massively. $500/month from age 25 to 65 at 7% = ~$1.3M. The same $500/month from age 35 to 65 = ~$610K. Ten years of compounding more than doubles the result. Start now.