Compound Interest Calculator
See how your investment grows with monthly contributions and compounding.
Interactive calculator
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$
%
Result
Future value
$300,851
Total contributed
$130,000
Interest earned
$170,851
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What it is
Compound interest is the magic of earning returns on your returns. Small monthly contributions over decades become large balances.
The formula
FV = P(1+r)^n + PMT × ((1+r)^n − 1) / r Where r is the periodic rate and n is number of periods.
Why it matters
Starting 10 years earlier often beats contributing twice as much. Time is the most powerful variable.
Worked examples
Start at 25
$500/mo at 7% for 40 years → ~$1.3M.
Start at 35
Same contribution, 30 years → ~$610k — less than half.
Common pitfalls
- Assuming a constant return — markets vary year to year.
- Ignoring inflation eroding the real value.
- Forgetting taxes on non-retirement accounts.
Frequently asked questions
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