Compound Interest Calculator
Free compound interest calculator. Enter principal, optional monthly contribution, annual rate, and time horizon. See the textbook A = P(1 + r/n)^(nt) result plus a reference table at 4/6/7/8/10% across 5/10/20/30/40 years.
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Power mode. Every input exposed, every assumption sourced, charts and shareables.
$1,000
$200
7.00% / year
30 years
Compounding frequency
Final balance (A)
$252,111
You contributed
$73,000
Compound growth
$179,111
ƒ(x)Show the formula(click to expand)
Compound interest formula:
A = P(1 + r/n)nt
- A — the final balance after t years
- P — principal (starting amount)
- r — annual interest rate as a decimal (e.g. 0.07 for 7%)
- n — number of times interest compounds per year (12 for monthly, 1 for annually)
- t — time in years
For ongoing monthly contributions, the calculator above also adds the future-value-of-an-annuity term to A. The reference table below uses principal-only growth so the textbook formula is what you see.
What $1,000 grows into at common rates
Principal-only growth (monthly compounding). Add a monthly contribution above to see your full projection.
| Rate | 5 yr | 10 yr | 20 yr | 30 yr | 40 yr |
|---|---|---|---|---|---|
| 4% | $1,221 | $1,491 | $2,223 | $3,313 | $4,940 |
| 6% | $1,349 | $1,819 | $3,310 | $6,023 | $10,957 |
| 7% | $1,418 | $2,010 | $4,039 | $8,116 | $16,311 |
| 8% | $1,490 | $2,220 | $4,927 | $10,936 | $24,273 |
| 10% | $1,645 | $2,707 | $7,328 | $19,837 | $53,701 |
💰 $252,111
$1,000 start + $200/mo compounded monthly at 7%
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The Compound Interest Calculator uses the textbook A = P(1 + r/n)^(nt) formula plus a reference table showing growth at 4%, 6%, 7%, 8%, and 10% over 5, 10, 20, 30, and 40 years. Compound interest is the most important concept in personal finance — Einstein supposedly called it the eighth wonder of the world — and seeing it in concrete dollars is what makes it stick.
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How this calculator works
- Enter the principal (starting amount).
- Add an optional monthly contribution. Even $50/mo matters dramatically over 30+ years.
- Set the annual interest / return rate.
- Pick the compounding frequency (monthly is most common for savings; annually for many investments).
- Set the time horizon. Notice the curve gets steeper later — that's compounding's gift to people who start early.
A = P × (1 + r/n)^(n × t)Compound interest is interest earned on prior interest, not just on the original principal. The formula compounds the rate over each period and applies it across the time horizon. Adding monthly contributions adds a future-value-of-an-annuity term to A.
- A
- Final amount after compounding
- P
- Principal (starting amount)
- r
- Annual interest rate (as a decimal)
- n
- Compounding periods per year (12 for monthly, 1 for annual)
- t
- Time in years
Frequently asked questions
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