Compound Interest Calculator
Textbook compound interest with monthly contributions.
Final balance (A) after 30 years
$252,111
- You contributed
- $73,000
- Compound growth
- $179,111
- Growth share
- 71%
of the final balance
You put in $73,000. Compounding added $179,111 — 71% of your final balance. Growth outpaces your deposits from year 11.
Try a what-if
Your numbers
$1,000
$200
7.00% / year
30 years
Compounding frequency
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 |
Next steps
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.
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) + C_period × [((1 + r/n)^(n × t) − 1) / (r/n)]Compound interest is interest earned on prior interest, not just on the original principal. The rate you enter is a nominal annual rate: with monthly compounding it's applied as r/12 each month (compounded monthly at r/12), with annual compounding as r once a year. Contributions go in at the end of each period — your monthly amount each month, or twelve months' worth at year-end with annual compounding — which adds the 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
What is the compound interest formula?
A = P(1 + r/n)^(nt). A is the final balance, P is principal, r is the annual rate as a decimal, n is compounding periods per year, t is time in years.
Does compounding frequency matter much?
A little. A one-time $10,000 deposit at 7% for 30 years grows to about $76,123 with annual compounding vs about $81,165 with monthly compounding (no further contributions) — a $5,042 difference that is useful but small next to a 1% rate change or a 5-year change in your horizon.
What rate should I plug in for stocks?
A common long-run assumption is 7-10% before inflation for diversified US stocks, or 4-7% after. The calculator lets you sweep across rates to see the dollar impact.
Sources used in this calculator
This calculator uses only the numbers you enter — no outside rates or tables. The formula it applies is shown above.
Reviewed by the Money Scale editorial team. How we source our data
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