Compound Interest Calculator
Watch your money snowball. See exactly how compounding frequency, monthly contributions, and time work together to build wealth — with a full year-by-year growth table.
Investment Details
Enter your numbers and click Calculate to see your growth.
A year-by-year table will appear below the summary.
The Power of Compounding
Albert Einstein allegedly called compound interest "the eighth wonder of the world." Whether or not he said it, the math is genuinely astonishing. Compounding means your interest earns interest — each period's gains become the base for the next period's growth, creating exponential rather than linear growth.
Formula
FV = P(1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) − 1) / (r/n)]
P = initial principal
r = annual interest rate (decimal)
n = compounding periods per year
t = time in years
PMT = contribution per compounding period
The Rule of 72
Divide 72 by your annual interest rate to estimate how many years it takes for money to double. It's a quick mental math shortcut that's surprisingly accurate.
4%
18 years
HYSA rate
7%
~10.3 years
S&P 500 avg
10%
7.2 years
Aggressive growth
At 7% compounded monthly, money doubles roughly every 10.3 years.
Why Starting Early is Everything
Two investors each contribute $200/month at 7% annually. Investor A starts at 25 and stops at 35 — 10 years of contributions ($24,000 total). Investor B starts at 35 and contributes every month until 65 — 30 years of contributions ($72,000 total).
At 65, Investor A still wins — with more money — despite contributing three times less. Time in the market beats timing the market, and it certainly beats waiting to start.
Investor A (starts at 25)
Total contributed: $24,000
Balance at 65: ~$264,000
Investor B (starts at 35)
Total contributed: $72,000
Balance at 65: ~$243,000
Compounding frequency matters — but less than you think
Daily compounding earns slightly more than annual compounding at the same rate, but the difference is small. On a $10,000 investment at 7% over 20 years, daily vs. annual compounding differs by only about $200. The rate and time horizon are what truly move the needle.