Compound Interest Calculator
Watch a starting deposit and steady monthly contributions snowball. Enter your numbers to see the future value, how much you put in, and how much growth came from compounding alone.
Compound interest means you earn interest on your interest. The initial deposit grows using the classic formula P(1 + r/n)ⁿᵗ, where n is how many times per year interest compounds — monthly compounding (n = 12) is the norm for US savings accounts.
Monthly contributions are treated as a series of deposits compounding monthly. The result shows three numbers that matter: the final balance, the total you actually deposited, and the interest earned — the part compounding did for free. Small rate differences become enormous over decades, which is why starting early beats saving more later.
Worked examples
Long-term saver
$10,000 plus $200/month at 7% for 20 years, compounded monthly.
→ ≈ $142,865 (about $84,865 from interest)
Lump sum only
$5,000 at 5% for 10 years, compounded monthly, with no contributions.
→ ≈ $8,235
The rule of 72
At 7% growth, money doubles roughly every 72 ÷ 7 ≈ 10.3 years — compounding in action.
→ Doubles about every decade