Investing

Compound Interest Calculator

Project the growth of an investment with monthly contributions. Watch the compounding curve redraw as you change rate or time horizon.

The math

We use monthly compounding for the base curve. The future value with regular contributions is:

FV = P × (1 + r)^n + C × ((1 + r)^n − 1) / r

P = initial principal
C = contribution per period
r = periodic interest rate
n = number of periods

Why a small rate change matters so much

Compounding is exponential. A 1-point change in annual return over 30 years can mean tens of percent in final balance. Try moving the rate slider 1% in either direction — the curve answers louder than any textbook.