Formula reference

Compound Interest Formula

These formulas explain the math used by compound interest calculators. They are planning formulas, not predictions of real investment returns.

One-time principal

A = P(1 + r / n) ^ (nt)

Use when there are no extra deposits.

Effective annual yield

APY = (1 + r / n) ^ n - 1

Use to compare rates with different compounding schedules.

Monthly projection

B = B(1 + monthlyRate) + C

Use when contributions happen monthly.