💰 Compound Interest Calculator

Compound interest — Quick answer

Compound interest earns interest on previously accumulated interest. The future value depends on principal, rate, time, and compounding frequency.

FV = P × (1 + r/n)^n·t
With monthly contributions: FV = P·(1+r/n)^nt + PMT·((1+r/n)^nt−1)/(r/n)

• P — principal (initial deposit)
• r — annual interest rate (decimal, e.g. 0.05 for 5%)
• n — compounding periods per year (12 monthly, 4 quarterly, 1 annual)
• t — number of years
• PMT — periodic contribution per period

Worked example: $1,000 principal at 5% APR compounded monthly for 10 years. FV = 1,000 × (1 + 0.05/12)¹²⁰ = 1,000 × 1.6470 = $1,647.01. Add $100/month: FV = 1,647 + 100 × ((1.00417¹²⁰−1)/0.00417) = $17,176.

$1,000 invested for 30 years at varying rates (monthly compounding)

Standard / source: Standard financial mathematics; SEC requires APR disclosure on loans (Truth in Lending Act).

Used for: Retirement planning, savings goal setting, mortgage interest projection, education-fund planning, FIRE-movement calculations.