Compound Interest Calculator

A = P(1 + r/n)^(nt), where P is principal, r is annual rate (decimal), n is compounding periods per year, and t is time in years. With regular contributions PMT per period: A = P(1+r/n)^(nt) + PMT × (((1+r/n)^(nt) − 1) / (r/n)).

Divide 72 by the annual interest rate to estimate how many years it takes to double your money. At 7% annual return: 72 ÷ 7 ≈ 10.3 years to double. It's a quick mental math shortcut that's surprisingly accurate for rates between 2% and 20%.

More frequent compounding increases returns, but the difference is smaller than most people expect. At 7% annual rate on $10,000 for 20 years: annual compounding gives $38,697, while daily compounding gives $40,138 — a difference of only $1,441. The rate matters far more than the frequency.