Rule of 72

The Rule of 72 estimates doubling time: years to double ≈ 72 ÷ annual rate (in %). It works for anything that compounds — investments, debt, even inflation. At 6% money doubles in ~12 years; at 3% inflation, prices double in ~24 years, meaning cash loses half its purchasing power. It's most accurate for rates between about 6% and 10%.