Rule of 72

At 12% a year, 72 divided by 12 gives 6 - so a lump sum roughly doubles in about 6 years. At 8%, it takes roughly 9 years (72 divided by 8). It's a mental-math approximation of compound growth, useful for a quick gut-check without reaching for a calculator, and reasonably accurate for rates roughly between 6% and 15% - it gets noticeably less precise outside that range. It's also a useful way to compare two return assumptions quickly: a fund assumed to return 15% doubles money nearly twice as fast (about 4.8 years) as one assumed to return 8% (about 9 years) - a gap that becomes very large over a multi-decade horizon.