Compounding, in one number
The Rule of 72 makes compounding intuitive β high rates double money fast, low rates take decades. It's a sanity check, not a promise. Model an actual plan on the compounding calculator.
The doubling shortcut
Divide 72 by your annual return and you get the years to double your money. 10% a year doubles in ~7.2 years. 24% doubles in three. The approximation is startlingly accurate between about 5% and 30% β inside that band the error is under a few months.
Where it gets useful in crypto: sanity-checking yield claims. A protocol offering "1% daily" implies doubling in 72 days and roughly 37Γ in a year. Nothing legitimate compounds like that at scale; the rule of 72 turns "1% daily" from tempting into transparently impossible faster than any due diligence thread.
It also works in reverse for inflation and fees. At 8% inflation, cash halves in purchasing power every 9 years. A 2% annual management fee halves your capital's compounding base in 36 years. Small percentages, big destinies β that's the entire lesson of the rule.