The Rule of 72 Explained — Mental Math Every Investor Should Know

The formula: Years to double = 72 ÷ Annual Interest Rate. It works in reverse too: Rate required to double in N years = 72 ÷ N. The rule is most accurate between 6% and 10%, where the error is less than 0.1%. At 1% or 50% the approximation drifts, but precision is sufficient for quick decisions.

• 1% (basic savings account): 72 years to double
• 4% (bonds / current HYSAs): 18 years to double
• 7% (conservative index fund): ~10.3 years
• 10% (historical S&P 500 nominal): 7.2 years
• 15% (optimistic): 4.8 years
• 24% (credit card APR): debt doubles in 3 years

That last entry is the most critical. $5,000 in credit card debt at 24% APR, making only minimum payments, will grow to $10,000 in about 3 years. Seeing debt rates through the same lens as investment returns often motivates faster payoff.

The Rule of 72 also applies to inflation eroding purchasing power. At 3% annual inflation, prices double — and uninvested cash halves in real value — in 24 years. At 7% inflation (2022 peak), prices doubled in about 10 years. A $50,000 cash emergency fund sitting in a 0.01% savings account loses half its real value in ~24 years at 3% inflation.

This is the mathematical case against idle cash in low-yield accounts. The current 4.5–5% HYSA rate actually beats inflation by about 1.5–2 percentage points — your real purchasing power grows. But once rates fall to historical norms of 1–2%, you're losing purchasing power again.

The Rule of 72 assumes annual compounding. For continuous compounding (theoretical maximum), use 69.3. For monthly compounding (most savings accounts and mortgages), 72 is accurate to within 0.3%. The difference between using 72 vs 69.3 is trivial for practical decisions.

For very high rates (payday loans at 300% APR), use 70 instead of 72 for better accuracy. At 300% APR: 70 ÷ 300 = 0.23 years (~85 days) to double. A $500 payday loan becomes $1,000 owed in three months — that's the math of predatory lending.

Use the Rule of 72 to quickly evaluate financial claims: "Our fund returned 6% last year." Doubles in 12 years — same as the S&P 500, so what are they charging for? "This REIT pays 9% yield." Doubles in 8 years — reasonable for real estate risk.

• Social Security delay: benefits increase ~8%/year from 62 to 70 — doubles in 9 years, making delay valuable for those with long life expectancy
• Mortgage cost: at 7% mortgage rate, the bank doubles its money from your interest in ~10 years
• Business investment: $20,000 in a business returning 20% annually doubles in 3.6 years
• Stock growth claims: "This stock is up 144%/year" — doubles every 6 months, which is never sustainable; be skeptical

For precise numbers — retirement planning, large investments — use the GoFinSolve Compound Interest Calculator. The Rule of 72 is for back-of-napkin decisions; the calculator is for actual planning.