Quick mental math for doubling time: 72 ÷ annual return = years to double. At 7% returns, money doubles every ~10 years.
The Rule of 72 is a mental-math shortcut: divide 72 by the annual rate of return, and you get the number of years it takes to double your money.
At 7% returns, money doubles in roughly 10 years. At 4% (CPF SA), it takes 18 years. At 2.5% (CPF OA), 29 years.
Doubling time = ln(2) / ln(1+r) ≈ 0.693 / r for small rates. 0.693 × 100 ≈ 69.3, but 72 is used because it has more useful divisors (2, 3, 4, 6, 8, 9, 12) for mental arithmetic.
The rule is most accurate for rates between 4% and 12%. Outside that band, more sophisticated formulas (Rule of 69.3 for continuous compounding, Rule of 70 for general use) are slightly closer to truth.
How many doublings until retirement? At 7% over 40 years, your starting investment doubles 4 times: 2 → 4 → 8 → 16. A S$10k investment becomes S$160k.
Inflation halving: at 3% inflation, prices double in 24 years. A S$1,500 monthly expense today becomes S$3,000 in 24 years.
Cost of fees: 1% in annual fees doubles in 72 years — meaning across 30 years you lose roughly half a doubling cycle in returns. Over decades, fees seriously compound.
CPF guarantees: SA at 4% doubles in 18 years. OA at 2.5% doubles in 29 years. The interest differential is why the SA → OA transfer (when allowed) is so powerful.
Anchors expectations: a young investor who realises their money will only double 4 – 5 times before retirement (rather than 'compound forever') makes saving more, earlier, an obvious priority.
Demystifies long-term math: people consistently underestimate compound growth in their 30s and 40s. The Rule of 72 makes it visible without a spreadsheet.
Trains rate intuition: 5% vs 7% looks like 2% difference but is actually a 30% difference in doubling time. Small rate differences compound to large lifetime outcomes.
A mental-math shortcut for doubling time. Divide 72 by the annual return rate to estimate how many years it takes to double your money. At 7%, money doubles in roughly 10 years; at 4%, 18 years; at 2.5%, 29 years.
Most accurate for rates between 4% and 12%. Outside that range, slight variants (Rule of 69.3 for continuous compounding) are closer to truth. For mental estimates in personal finance, the Rule of 72 is plenty accurate.
Count doublings. From age 25 to 65 at 7% returns, money doubles ~4 times (every 10 years). S$10,000 at 25 becomes S$160,000 at 65. Starting at 35 cuts you to 3 doublings — only S$80,000. The extra decade of compounding is worth more than the contributions themselves.
Yes — and it's sobering. At 3% inflation, prices double in 24 years. A S$2,000 monthly expense today becomes S$4,000 in 24 years. Use this to size retirement income targets in nominal vs real terms.