The Rule of 72 Makes Doubling Time Feel Obvious
How long until your money doubles — five years, ten, twenty? The Rule of 72 answers that with one division problem, and it works on savings, inflation, and debt alike.
No math degree required — just one number and the 72.
1. Run the Basic Math
The whole rule: divide 72 by your yearly return, and out pops the years to double.
As a formula: 72 ÷ x = y, where x is the annual rate and y is the doubling time.
Example: money tracking the S&P 500's long-run average return of about 9.8% doubles in 72 ÷ 9.8 = 7.34 — call it 7 years and 4 months.
One catch: the rule assumes compounding, meaning you reinvest every gain and dividend.
Spend the earnings as they come, and the doubling magic — and the rule — stops working.
How to Do It:
- Divide 72 by any rate you're offered; if the answer disappoints, so will the investment.
- Turn reinvestment on so the dividends compound instead of leaking out.
2. Adjust for High or Low Rates
The rule is sharpest for rates between 6% and 10%, with 8% as the bullseye.
Outside that range, tune it: for every 3 percentage points your rate sits from 8%, add or subtract 1 from the 72 — add for higher rates, subtract for lower.
A calm 5% investment: 5 is 3 below 8, so use 71 ÷ 5 = 14.2 years to double.
A hot 20% bet: 20 − 8 = 12, and 12 ÷ 3 = 4, so use 76 ÷ 20 = 3.8 years.
If the difference doesn't divide evenly by 3, just round like normal — for 22%: 14 ÷ 3 = 4.67 rounds to 5, giving 77 ÷ 22 = 3.5 years.
The further from 8% you stray, the more those little adjustments matter.
How to Do It:
- Memorize the one-liner: "±1 on the 72 for every 3 points away from 8%."
3. Swap the Time Units When Needed
The rule isn't married to years — it works on any period, as long as both sides match.
Charging a friend 5% monthly interest on a loan? Adjust to 71 (5 is 3 below 8), and 71 ÷ 5 = 14.2 — his balance doubles in about 14 months, not years.
Monthly rate in, monthly answer out — that's the whole trick.
How to Do It:
- Before dividing, say the unit out loud: "percent per what?" — the answer comes back in that same unit.
- 130+ functions cover loan payments, bond prices, depreciation, NPV, and IRR
- Switch between classic RPN and algebraic entry — with an undo key
- Stores 30 cash flows and is approved for finance certification exams
4. Point It at Inflation
Flip the rule around and it shows how fast rising prices halve your money's buying power.
At 2% inflation: 2 is 6 below 8, so adjust 72 down by 2 to get 70, and 70 ÷ 2 = 35 — your cash loses half its punch in 35 years.
Run it at 5% inflation and the answer drops to about 14 years. Ouch!
That's why parked cash needs a plan — here's where money can hide when inflation runs hot.
How to Do It:
- Divide the adjusted 72 by the current inflation rate to see your cash's half-life — then invest accordingly.
- Practice investing, real estate, and passive-income decisions with play money
- Built by the Rich Dad team to drill cash-flow thinking
- For 2-6 players ages 14 and up, refreshed with modern money concepts
5. Point It at Your Debts
Interest doesn't care whose side it's on — the rule also shows how fast your lender doubles their money on you.
New credit cards lately carry average rates around 24%: adjust to 77, and 77 ÷ 24 ≈ 3.2 — a carried balance doubles in barely three years.
Even a 30-year mortgage at 6% doubles the lender's money in about 71 ÷ 6 ≈ 12 years — well inside the loan's own term.
Knowing your real rate versus yield makes these numbers honest — and motivating.
How to Do It:
- Run the rule on every debt you carry; the scariest answer is the one to pay off first.
Wrapping Up
The Rule of 72 is a pocket flashlight: one quick division and you can see where your money's headed — growing, shrinking, or feeding someone else's snowball.
These are ideas to learn from, not personal instructions or individualized financial advice.
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