Week 4 Day 4: The Rule of 72 Works Against You Too

72 / 24 = 3 years. A $5,000 credit card balance left unpaid at 24% becomes $10,000 in 3 years. Then $20,000 in 6 years. The math that builds wealth in investments destroys wealth in debt. This is why paying off high-interest debt is the first investment you should make.

Every dollar you pay toward high-interest debt earns you the interest rate of that debt in guaranteed, risk-free return. Paying off a 24% credit card is equivalent to making a 24% return on your money -- guaranteed. No investment in the world offers that. This is why most financial advice says: (1) Build a small emergency fund ($1,000). (2) Pay off all high-interest debt (anything above 6-7%). (3) Then start investing. The order matters because the Rule of 72 is working against you on the debt faster than it can work for you on investments. A dollar that eliminates 24% debt saves more than a dollar that earns 7% in the market. Math does not care about your feelings -- it cares about the rate.

The mathematical comparison between paying debt versus investing is straightforward but often obscured by tax considerations. For after-tax analysis: paying off 24% credit card debt at a 22% tax bracket is equivalent to earning a 30.8% pre-tax return (since debt payments are made with after-tax dollars and are not deductible, while investment gains may be taxable). For mortgage debt at 4% with tax-deductible interest, the effective rate for someone in the 22% bracket is 3.12%, making it rational to invest instead since expected market returns (7-10%) exceed the after-tax mortgage cost. The breakeven point is roughly 5-6% -- debt above this rate should generally be paid before investing beyond an employer 401(k) match.