The headline projection was the national debt will almost double in 30 years. Using the rule of 72, T=72/r, where T is the time in years required for principle to double and r is the annual interest rate compounded, a 30-year doubling time implies the debt is growing at 2.4 percent annually (30=72/2.4). However, the national debt almost doubled during George W. Bush’s two terms, and almost doubled again during Barack Obama’s two terms. That implies a T of a little more than 8 years. To be conservative (because debt almost doubled but not quite), round the T up to 9 years. Plug that into the rule of 72, and you have the debt growing at 8 percent per year (9=72/8), or over 3.3 times the rate the CBO is assuming. Scary as that 30-year doubling sounds, simply extrapolating the reality of the last 16 years projects another doubling in not 30, but 9 years, or a year longer than Donald Trump’s potential two terms.

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