WASHINGTON - How big is the national debt? You'd think this would be an easy question. Surely we know how much the government owes.
Unfortunately, it's not that simple. The true national debt could be triple the conventional estimate, anywhere from $11 trillion to $31 trillion by my reckoning.
The differences mostly reflect explicit and implicit "off-budget" federal loan guarantees.
In another economic downturn, these could result in large losses that would be brought "on budget" and worsen already huge deficits. That's the danger.
My purpose is not to scare or sensationalize. It's simply to illuminate the problem.
Broadly conceived, the national debt covers all debts for which the federal government assumes final responsibility. For politicians, the appeal of "off-budget" programs is that they allow the pleasure of spending without the pain of taxing.
But they also create massive exposure.
Let's see why. Below are five estimates of the national debt. I compare each with our national income (gross domestic product), which is the economic base to service debts. In fiscal 2012, GDP was $15.5 trillion.
Some economists say a debt ratio exceeding 90 percent slows economic growth. The United States already exceeds this threshold on four of my five measures.
* (1) Treasury debt held by the public: $11.3 trillion, 73 percent of GDP for fiscal 2012.
This is the most common measure of the national debt. Reflecting past annual deficits, it represents what must be borrowed through sales of Treasury bills, notes and bonds.
In 2007, the figures were only $5 trillion and 36 percent of GDP.
Today's levels - as a share of GDP - are the highest since World War II's immediate aftermath.
* (2) Gross federal debt: $16 trillion for 2012, 103 percent of GDP. This definition includes the "debt held by the public" (above) plus the Treasury securities issued to government trust funds, the largest being Social Security.
Economists dislike this debt concept, because the trust fund Treasury securities represent one part of the government owing another. It's comparable to lending yourself money.
Congress could cancel these debts, though it almost certainly won't. The trust-account Treasury securities represent political commitments more than financial obligations.
* (3) Federal loans and loan guarantees: $2.9 trillion in 2011, 19 percent of GDP.
The government makes or guarantees loans to college students, farmers, veterans, small businesses and others. The face value of most of these loans don't show up in the budget, but the government is on the hook if borrowers default.
Adding this debt (19 percent of GDP) to gross federal debt produces a total debt ratio of 122 percent of GDP.