Let me show you how this works, using numbers from the recent update issued by the nonpartisan Congressional Budget Office. (I'm giving you the simplified version to keep your eyes from glazing over. You can find a detailed version in the accompanying box.) We'll start with Social Security, which will take in about $78 billion more in payroll and income taxes than it shells out. The Treasury takes that cash, gives the trust fund IOUs for it, and spends it. That $78 billion isn't in the stated deficit.
Wait, there's more. The Treasury will fork over $108 billion of interest on the trust fund's $2.2 trillion of Treasury securities, but will give the trust fund IOUs, not cash. They won't count in the deficit either. Add that $186 billion to the stated budget deficit, and it more than doubles, to $344 billion. The stated deficit, you see, measures how much less cash Uncle Sam takes in than he spends. That's fine for gauging the deficit's impact on the economy, which is what budget experts generally do. But if you're trying to assess Uncle Sam's overall fiscal condition, as I am, you should count those IOUs in the deficit because they have to be paid someday.