We’re seeing a lot of debate in Washington about what is commonly referred to as the "national debt ceiling." This post is an attempt to shed some light – and provide some good resources for further information – on what this really means. National debt is not the total future obligations of the federal government to pay. It is basically all the public debt (like Treasury bills) plus money we owe to other governments – in other words this ceiling only puts a limit on how much the federal government can borrow, not on how much they can spend.
The national debt number is available "to the penny" at the Treasury Direct website. There are only a few categories of debt that are not subject to the limit, mostly having to do with the way that Treasury Bills are issued to pay all the interest up front (discounted) and the way that payment is handled in accounting terms. Raising the National Debt Ceiling involves raising the limit on the public debt ceiling.
There is a bigger number that most other countries use to define “debt”. The official definition for “debt” used in the European Union, for example, includes obligations to Social Security, Medicare, etc. at the national level, plus regional and local government debt. (Thanks to Yannick for initiating a discussion of the distinction with his comment to my 2009 piece on Public Debt Crisis.) In the U.S., the larger number is usually referred to as "total indebtedness". There is no limit set on the promises of the US government to spend money -- for example, the almost $13 trillion committed to the post-crisis bailouts and stimulus was not subject to the debt limit despite that number being almost equal to the total national debt. The limit only applies to how much the Treasury can borrow to meet its obligations. So if the question is “should the ceiling be raised?” then my answer is “it doesn't really matter.” Congress can keep spending without it.