Congress sets a ceiling on how much the Treasury may borrow. Not every dollar of debt counts against it: unamortised discount, Federal Financing Bank debt and a few other lines are excluded. The Daily Treasury Statement publishes the debt subject to the limit and the limit itself, so the headroom is a subtraction anyone can do.
On 9 September 2026 the limit stood at 41,104 billion dollars and debt subject to it at 39,886 billion: 1,218 billion of room. A 1,226 billion dollar dividend would have used all of it.
The number is the difference between what a politician can promise and what the Treasury can currently issue.