What the federal debt costs, and what it now outranks

The Treasury reported $689.2 billion of interest expense on public issues of the federal debt in the last complete fiscal year, 2025. In fiscal year 2015 — the first year of the same published series — it reported $237.3 billion. That is 2.9 times as much over 11 fiscal years.

The debt itself grew over the same window, from $18.15 trillion outstanding at the end of fiscal year 2015 to $35.46 trillion at the end of fiscal year 2025 — 2.0 times as much. Interest grew faster than the amount borrowed.

What it now outranks

The federal budget is organised under a set of purpose headings, and the publisher reports obligations and outlays against each of them. In fiscal year 2026 to date there are 18 of them, and interest is the 3rd largest at $1.18 trillion.

Ahead of it are Medicare at $1.71 trillion and Social Security at $1.45 trillion. Behind it, narrowly, is national defence at $1.17 trillion.

Servicing the debt is not a programme. Nobody receives a service from it and no agency administers it as a benefit. It is what the government pays for decisions already taken, and by the budget's own headings it is now among the largest things it does.

Two official figures, and this brief does not reconcile them

The two interest numbers above are not the same measure and should not be read as a series.

$689.2 billion is the Treasury's own reporting of interest expense on public issues of the debt, on an accrued basis, for a complete fiscal year. It is published monthly with a year-to-date column, and the figure here is the year-to-date reading at the end of September, which is where the federal year ends.

$1.18 trillion is what the budget function called net interest carries in the account data agencies submit, for the open fiscal year to date. It is a different agency, a different definition, and a different window.

Both are official and both are current. The Factbook has not established what accounts for the distance between them, and says so rather than picking the one that makes a tidier sentence. The debt page carries the Treasury series with the basis named on it; the budget functions ranking carries the other.

What this does not establish

It does not say why interest rose. Interest on a debt is the amount borrowed multiplied by what it costs to borrow, and this brief measures the product rather than either factor. A rise in one, the other, or both would look identical here.

It does not say the debt is unaffordable, or affordable. That is a judgement about the economy the debt sits in, and nothing in the Treasury's tables speaks to it.

It does not cover the open fiscal year in the first two figures. A year that is not finished has a year-to-date total and not a year, and putting one beside eleven complete years would show a fall that is only the calendar.