The Bill Trap: Why Treasury Keeps Borrowing Short
One of the quieter findings in the GAO’s March 2026 federal debt management report (GAO-26-107529) is the degree to which the U.S. government has increased its reliance on short-term borrowing — and what that implies for fiscal exposure to interest rate movements.
In fiscal year 2014, Treasury bills accounted for 13 percent of marketable debt held by the public. By fiscal year 2025 that share had risen to 22 percent, against a long-term historical average of 20 percent. Notes declined as a share but still constitute over half of all outstanding debt. Bonds increased from 12 to 17 percent, partly due to the reintroduction of the 20-year bond in 2020.
The shift toward bills is operationally rational. Bills are the instrument Treasury uses to absorb shocks — when borrowing needs spike unexpectedly, bills can be issued rapidly without the market disruption that sudden increases in note and bond supply would generate. The GAO quotes Treasury officials describing bills as “shock absorbers.” During the COVID-19 pandemic, Treasury rapidly scaled bill issuance to finance the emergency response before gradually refinancing that debt into longer-term instruments. Average bill auction sizes jumped from about $43 billion in the quarter preceding the pandemic to roughly $64 billion in the April–June 2020 period.
But shock absorbers carry a cost: they absorb future rate uncertainty as well as current financing needs. A bill matures in weeks. When it does, Treasury must refinance it at whatever interest rate the market clears. With 33 percent of all federal debt maturing within 12 months as of September 2025 — up from 24 percent in September 2014 — federal interest costs have become substantially more sensitive to short-term rate fluctuations than they were a decade ago.
The rollover arithmetic
The scale of the refinancing obligation is not intuitive until the numbers are examined directly. In fiscal year 2025, Treasury refinanced $9.1 trillion of maturing securities. In fiscal year 2026, the estimate is $9.7 trillion — before new borrowing to fund the deficit is factored in. This is not gross issuance of new debt; it is existing debt coming due and needing replacement, at whatever rates prevail at the time.
Bills compound this dynamic because of their roll frequency. A 4-week bill issued and refinanced for a full year generates 13 refinancing transactions. During each of those transactions, the rate resets. If the Federal Reserve has tightened or market conditions have shifted, the new rate applies to the entire stock of maturing paper.
TBAC’s assessment
The Treasury Borrowing Advisory Committee’s July 2024 analysis concluded that a bill share averaging around 20 percent represents an appropriate trade-off between cost and rollover risk. In August 2023, TBAC had indicated comfort with bills temporarily running above 20 percent while Treasury gradually increased note and bond auction sizes — consistent with its regular and predictable framework. By fiscal year 2025, note and bond sizes had been raised and the bill share stabilized.
The November 2025 TBAC analysis found the current debt mix well-positioned but noted that higher debt levels, larger deficits, and higher investor-demanded interest rates since 2019 have raised the baseline level of expected debt service cost and volatility. That is the new floor.
A government that owes a third of its debt in the next twelve months is a government betting heavily that the next twelve months will be calm. That bet has been winning. The payoff when it doesn’t is compounding interest at the worst possible moment.