The war is expected to increase US inflation by 0.5 per cent in the first three months of 2027, according to the Congressional Budget Office accounting, which totalled expenses through to August 1.
The mounting costs have strained US munitions stockpiles and driven up energy prices, raising concerns about America's ability to respond to other potential conflicts and adding pressure on an already stretched federal budget.
The cost was close to the $US37.5 billion that Defense Secretary Pete Hegseth reported at a Senate hearing on July 21.
The budget office said the majority of the costs stem from the rapid drawdown of munitions, estimating that it could take five years to replenish US stockpiles. Reuters reported in August that the US had used "virtually all" of its long-range precision missiles during the war.
The Department of Defence did not co-operate with congressional financial auditors, CBO reported.
White House spokeswoman Anna Kelly described President Donald Trump's attacks on Iran as "decisive action" to prevent Tehran from harming US personnel or interests.
She said in a statement the US military has "more than enough munitions, ammo, and stockpiles to serve all of the Commander-in-Chief's strategic goals and beyond."
Chief Pentagon spokesperson Sean Parnell disputed the report's findings of munitions shortages and said: "We have everything required to strike at the time and place of the president's choosing."
The cost inquiry was conducted at the request of the top Democrat on the US House of Representatives Budget Committee, Brendan Boyle of Pennsylvania.
"Donald Trump's disastrous war in Iran has already taken the lives of brave American service members and left others wounded," Boyle said in a statement.
"Beyond that human toll, this nonpartisan CBO report makes clear that the war has also cost American taxpayers tens of billions of dollars and counting, while continuing to drive up costs."
Eighteen US military service members have died in the Iran war.
The report did not include costs of recent strikes on commercial vessels in the Strait of Hormuz and attacks on US military installations in the region. It also did not factor in the borrowing expenses associated with financing the war, which could add tens of billions of dollars to the total cost.
The CBO accounting found the war could have sprawling consequences for the US defence posture and the US economy.
The deficit in critical munitions, including missile interceptors, risks hampering the Pentagon's response in the event of another major conflict, such as potential hostilities involving China and Taiwan, CBO said.
At the July hearing in the Senate, Hegseth pleaded for nearly $US90 billion in emergency funding to restock munitions supplies.
"Without these funds, we face critical shortfalls," Hegseth said.
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