Pentagon budget documents forecast the number of U.S. troops in Afghanistan will drop to an average of 98,250 in fiscal year 2012, compared with an estimated average of 102,000 soldiers and support personnel in fiscal 2011.
Fiscal 2012 starts in October.
President Barack Obama has announced plans to start drawing down troops from the nearly decade-old war in Afghanistan in July, with the goal of passing lead security responsibility to Afghan forces by the end of 2014.
Gates, briefing reporters on the budget request, said he explained to lawmakers the 98,250 figure was a conservative approach to budgeting ... since we don't know how many troops will be reduced during the course of FY '12.
Depending on the size of the drawdowns, that (funding earmarked in the budget) may be money that we just don't spend, he added.
Pentagon officials say the pace and scale of the actual drawdown will depend on conditions on the ground and they refuse to give any definitive idea about the size of the initial withdrawal. Gates renewed that message on Monday.
But that's not to say that we will have 98,000 troops at the end of FY '12, he said. In fact, it's a lead-pipe cinch we won't.
The documents were released as part of the Obama administration's roll-out of its spending requests to Congress, which includes $118 billion in funding for the Iraq and Afghan wars as well as other overseas efforts.
(Reporting by Phil Stewart; Editing by John O'Callaghan and John Whitesides)