Lisa Rein writes many managers were already preparing plans for months under low profile.
In late July, the Office of Management and Budget sent agencies a two-page memo saying calculations of cuts under the sequester can only occur once spending levels for fiscal year 2013 are known.
Citing analysis from budget experts, Rein writes the cuts must be compressed within nine months as January is three months into fiscal 2013.
That equates to reductions between 10 and 11 percent, she writes.
Army Lt. Col. Elizabeth Robbins, a Pentagon spokeswoman, told the Post the Pentagon would likely implement an immediate freeze on hiring civilian workers.
OMB instructed the Pentagon late last week to start planning for the sequester.