Page "President of the United States" Paragraph 62

The Constitution states that the vice president becomes president upon the removal from office, death or resignation of the preceding president.

If the offices of president and vice president both are either vacant or have a disabled holder of that office, the next officer in the presidential line of succession, the Speaker of the House, becomes acting president.

The line then extends to the president pro tempore of the Senate, followed by every member of the Cabinet in a set order.