Place a measuring tape across the vanity top, and measure from one side to the other. This measurement is the width of the vanity top.
Measure the depth of the vanity top. Lay the measuring tape on the top of the vanity, and measure from the back to the front.
Measure the depth of the vanity itself. Run the measuring tape along the side of the vanity, from the wall to the front of the piece to determine the depth.
Evaluate the width of the vanity. Run the measuring tape across the front of the vanity, from one side to the other, to get this measurement.