What screw size is this?

[Lou Cipher]

In the installation instructions for my projector ceiling mount (Chief CMA105), it states to use screws sized 5/16-18" x 2-1/2" (minimum) Grade 2 lag screws. What size in mm and type of screw is this? The mount will be screwed into a visible ceiling joist.

 

[Peter2400]

You can use any screws you want for this, a projector doesn't weigh much.
But according to your measurements, the screw should be at least 8mm thick and 63.5mm long.