How do I measure the size of the door?

For single and double door configurations

Subtract 2" from the height. This will determine the door slab size.

1

Let's imagine that we have measured the width of the rough opening and it equals 35"

In order to determine the width of the door slab, we first of all need to subtract 2" from the width of the opening. So where do these 2 inches actually come from?

2

The standard thickness of our jamb is 3/4".
3/4" + 3/4" = 1 1/2" is the total thickness of 2 jambs.
And we leave the rest of the space (1/2") as technological gaps to be filled in construction foam.
So this forms the 2 inches' width that needs to be subtracted from the overall width of the opening.

3

35" - 2" = 33"
Now we need to determine which standard width is suitable for the 33" wide opening.
This is obviously 32".
And the remaining space will be filled width construction foam and covered with prefinished casings.

4

Standard width of Modern Doors casings is 3 1/2"

For barn, magic, bi-fold, bypass and pocket door systems

Please refer to the manuals provided by the hardware manufacturers.