Open Area is the ratio between the total area of the holes and total sheet area, normally it is expressed by percent. For example, for a perforated sheet with the following specifications:
Round hole 2mm hole size, 60 degree staggered, 4mm pitch, sheet size 1M X 2M
According to above information and based on the formula below, we can get the open area of this sheet is app 23%. It means the total holes area of this sheet is 0.46SQM (1M X 2M X 23%)

Open Area Formula:

Round Hole 60°Staggered

Round Hole 60°Straight Row

Round Hole 45°Staggered

Square Hole, Staggered

Square Hole, Straight Row

Round End Slot, Z Type Staggered

Round End Slot, Straight Row

Round End Slot, K Type Staggered

Square End Slot, Staggered

Square End Slot, Straight Row

Square End Slot, Staggered