Surface area is length times width,,

so I'm guessing that the 4mm is depth, not needed

140 x 120 = 16800mm squared ( surface area)

if its volume you are after

then

140 x 120 x 4 = 67200mm cubed ( volume )

so I'm guessing that the 4mm is depth, not needed

140 x 120 = 16800mm squared ( surface area)

if its volume you are after

then

140 x 120 x 4 = 67200mm cubed ( volume )