A portion of this duct has split in half in my attic, and I wanted to repair it with a new R6.0 duct. I just can't tell what size this is based on the tag. Is it 8'' or 10''?

The tag says:

Max. Velocity : 6000 F.P.M
Max Pos. Pressure : 10 IN W.C.
Max Neg. Pressure : 1/2 IN W.C.


Thanks. Here is a link to the picture :

http://www.evernote.com/l/AW6A-SfUU9VHW5bpbqQnH6NaIS30X64TEnw/

That is a performance tag. 6,000 lineal feet of air per minute. 10" water column positive pressure, 1/2" negative pressure. I see a D 4338 so my guess would be 8"