SunnySports S Sorry, but Marmot doesn't give inside dimensions, the outside dimensions are 19" x 8.6" x 6.7"and the capacity of the pack is 1100 cu in / 18 liters. The organizer is not really for pens and pencils, it is more like some zipper pockets and I believe a key loop. If you really need the inside dimensions of the pack your best bet would be to call Marmot directly at (888)357-3262. Stay safe and have fun out there! Neal