Timbuk2 Bicycle Seat Pack 900D Polyester HD Material: ballistic nylon Volume: [S] 31 cu in, .5 L, [M] 55 cu in, .9 L Mount: seatpost Pockets: 1 rear Dimensions: [S] 5.5 x 2.6 x 2.0 in, [M] 6.7 x 3.9 x 3.0 in