This is a pretty big bag. 31" x 12" x 13" are the dimensions. So you would have a lot of excess room for a day pack. Great for weekend or week long excursions..but for one day..a lil much.

One of the reasons I bought this bag was because I can take the top back off completely and presto ... you've got a nice day-pack. The top bag also has a built-in lumbar strap so you can use this as a belly-bag.