A Rucksack, by definition, is a bag that will allow you to transport your equipment on your back, in a comfortable and space efficient way. The size / capacity of a Rucksack is measured in LITRES.

In general, Rucksack sizes fall into three categories

“Daysack”

Capacity is between 10-30 litres.
Features include a single compartment, and may have external pockets in the sides or front.  Ideal for day-trips / walks, when size and weight are important.  Adequately stores waterproofs, maps, food, and other essentials.

“Rambler”

Capacity for a rambling sack is between 30-60 litres.
Features will include padded backs for comfort, with air channels to aid ventilation.  1 or 2 compartments, with at least 2 external pockets.  Ideal for weekend camping and hostelling.

“Backpacker”

Around 60-80 litres.
Features include, 2 compartments, at least 3 external pockets.  Some may have an adjustable harness for comfort and a tailored fit.  Extensive padding throughout.  Will accommodate everything for a successful backpacking / camping trip.