A litre IS a measure of volume - by definition its the volume 1kg  of water will occupy.

One way of picturing volume by size in your head is to compare with known volumes of liquid produce -  for example a 2litre fizzy drink bottle or a 4litre milk carton gives you a fair idea what 2/4 litres actually looks like.

Another think you could do is email Alpkit asking what might suit specific kit - they have respnded quickly when I've emailed them a couple of queries in the past.

Hope that helps.

240 litres

Or work it the other way... to be on the safe side put your camera gear in the smallest box(es) you can get them in.  Then measure height, width and depth of the box(es) in cm and multiply all 3 together to get the total in cubic entimetres.  That'll be an overestimate thanks to dead space in the box(es), but better a little too big than too small.

 1000cc is one litre, but of course it'll need to be the right shape as well as the right volume  as the bags aren't stretchy, so check the maximum dimensions of your camera against the bag (i.e., if a one litre bag is 1000 cm long, one cm high and 1 cm wide then desite having more volume than a 350 cc camera, you won't get the camera in therough the 1 x 1 cm opening)

Pete.