We are often asked how we consistently measure the size or capacity of our bags when they are complicated shapes and it would be impractical to actually fill them with water to measure the total capacity.
In simple terms there is an international standard which we work to that is given the catchy title F2153 "standard test Method for measurement of backpack capacity". We prefer to call it the ping-pong ball test.
We have a large cylindrical barrel in our office which we poured litre jugs of water into and marked off on the side of the barrel the height of the water at every 5 Litre interval.
The most accurate process would have then been to fill our bags with water and tip them into this barrel to show us exactly how much water had gone in on our scale... But that is extremely messy and heavy and time-consuming so the ASTM Method says that we can use any small object of a fixed size which will not compress and can be used to fill a bag instead of water.
We got our hands on several thousand ping-pong balls and we use these to fill our bags. Once everything is packed full we can then tip the ping-pong balls into the barrel place a piece of cardboard on top of them give everything a good shake so it settles and then read off on our scale the capacity of the bag.
It sounds a little bit basic but actually this is the industry standard way of doing these things and the result is surprisingly accurate. In fact, it's so accurate that every manufacturer of bags uses it so we all talk the same language. For more information, see here