9

Backpacks that we use are typically categorized in litres (Or liters if you are an American English follower).

Whats is the point?

Why don't we have backpacks that are categorized as for example 70 kg instead of 70 L?

  • 3
    Backpacks certainly also have a limited weight they can hold. But that limit is not relevant, because the human carrying limit is much lower. – gerrit Mar 17 '18 at 18:32
  • if you're just carrying liquid water on earth the max volume and weight will be the same for your pack. but when you make that inevitable trip to mars, you'll be very confused. – ldgorman Mar 20 '18 at 16:30
20

Because the characteristic of a backpack in question is volume or how much content it can hold, not mass. An extreme example: If I fill lead into a 20l pack, I get ~225kg, if I fill styropor into a 50l pack, I get about 2.5kg. Mass is still specified sometimes to give the empty mass of a pack.

You might want to specify max loaded mass as either a limit for what the backpack can structurally take without taking damage or a limit for what the carrying system (shoulder, hip straps) are designed for. But these are different concerns and both are very imprecise. Maybe the max load without the pack taking damage could be estimated and the producer could recommend a max load to be carried with the pack, but as with any "soft manufacturer recommendations", that wouldn't be much help, as the human carrying the weight will be the limit.

  • Closer to 200 kg for the 20 liters of lead, 40 for stone, 20 for water. But your point stands. – Monster Mar 17 '18 at 16:02
  • @Monster Uh, zeros - almost as bad as signs... Thanks for noticing :) – imsodin Mar 17 '18 at 16:22
  • 4
    Easier said, kilos are a measure of weight, liters are a measure of size. It is size what you want to know when you compare different packs. – Willeke Mar 17 '18 at 18:30
  • @Monster are you saying 20 litres of water weighs 200kg? – ldgorman Mar 20 '18 at 16:42
  • No, I'm saying 20 liters of water weights 20kg, 20 liters of stone weights roughly 40kg and 20 liters of metal weights about 200 kg (very roughly, aluminum, titanium and even steel weigh less, lead, uranium, gold and platinum more to much more). – Monster Mar 20 '18 at 21:28
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Luggage is usually chosen on its size, not on the weight it can transport. Liters are a measure of size, kilos are a measure of weight.

To me it is harder to convert a given weight in a certain substance into a real size than it is to convert a given number in liters to a real size.

If you want to know how many kilos of water a given pack can move, you can just replace liters with kilos.

Good pack makers will also tell you the weight limits of your pack if you are likely to reach them in normal use, but as very few people will be able to lift and carry backpacks filled to the limits with heavy items, a well made pack should be able to withstand more kilos than the average human can carry.
The point where packs fail is mostly after use, so a weight limit or indication will not be very helpful.

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