Backpack’s volume capacity and weight capacity are very different features. The former can be measured and it is exact. The latter describes the comfort level, it is based on feeling and therefore it is not exact.
Specifications of a backpack include parameters like weight capacity (or carry load), and volume capacity (or simply capacity).
These terms may sometimes be mixed but they describe completely different aspects of a pack, and they have nothing to do with each other.
The volume describes the total inner size of all cavities of a pack, and it is given in liters or in cubic inches. In principle, this parameter you can determine yourself or rely on what specifications of a pack show you.
The weight capacity is a parameter that describes the maximum load (in kilograms or pounds) that you can carry comfortably in one specific pack. So this has lots to do with the construction of the pack, its frame, padding, hip belt presence or absence, etc.
However, the feeling of comfort is individual, yet weight capacity comes in specifications, so it is determined externally. How? By some “average” or “typical” testing people. This is why it is not truly reliable and it should not be taken very strictly.
How To Determine the Weight Capacity of a Backpack?
The answer to this question is simple – put some extremely heavy stuff in the pack and carry it around. Ask somebody to keep adding weight till the shoulder harness straps tears of and break apart.
I am joking of course. But seriously, you do not determine the weight capacity of a pack. This parameter is normally given in specification for almost every pack of top brands.
Here I have to correct myself. Surprisingly enough, Deuter is the top brand, but you will not find such information in specifications of their packs. Once I asked about one specific pack and they replied that they simply do not provide it.
But knowing how reliable Deuter packs are, you can simply compare it with some other brand’s pack of the same volume where the weight capacity is given. You can be sure that Deuter’s pack will be able to support at least the same weight. So its carry load will be at least the same or better.
But what is a pack’s weight capacity? How much weight can a backpack hold?
It is best to take some examples:
- My Osprey Stratos 24 pack (shown in the top picture above): official carry load range is 10-25 lb (5-12 kg). Observe the lower limit here.
- Gregory Katmai 55 pack for men: official load capacity is 45 lb (20.4 kg).
Now, what is the meaning of this? Does this mean if you have 25 kilograms in your Katmai pack that it will fall apart? Of course not.
The meaning is that some “average” or “typical” person (but not necessarily you) can carry up to 45 pounds (20.4 kg) comfortably. Observe those 0.4 kg and think if this is really a big deal. Could you set the carry load to 44 pounds (20 kg)? Of course you can, there is nothing exact here.
I am sure you can carry 30 kilograms in the pack at least for a while, the pack will be fine, yet I am not sure how this would be with you. So you see the point?
But what about the mentioned lower limit in my Osprey pack, those 10 pounds (5 kg)? Frankly speaking, I have no idea what they mean by this. I have used that pack many times with just a few items inside, so this was well below the mentioned lower carry load limit.
My guess is that this may have something to do with how the pack’s harness wraps around your body under an appropriate load. So if the weight is too small, it may not hug your body in an optimal way, or something along this line of thinking. Nevertheless, I am sure it is best to disregard this parameter, in my view it has no practical meaning.
What is volume capacity of a pack?
If a pack would be a rectangular object, its volume would be simply the product of its three dimensions (height, width, and length). As you know packs usually come with such dimensions indicated.
But the reality is that a pack has no regular shape. Quite generally, volume capacity or simply capacity refers to the inner dimensions of all its cavities, and this means inner compartment(s) and pockets.
So this is about bulk of the pack when all its cavities (including the lid as well) are filled with stuff. Now, you can fill those compartments and pockets with something very light to measure the volume, and, consequently, this has nothing to do with carry load weight.
In other words, a 50-liters pack does not necessarily mean that you can carry 50 kg of weight in it.
How is a pack’s volume measured?
I admit, I do not have a precise information on how various brands determine the volume capacity or volume of their packs.
In any case, this is something that can be measured, so this is more or less an exact parameter. One simple way would be to use some small identical objects that would be used as units of volume, assuming that we know exactly individual volume of each of them. Imagine that these are simple balls with 1 cm diameter.
The volume of such a ball is then 3.14/6 cm³ = 0.52 cm³. But when these balls are packed together and they are in contact with each other, each of them will need around 1 cubic centimeter of space (volume).
Now fill the pack, check how many balls fit in (say this is N balls), and the volume in cubic centimeters will be close to N. Easy! Then you can recalculate, 1 liter equals 1000 cm³ and this equals 61 in³.
This is a bit rough estimate, and this is probably the minimum volume of the pack. You can use some other size balls, for example those that are used for table tennis or tennis. It is important that they are all identical and that you know the diameter.
The guy in this video shows more or less the same:
There are some other clever ideas, see one here:
Why is the capacity of backpacks always measured in liters?
This is a real question that I have seen as being asked, and I can only say it is misleading. In the US area, backpacks volume capacity is mostly given in cubic inches.
As mentioned above, you can easily recalculate, one liter corresponds to 61 cubic inches.
How much weight can a 30 L backpack hold?
I have seen a number of questions of this type, for a variety of volumes. In fact, this and similar questions reflect the real issue, people get confused with the volume and the carry load capacity.
So to properly answer this particular question, this is about a backpack that has a volume of 30 liters. Now, the actual official carry load of such a pack will depend on its construction and on the quality of the pack in general, an don their testing people as mentioned above.
Let’s see some examples.
- Gregory Mountain Products Icarus 30 Liter Kid’s Hiking Backpack has an official 20 lb (9.1 kg) load capacity.
- Gregory Mountain Products Zulu 30 Liter Men’s Hiking Daypack’s official load capacity is 35 lb (16 kg).
- Gregory Mountain Products Juno 30 Hiking Backpack has a load capacity of 25 lb (11.3 kg).
So as you see, there is no strict rule, all three packs have the same volume but the official weight capacity is very different. As I said earlier, this is about carrying the weight comfortably, so this is both about the pack and about you.
In summary, backpack’s volume capacity and weight capacity are two different features of a pack, and I hope I was able to explain this topic clearly.
Let me know if I missed to say something important or if I said something that was not clear. There is a comment box below. Check also my text about some other specifications of a good pack. Thank you for reading and have a nice day.