Backpack’s Volume Capacity vs Weight Capacity

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.

Backpack's Volume Capacity vs Weight Capacity - top picture showing my Osprey Stratos 24 pack on a mountain summit.
My Osprey Stratos 24 pack on the summit of Geisterspitze (Punta degli Spiriti) in the Italian Alps.

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:

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). 

Such balls can be used to measure the volume of a pack.
Such balls can be used to measure the volume of a pack.

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:

You can also put some waterproof plastic sack in the pack and fill it with water. Then measure the weight of this water, 1 kg is 1 liter, so you will know very exactly the volume of the main compartment.

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.

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.

 
Me on Jalovec.Hi, I am Jovo, the founder of this OutdoorsFAQs site and several other outdoor sites. I have been mountaineering for almost 40 years already, and I have created this site to use as a reference for various questions that I receive in my sites. Being a theoretical physicist by profession, I tend to base my answers on facts and on my own personal experience.

2 thoughts on “Backpack’s Volume Capacity vs Weight Capacity”

  1. hi, I am interested in knowing an answer to a different question. let’s say we take a 30L backpack and fill it with such material as required on a typical trek how much will it weigh? this will be the weight that I will be carrying. also airline have certain kg allowance (checked in or carry on). say 15kg. for the kind of typical stuff one takes with on travel what size bag is optimal? my general observation is that every 6 liters holds about 1kg of travel material. my guess is that when people are asking questions on weight capacity they mean the typical weight when filled with travel stuff.

    Reply
    • Hi, I am afraid there is no simple answer to your questions. Regarding 30 liters pack, there is no typical trek, it depends on what you do and how much water you carry. Water is usually the heaviest stuff. A pack with such a volume is for day tours, so water may be up to 3 liters or so. The rest will depends on the stuff you need, imagine this is a ferrata, then you will have some metal stuff, a torch, food, etc. You can easily have up to 7-8 kg or so.

      It is best to check in specifications for the weight capacity, this is the maximum suggested, but I am sure that good packs can carry far more. Take Big Agnes Ditch Rider 32L Pack as an example, its carry weight is 25 lb (11 kg). The same recommended weight is in Osprey Talon Pro 30 pack.

      As for travel packs, you seem to know more than me. But if 6 liters of volume result in 1 kg of weight, for a 30 liters pack this will give around 6 kg in total with the pack’s weight included. Sounds too much to me because you do not carry water which adds most of weight.

      Reply

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