There are three types of backpacks: Small, Medium, and Large. All three will increase the player's max weight value by a certain percentage, which is also configurable in the config file for the mod. When wearing one of the backpacks, a custom model will be rendered on your player's back: 

You can craft the Small Backpack with some leather, sticks, and a chest: 

The Medium, and Large backpacks can only be found in dungeon chests, or can be acquired by using the Upgrade Station (see below). Small Backpacks are about as rare as iron, Medium are as rare as diamonds, and Large are as rare as Golden Apples.

The Upgrade Station is used to upgrade backpacks into higher quality, more efficient ones. It is crafted with six Spruce Logs, one Crafting Table, one Glass Pane, and one Obsidian:


 
Once made, it will look quite inviting when placed: 
 
Next, just open it, and you'll be greeted with this screen: 


Once there, simply follow these steps: 

Step 1 Place a backpack in the first slot: 

Step 2 Put the material, and amount required to upgrade the backpack into the second slot: 

Step 3 Enjoy your new backpack! This can also be done on the Medium Backpack, and soon to be Large! 

This mod also adds a custom inventory, which is accessed by pressing the 'R' key (changeable): 

To the left of the inventory is your current weight over max weight. This indicates how much you're carrying, and how much you can carry before being slowed down. The first, and only slot on the inventory is where your backpack will go. When placed in this slot, the backpack will upgrade your max carry weight by a certain amount; and render a model on your player's back!

Weight of all items is stored within a json file that is created when your first load up your game with the mod. This file will load all blocks and items from the vanilla game, and any mods you have loaded with this one. The file can be located within the folder that the config would be, and can be changed to your liking. When running this mod from a server, the file will be called 'WeightLimitServer.json', and any values set in that file will be synced with the client.

NOTE: THE ONLY WAY FOR THE CLIENT TO BE SYNCED, IS IF YOU DO A FULL SERVER RESTART! 

Let's take a look at how tooltips work: 
Each item has a new variable on their tooltip called 'Weight'. This determines the item's weight multiplied by the amount of the item in the stack.The pictures indicate how the color will change with the more of said item you have in a stack. Green means light, dark green means slightly-light, orange means heavy, and red means very heavy:
 

Next, is how weight is displayed. As shown in the inventory section, your weight is displayed in the custom inventory. It can also be displayed on the HUD, which is configurable within the wl.config file: 

When you become over encumbered, the letter's on the HUD will turn red: 

If you turn on advanced tooltips (Holding F3 + H), you will get an advanced view of your current weight (NOTE: This is only available on the HUD display of your weight. If you toggle it off in the config, you will not see advanced weight!): 

There is a small bug with the advanced HUD display that I am currently working to fix.

Currently, there is only one enchantment in this mod, Strong Back. This Enchantment can be applied to chestplates only, but has quite a big effect on the player's max weight:

When worn, it will increase the player's max weight by 30 (multiplied by the Enchantment's level). The Enchantment is as rare as getting protection, and only has three levels. As shown in the screenshots, Strong Back I, II, and III increase the player's max weight by the amount above, multiplied by the Enchantment level (all amounts can be set in the config file if wanted):


Strong Back I


Strong Back II


Strong Back III

A new feature added in v1.0.2 introduces a mechanic that affects the player's max weight when a Strength, or Weakness potion is active. With Strength, the player will experience increased max weight for the duration of the potion, multiplied by the potion's amplifier:

With Weakness, the player will experience decreased max weight for the duration of the potion, multiplied by the potion's amplifier:

All of the variables are changeable within the config file, and sync between server and client.

Version 1.0.2:
+ Added Strong Back Enchantment (I, II, III) for body armor, which increases your max carry weight by 30 multiplied by the Enchantment level.
+ Added max weight buff of 40 when having the Strength potion active (multiplied by it's amplifier).
+ Added max weight debuff of 30 when having the weakness potion active (multiplied by it's amplifier).
+ Added more config switches for better customization.
* Reworked max weight system to run faster.
* Reworked Backpack rendering to run faster.

Version 1.0.1:
* Fixed weights not being loaded for mods added after .json file was created
* Fixed Backpack's not rendering correctly when player starts sneaking
* Fixed Backpack's not rendering when restarting your game
* Fixed bug that would crash the game if attempting to upgrade the Large Backpack

Version 1.0.0:
Release!

If you make one, inform me through this thread and it will be posted here!

The API for this mod can be used to add Weight to your own mod items, and to make other items that modify the player's max weight. I do plan to add an easy way of creating new backpack's through json in later updates.

When first loading up the mod, a file will be created in your config directory called "WeightLimit.json" (WeightLimitServer.json on servers). This file holds all the weight values for the items in the game. If you decide to add more mods to your game after creating this file, the game will create a backup of your previous WeightLimit.json file (in case you changed the values), and create a new one with the updated items. In addition, there is a file called "WeightLimitDefaultBackup-DO-NOT-MODIFY.json" and, as it states in the name, you SHOULD NOT delete nor modify this file in anyway. If you do, there is a chance the game will not create a backup when needed.

The source of my mod can be downloaded here: Latest Version | Earlier Versions: 1.0.0, 1.0.1

 - When adding ".6" to any weight value, the numbers mess up in advanced display mode.

You may use my mod in your modpack if you meet these requirements:

1: You must give full credit to me (HappyKiller1O1) for creating this mod, and provide a link to this thread.
2: You must not EVER state you helped in the creation of this mod.
3: You must not make money of your modpack.
4: Any issues or bugs must be handled internally first. I do not want people replying to this thread about my mod not working in a mod pack because of a bug I might have fixed ages ago.

If you meet these, you may use my mod in your modpack.

You DO NOT have permission to:
Redistribute my mod on any other site, or separate post.
Copy the source of my mod, and claim it as your own.

Minecraft is a trademark of Mojang, and I am in no way affiliated with them.