The common mistake most people make when buying a backpack is buying something way too big that doesn't fit! Anything above 60 liters it's usually overkill because the larger the backpack the more irrelevant stuff you are likely to pack, carrying extra baggage with you, which is never fun!
If you're an "ultralight" type of person, the Thule Stir 20 liter backpack will be right down your alley. The Stir 20 liter backpack is a minimalist, efficient pack that weighs just 570 grams, yet it can pack way above its weight.
The outershell is made of ripstop nylon, while the inner is made of a thinner denier nylon (70D). There are three main types of ripstop nylon available, including Robic, Dyneema X Gridstop and X-Pac. The Thule Stir 20L backpack uses 210D Robic with PU/DWR coating that makes it more hard wearing and water resistant than nylon 6 fabric.
The mouth opening of this side pocket is small (14cm long and 9cm wide), although the compartment is 20cm deep and very spacious for storing items that you need quick access to like a phone, house keys, a mini folding umbrella or even a powerbank connected to a cable (for charging external devices). Inside this compartment, there is also a non-detachable 7cm long fabric cord that is attached to a plastic keyring.
The Thule Stir 20L backpack has two water bottle side pockets that you can also use to store other things like a small tripod. The side pockets have good sizing in proportion with the size of the bag.
The integration of elastic bands for the side pockets is neat because it helps compress things down so, they don't come out accidentally from the side pockets. It really is remarkable how the Stir 20 liter backpack can pack almost as much as the bulkier 27 liter Paramount backpack.
The Thule Stir 20L backpack has several webbing loops sewn on to various parts of the backpack for daisy chaining various items such as a cycling helmet. There are 9 daisy chains on the front and 4 daisy chains on the backside. Two of those rear webbing loops are designed for looping the 1-meter long waist strap, which is detachable. The longest front-facing daisy chain contains 6 webbing loops sewn onto the same strip, alongside a thin webbing grab handle. There is a second grab handle located on the opposite side.
More daisy chains strips can be found on the front of the shoulder straps. There are 6 elastic webbing loops (three per shoulder strap). Further down each shoulder strap, there is also a daisy chain strip with 4 fabric webbing loops, which are primarily designed for attaching the sternum (chest) strap at different heights. The chest strap measures 14cm long and also integrates elastic retention bands for managing excess webbing.
The left shoulder strap integrates a small zip opening with a large mesh pouch that is deep enough to store small bulky things like an apple or a pair of true wireless earbuds. The Thule Stir 20L backpack uses quick release D-Flex plastic buckles, as well as metal sliders with fabric cord pull tabs.
The Thule Stir 20L backpack has several webbing loops sewn on to various parts of the backpack for daisy chaining various items such as a cycling helmet. There are 9 daisy chains on the front and 4 daisy chains on the backside. Two of those rear webbing loops are designed for looping the 1-meter long waist strap, which is detachable. The longest front-facing daisy chain contains 6 webbing loops sewn onto the same strip, alongside a thin webbing grab handle. There is a second grab handle located on the opposite side.More daisy chains strips can be found on the front of the shoulder straps. There are 6 elastic webbing loops (three per shoulder strap). Further down each shoulder strap, there is also a daisy chain strip with 4 fabric webbing loops, which are primarily designed for attaching the sternum (chest) strap at different heights. The chest strap measures 14cm long and also integrates elastic retention bands for managing excess webbing. 
Most of the components of the bag are made of plastic aside from the metal G-hook that is attached to the hood of the backpack. The G-hook can be adjusted, thanks to a five-loop daisy chain strip, which allows you to keep the top of the backpack securely compacted. Just above this five-loop daisy chain strip, there is a smaller grab handle. There is a total of 24 webbing loops of various sizes, which is an incredible amount given the small size of the Stir 20L backpack! Well, as the saying goes "the more, the merrier"!
The Thule Stir 20L backpack uses an expandable cuff drawstring mechanism to fasten the main compartment. The drawstring mechanism uses an unconventional cordlock that works differently to the standard barrel cord lock spring toggle.
Instead of pressing down on the cordlock to cinch and loosen the cord, you have to pull the cordlock apart. It looks complicated at first but it actually works just like a standard drawstring cord lock. Not all top loading backpacks have this "cuff neck" at the top, which provides a useful expansion in height and volume capacity. If you do want a proper expandable backpack though, check out the Enroute Escort 2 backpack.
Reinforcing the backside of the Stir 20L backpack, it's a semi-rigid hard plastic panel that has been layered with a thin layer of soft foam padding covered with mesh. The foam padding has an 8-shape design with small cutouts throughout, which are mostly spread across the top and middle section and provide excellent backside ventilation. The 8-shape foam padding measures 22.5cm wide at the top and 18cm wide at the bottom. The height of the back panel is 45cm long so, it will fit perfectly if your torso is similarly sized.
The shoulder straps have springy foam padding, which is perforated all the way through, making the should straps extremely breathable and covered with mesh. The shoulder straps sit comfortable on the shoulders, thanks to their long width at the top (7cm). The shoulder straps have an S-shape and become progressively narrower towards the bottom end. The shoulder straps have good separation in between to accommodate broad shoulders and prevent rubbing against the sides of the neck.
There is a passthrough opening on the top backside of the backpack too for attaching a drinking tube to a hydration bladder that you can store inside the large elastic inner panel pouch. You can buy the Thule Stir 20L backpack from amazon or Thule. Check out the Thule Chasm backpack.
Reinforcing the backside of the Stir 20L backpack, it's a semi-rigid hard plastic panel that has been layered with a thin layer of soft foam padding covered with mesh. The foam padding has an 8-shape design with small cutouts throughout, which are mostly spread across the top and middle section and provide excellent backside ventilation. The 8-shape foam padding measures 22.5cm wide at the top and 18cm wide at the bottom. The height of the back panel is 45cm long so, it will fit perfectly if your torso is similarly sized.The shoulder straps have springy foam padding, which is perforated all the way through, making the should straps extremely breathable and covered with mesh. The shoulder straps sit comfortable on the shoulders, thanks to their long width at the top (7cm). The shoulder straps have an S-shape and become progressively narrower towards the bottom end. The shoulder straps have good separation in between to accommodate broad shoulders and prevent rubbing against the sides of the neck.There is a passthrough opening on the top backside of the backpack too for attaching a drinking tube to a hydration bladder that you can store inside the large elastic inner panel pouch. You can buy the Thule Stir 20L backpack from amazon or Thule. Check out the Thule Chasm backpack.