20–35L · Short tripsFor day hikes or very compact gear.35–50L · Lightweight trekkingFor several days with optimized gear.50L+ · More gearFor longer trips or more gear.Frameless · Maximum lightnessFor light loads and highly optimized gear.With a frame · More comfort under loadBetter if you're carrying more weight or on longer routes.
A frameless backpack prioritizes lightness and is designed for light loads, usually up to 5–8 kg in total weight. In this type of backpack, the gear itself (such as a sleeping pad or proper internal organization) provides the necessary rigidity. They work best when the load is compact and well-distributed.
A backpack with a frame incorporates a structural system (stays, frame, or plate) that improves weight transfer to the hips, increasing stability and comfort with medium loads, generally between 9 and 15 kg, depending on the model. In return, it adds a bit more weight to the backpack.
The choice depends on the total weight you carry, the level of compactness of your gear, and the balance you seek between lightness and comfort on long routes.
Most ultralight backpacks are water-resistant but not completely waterproof. While some fabrics repel rain well, seams, zippers, and connection points still remain critical areas for leaks, especially in prolonged rain.
Materials such as Dyneema Composite Fabric (DCF), Ultra™, or X-Pac offer high water resistance and absorb very little moisture, whereas fabrics like nylon or Robic® can get soaked over time if they lack coatings. In all cases, total waterproofing depends more on the design and construction than on the fabric alone. Here, UL models have an advantage.
For secure protection of gear, using dry bags or waterproof liners remains the most effective method, providing real waterproofing with minimal impact on weight—just a few grams.
In ultralight backpacks, the most effective option is usually an interior waterproof bag (nylofume). By directly protecting critical material, it offers more reliable waterproofing with less weight and without relying on the external fit.
Pack covers can reduce direct rain entry, but they are often less effective in the wind, can shift while walking, and do not completely protect areas like the back or the bottom.
For routes with frequent or prolonged rain, a waterproof liner combined with waterproof bags for sensitive items (sleeping bag, dry clothes) provides the best protection with the least impact on weight and system simplicity.
More than focusing on the declared liters, the important thing is the actual volume of your system: shelter, insulation, kitchen, and food. Ultralight backpacks work best when the gear is compact and compressible. If your gear takes up too much volume, the backpack will be strained, lose stability, and be less comfortable, even if the weight is within the recommended range. One way to determine this is by using trash bags of different sizes to know the actual capacity of the gear.
On technical, uneven, or steep terrain, load stability is key. A well-adjusted backpack with a balanced weight distribution reduces swaying and improves body control, especially on descents.
On smoother routes, like the Camino De Santiago, extreme lightness can be prioritized. The more demanding the terrain, the more important it is to have a backpack that keeps the load stable and close to the center of gravity.
The change usually makes sense when the base weight of the gear is optimized, and the goal is to improve efficiency and reduce fatigue on long routes. Switching to an ultralight backpack without first reducing the volume and weight of the gear often leads to frustration. Let's say that the backpack is one of the last things to change.
When the complete system is balanced, a UL backpack allows for more comfortable walking with less accumulated weight day after day.
Backpacks with adjustable sizing incorporate additional mechanical systems (rails, movable panels, or adjustable anchors) to accommodate different torso lengths. This adds weight, complexity, and rigidity to the setup.
In ultralight philosophy, it is common to choose the correct size from the start and eliminate unnecessary adjustment mechanisms. Remove what is superfluous. With low loads and optimized gear, a well-sized backpack efficiently transfers weight to the hip belt without the need for adjustable systems.
Adjustable sizing makes sense for backpacks intended for heavier loads or general use, but it is not typical in UL backpacks, where simplicity, lightness, and efficiency of the complete system are prioritized.
