Ultralight as a philosophy has grown over time due to the availability of materials which lends to a better hiking experience. The ability to stop at places on the route to resupply has also increased the ability to look deeper into an ultralight travel methodology.
Ultralight backpacking base weight is when your backpack weight in under 10 lbs without food and water counted in the total. This weight class helps give thru-hikers and long-distance hikers, in general, less wear on their muscles and joints.
There are other things that can help you lower your base weight and get you in that "ultralight" category. Worst case these are methods you can start instituting to start a progression in knowledge and gear towards getting to lighter pack weights and learning the amount of enjoyment it provides!
Trail weight is everything in your pack including food and water. The average food carried is 2 pounds per day in-between your resupply points. Your weight in water is measured based on the liters you carry, one liter will weigh 2.2 pounds.
This is why as an ultralight backpacker you want to weigh the distance you are hiking with the food and water weight to make it through the current section. Many times this will be less water weight as you can bring a filter to get additional water when you reach additional available places along the trail.
The best way to lower your base weight is to shakedown your gear and remove the unessential items as your first priority as this has zero incurred costs. This process of completing shakedowns should be done as trial shorter hikes meant to get out and evaluate your gear and needs in the field.
The next method will help you drop your weight the fastest, this method is to aim at purchasing replacement gear for your big three gear. The big three are your tent, backpack and sleeping bag/pad. These will typically amount to 1/3 to 1/2 your base pack weight and are the simplest way to dramatically lower weight efficiently.
A gear shakedown is when you fill and pack your backpack like you would for your long-distance hike. The point of this is to go take a hike from a day to a week and validate the gear in your pack is necessary.
Many times you will believe that the items you pack are invaluable and necessary for your thru-hike. What you want to do is look to eliminate these extra items from your pack, please don't drop safety items without a decent amount of real validation to need.
What you are looking to learn is that most thru-hikes are really a series of 5-10 day hikes with resupply points in-between. Much of the items you pack will be something you can get down the trail if you find a need, you are aiming to remove as much of this useless crud as possible at the start of your hike.
This is more than likely a large point of focus in getting your base weight down. What you should be looking for is a tent which should weigh no more than 2 pounds, though lighter options are available the cost will tend to rise in parallel to weight drop.
This is why many in ultralight backpacking will start to look into alternative sleep systems, for example, hammock or tarps. Hammocks and tarps have their drawbacks and issues but depending on hiking could be a good fit.
Hammocks, for example, require trees or similar methods to support hanging correctly to get them off the ground. Tarps meanwhile, have open sides and front typically leaving you exposed to critters and wildlife. Both can drop your base weight, but you need to find the right tool for the trip.
For your backpack itself, you would like to aim for under 2 pounds as this allows you to almost halve the weight of a typical hiking backpack. This will mean much less on your shoulders and hips over the course of months which should help you thrive in a thru-hike.
Make sure when finding the backpack you want to take that it has all the pockets and extras you need. There are many very lightweight backpacks that eschew these outer pockets and strap areas that may impact you carrying all the gear on the easiest way possible.
When looking into sleeping systems most people are familiar with sleeping bags but sleeping quilts are growing in popularity sdo to being open on one side and allowing for better airflow. I have always used mummy bags and similar sleeping bags, although lately, I have been looking for a quilt that won't bust the bank.
Sleeping pads though come in many different forms depending on the need you are trying to fill. If you are going to be in the snow and ice where insulation value is of more a concern then aim for the high R-Value of this pad. If more of the hike is from summer into fall and winter weather isn't as large of a concern then you can seriously dump weight by looking at the Uberlite.
When looking to start hiking on a long distance you want to lose as much excess weight as you can without impacting your personal safety. Many times this won't be done ahead of the hike and only happens after the first section is completed and you find a list of things you don't want to carry any longer.
Instead, I would recommend you start this process well before you begin your thru-hike as a means to be better prepared for your thru-hike. Doing short hikes and dropping useless gear will help you know your limits and the limits of your gear along with finding any needs you may have forgotten prior.
Prior to your hike is also the best time to replace any heavy gear with lighter alternatives as once you are out on the trail you will be limited to choices in the shops along the route and any imposed price hikes that come with it.