If you’re new to thru-hiking, your first instinct might be to start with the backpack. After all, it’s the most visible piece of gear, the thing that literally carries everything else. But here’s the truth that experienced hikers wish they’d known earlier: your backpack should be the last major gear purchase you make, not the first.
This might seem counterintuitive, but by the end of this guide, you’ll understand exactly why pack sizing is far more complex than grabbing whatever looks cool at the outdoor store.
Why Pack Sizing Matters More Than You Think
Your backpack is not just a container. It’s a load-bearing system that will sit on your shoulders and hips for eight to twelve hours every single day, potentially for months. The wrong size creates a cascade of problems: chronic pain, inefficient hiking, and even injury.
But here’s the complexity: pack size isn’t just about your body measurements. It’s about the volume of gear you need to carry, which you won’t know until you’ve selected all your other equipment first.
The Two Critical Dimensions of Pack Sizing
When we talk about pack sizing, we’re actually discussing two completely different measurements that beginners often confuse.
Torso length refers to the physical fit of the pack on your body. This is measured from the C7 vertebra (that prominent bone at the base of your neck) down to the top of your hip bones. Most quality packs come in multiple torso sizes, typically small, medium, and large, corresponding to torso measurements roughly between 15 to 22 inches. This measurement determines how the shoulder straps and hip belt align with your body’s frame.
Pack capacity refers to the internal volume, measured in liters. This determines how much stuff you can physically fit inside. Common thru-hiking packs range from 40 liters on the ultralight end to 65+ liters for traditional backpackers.
Both dimensions matter enormously, but they’re determined by completely different factors.
Why Torso Length Trumps Height
Here’s a mistake nearly every beginner makes: assuming that because they’re tall, they need a large pack, or because they’re short, they need a small one. Your overall height has surprisingly little correlation with your torso length.
You might be six feet tall but have a relatively short torso with long legs. Or you might be 5’4″ with a longer torso and shorter legs. Two people of identical height can need completely different pack sizes based solely on their torso-to-leg ratio.
This is why trying on packs with weight in them at a quality outfitter is non-negotiable. A properly fitted pack should place about 80% of the weight on your hips, not your shoulders. The hip belt should sit directly on top of your hip bones, and the shoulder straps should hug your shoulders without gaps or pulling.
The Volume Question: Why You Can’t Answer It Yet
Now for the second dimension: how many liters do you actually need?
This is precisely why your backpack must be your last purchase. The volume you require is entirely dependent on the bulk and compressibility of every other item you’re carrying.
Consider two sleeping bags: a traditional synthetic bag might pack down to 12 liters, while a high-quality down quilt compresses to just 4 liters. That’s an 8-liter difference from a single item. Your sleeping pad, tent, clothing layers, cooking system, and food storage method all contribute to your total volume needs.
If you buy a 65-liter pack first, you’ll almost certainly fill it because it’s there. This is known as Parkinson’s Law applied to backpacking: gear expands to fill the space available. You’ll end up carrying more weight than necessary simply because you have room for it.
Conversely, if you buy ultralight gear that compresses well, a 40-liter pack might be perfectly adequate, even for a six-month thru-hike. But you won’t know this until you’ve laid out all your other gear.
The Base Weight Calculation
Before you can intelligently choose pack capacity, you need to calculate your base weight. This is the weight of everything in your pack excluding consumables (food, water, and fuel).
As a general guideline:
- Base weight under 10 pounds: A 40-50 liter pack is typically sufficient
- Base weight 10-20 pounds: A 50-60 liter pack is usually appropriate
- Base weight over 20 pounds: A 60-65+ liter pack may be necessary
But weight and volume don’t always correlate perfectly. Lightweight gear is often highly compressible (like down insulation), while some lightweight items are bulky (like foam sleeping pads). You need to see the actual packed volume of your gear, not just weigh it.
The Critical Test: The Kitchen Table Method
Once you’ve purchased all your other gear, perform this essential test before buying your pack. Lay everything out on your kitchen table or living room floor. Actually stuff your sleeping bag into its compression sack. Roll your clothing. Pack your tent. Arrange your food (using bags of rice or beans to simulate a week’s worth if needed).
Now measure or estimate the total volume. Be honest about how much compression is realistic for daily use on trail. Just because your sleeping bag can compress to 4 liters doesn’t mean you’ll compress it that tightly every single morning at 6 AM in the rain.
Add a buffer of about 5-10 liters beyond your measured volume. This accounts for items that don’t pack efficiently, unexpected gear additions, and the reality that you won’t achieve perfect packing every time.
Features to Consider After Size
Once you know what size you need, then and only then should you evaluate other features: hip belt pockets, stretchy side pockets, top lid versus roll-top closure, internal or external frame, and load lifter adjustments. These features matter, but they’re meaningless if the fundamental size is wrong.
Also consider whether you need a men’s or women’s specific model. Women’s packs typically feature shorter torso lengths, narrower shoulder straps, more contoured hip belts, and a different center of gravity to match typical anatomical differences.
When to Seek Professional Fitting
Don’t rely on online sizing charts alone for your first backpack. Visit an outfitter with knowledgeable staff who can properly measure your torso, load test packs with 20-30 pounds of weight, and watch how you move with the pack on.
A good fitter will examine how the hip belt sits, whether the shoulder straps gap, how the load lifters angle, and whether the pack’s length matches your torso. They’ll make micro-adjustments to straps and ensure the sternum strap hits at the right height (typically between your armpits and collarbone).
This professional fitting typically costs nothing beyond purchasing the pack itself, but the value in preventing months of discomfort is immeasurable.
The Bottom Line
Your backpack is simultaneously the most important and most likely the last piece of gear you should choose. It must fit your body precisely and accommodate your gear’s volume exactly. Neither dimension can be accurately determined until you’ve made nearly every other gear decision.
Start with your shelter, sleep system, clothing, and cooking setup. Calculate your base weight and measure your actual gear volume. Only then can you intelligently select a pack that will carry you comfortably through hundreds or thousands of miles.
The right pack size is the one that fits your body and your gear, not the one that looks impressive or carries the most. By making your backpack decision last, you ensure that every mile on trail is carried in comfort rather than regretted in pain.
Your thru-hike begins long before the trailhead, in these careful gear decisions. Take the time to get your pack sizing right, and your body will thank you every single day on trail.