Backpacks by the Numbers: Sizing and Buying Guide 
small husky in a backpackby Lisa Maloney from Thought.co

Hiking packs are sized in two ways: The internal volume (how much it can carry — given in cubic inches or liters) and the torso length (or range of torso lengths) the pack is designed to fit.

Leave the random the trial and error to Goldilocks, though — you can shortcut straight to the best backpacks by learning how to measure your torso length and estimate how much carrying capacity you need for a given trip.

HOW TO MEASURE YOUR TORSO LENGTH FOR A BACKPACK

To measure your torso length, you’re going to need a flexible measuring tape and some help from a friend. Ask your friend to measure from the prominent bump at the base of your neck down to a point between the crests of your hipbones. The resulting measurement is the torso length you’ll shop for when sizing backpacks.

(Find the point between your iliac crests — that is, the high points of your hipbones — by putting your palms on your hips, index fingers across the crests of your hipbones. Keep your thumbs horizontal, pointing straight across your hips toward your spine. Your friend can then measure down to the point between your thumbs.)

HOW MUCH CARRYING CAPACITY DO I NEED?

All manufacturers designate pack size in terms of liters or cubic inches (usually both); but what does this actually mean? The following list is meant to give you a ballpark idea of what size pack you need for what type of hike.

Of course, your ideal pack size is going to vary according to your packing habits. A hardcore ultralight backpacker can fit several days’ worth of gear into what’d be a large daypack for anyone else. Your typical hiker, however, might not have the budget or know-how for ultra-small, ultra-lighter, ultra-expensive gear — which means a larger pack to make room for larger gear.

If in doubt, put your typical hiking load into a few stuff sacks (so it’s easy to handle) and haul it down to your local gear store to test-pack a few packs. (Most gear stores will have sandbags you can fill the pack with to test how it feels underweight — but this won’t address the size issue.)

DAY HIKING: 500 TO 1,800 CUBIC INCHES (8 TO 30 LITERS)

To put these numbers into perspective: A 500-cubic-inch pack works out to about the amount of packing volume in a kindergartener’s backpack, which usually isn’t small enough to hold a full-size folder or binder. When packed to the gills, a 500-cubic-inch pack can just hold a smallish (2L or smaller) hydration bladder; a super-lightweight insulating layer (top and bottom); lightweight/packable wind jacket and pants, light hat and gloves, quart-size emergency kitice grippers, and a couple of snack bars. I usually end up carrying my camera in my pocket.

I love this sort of pack for short day hikes in mild weather — but if conditions aren’t great or you don’t have your packing system dialed in just yet, I’d recommend a larger pack. (I appreciate having a 1,000- to 1,700-cubic-inch pack, depending on the weather and length of the hike I’m tackling.)

OVERNIGHT HIKES: 2,400 TO 3,000 CUBIC INCHES (40 TO 50 LITERS)

I try not to festoon my pack with lots small items dangling off the outside — but strapping large items like your sleeping pad or bag to the top or bottom of your pack is an easy way to make a (relatively) small backpack work for overnight hikes. Bump up to a larger size if you’re hiking in winter or won’t be camping near a water source, so you can carry extra layers and water respectively.

And of course if you have the money to invest in ultralight gear (or the know-how to create your own), you can get by with a much smaller and lighter pack.

MULTI-NIGHT BACKPACKING TRIPS: 3,000 CUBIC INCHES (50 LITERS) AND UP

A multi-night trip requires all the staples of an overnighter.

Your tent, sleeping pad, sleeping bag and cooking system (including stove, pot(s), utensil(s) and fuel) are the bulkiest items. Because you’ll be out for longer, you’ll need more of anything that can be consumed or used up in any way: more fuel, more food, and potentially more layers of clothing.

Other good reasons to opt for a larger pack include if you’re packing gear for more than one person, experiencing unpredictable weather, hiking with kids or pets, or maybe it just plain fits your style better. Remember, there’s no right or wrong when it comes to this — there’s only what works for you.

Some larger packs — especially once you get into this expedition-size range — offer top lids that detach and convert into lumbar packs for day hiking. This is a great feature, especially if you’re going to set up a base camp in the mountains and then explore from there.

A WORD ABOUT COMPRESSION STRAPS AND EXTERNAL ATTACHMENTS

Compression straps are great. They keep partial loads from flopping around in large packs and help keep fully-loaded small packs at a manageable size that can ride close to your body. In a pinch, you can also use compression straps to strap extra items to the outside of your pack.

Other external attachment points can come in handy. I’d look for a pack that has two attachments for your trekking poles, should you decide to carry them. External bungees come in handy for holding a spare outer layer; side pockets are great. I’m not terribly crazy about daisy chains on a hiking pack.

If you plan to do a lot of winter hiking, look for a pack that also has quick-access pockets for essential avalanche survival gear like a probe and shovel.

A FEW MORE TIPS ON CHOOSING A PACK

  • DO dress like you’re going for a hike when you go to the store. If your pack’s hip belt is going to pinch or chafe when paired with your favorite hiking pants, you want to know in advance.
  • DO fasten the hip belt around your hipbones, not your waist.
  • DO take the salesperson upon his or her offer to load the pack with sandbags and let you walk around with the loaded pack for a while.
  • DO pay attention to where you feel the pack’s weight — the vast majority of it should rest on your hips, not your shoulders.
  • DO use the pack’s sternum strap to help shift weight off your shoulders (by drawing the straps together slightly).
  • DON’T be shy about asking for help with the pack’s suspension system. It should keep the pack balanced and close to your body.
  • DO check for enough quick-access, exterior pockets to store things you want ready access to Bear spray, water bottles, your camera, and so on.
  • DO feel free to shop for “climbing” packs too — they’re often sturdily built and light-weight.
  • DO try on at least several packs before you buy.
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