Choosing the right backpack size can be very simple in an outdoor adventure gear store serious about their backpacking equipment.
They usually have staff available who are experts in fitting backpacks and can set you up with the perfect pack pronto.
But you can't always count on that.
So your best bet is to know exactly what size pack you'll be looking for before you even hit the shops.
Now, if you haven't already measured your torso length and hip size you'll need to do that first.
Go to the page on how to fit a backpack, take your measurements and then pick up where you left off here.
Some manufacturers' sizing guides might be a bit different from this one and not all backpacks come in all sizes. But in general, your best backpack fit will be:
If you're right on the border of one backpack size or another, try both backpacks on and choose the one that fits and feels better on your back.
It gets a bit tricky here because hip belt sizes vary a lot more from manufacturer to manufacturer than torso sizes. Also, depending on the make and model of your backpack, men's and women's hip belts might be sized differently.
But generally, hip belt sizes are:
If you're in between two overlapping sizes, try both belts on and go for the one that sits more comfortably on your hips.
Backpack volume is measured in litres or cubic inches. In around-the-world-for-a-year terms:
Before deciding what backpack size to buy, you need to have some idea as to how long you'll be travelling, where you plan to go and what you plan to do when you get there.
That way you'll know roughly what gear you'll be bringing with you and what size pack you'll probably need to fit it all in.
Take my word for it though. Chances are you'll manage to fill your backpack no matter what size you buy, so try to get the absolute smallest one you think you can live with.
If you want to travel ultra light, a 35- to 45-litre backpack will force you to cut your gear down to the bare bones minimum.
In that case, head over to the packing tips and packing list pages for ideas on how to get ruthless with your stuff.
But you don't need to overdo it. If you're attaching bits and pieces of gear to the outside of your pack before you even step out your front door, you know it's not going to work.
Depending on your packing skills and travel experience, 50 to 55 litres is probably a more realistic backpack size to aim for.
And a 60- to 65-litre pack should really be more than enough for most people on a classic one-year around the world backpacking trip.
Still, you might decide that you really do need a bigger pack. Maybe because you're bringing camping equipment along or you plan to stay in one place for a while and look for work.
That's perfectly okay.
The main thing is to be comfortable with the backpack size you choose and not to injure yourself (or others!) while carrying it.