What Size Backpack Do I Need?

By Mark Roberts

There are multiple factors which determine what size rucksack you need: length of activity, type of activity, weather conditions, the amount of safety gear you need. But there are some good general rules to make choosing easier.


Trekking in the french Alps above Chamonix with the Pacific Crest 65L rucksack

How is backpack volume measured?

Backpack capacity is usually measured in terms of litres. Every outdoor company will measure this slightly differently so there’s no guarantee that two different 35L rucksacks will have identical capacities.

At Alpkit, our litre capacity refers only to the main compartment of the rucksack. We calculate this by pouring small polystyrene balls into the rucksack and measuring how many litres of these balls it takes to fill the rucksack in a big measuring cylinder.

Even though litre capacities can be a bit misleading, brand to brand, they do give you a pretty good idea on how much your rucksack can carry.

Two people walking with Presta 25 rucksacks across the Kinder Plateau in the snow

Factors to consider

There’s a reason so many of us have cupboards overflowing with different sizes of rucksacks. The huge range of different activities we use rucksacks for, means there’s usually a perfect size for each eventuality. You may want to buy a rucksack to cover a few different uses though so here are some factors to consider:


How long will you be out for?

A short morning run, ride or hike will require far less food, water and equipment than a full day hike or even a multi-day backpacking trip. On long days you will often want to carry at least 2 litres of water, more food and probably more layers for morning and evening temperatures. A multi-day trip will require you to carry a sleeping bag, mat, stove, pot, shelter and even more food.

What type of activity will you use it for?

You usually carry less with you for high intensity activities like running and mountain biking where packing is kept to the bare minimum. Fast-packing – a mix of running and backpacking – requires a lightweight rucksack with easy access to snack and water. Climbing and mountaineering on the other hand will require lots of extra space for carrying heavy and bulky climbing equipment. For winter mountaineering you may need to fit crampons and emergency storm shelter in too.

Mountaineering in the Slovenian Alps

What weather conditions will you use it in?

It stands to reason that outdoor activities in hot and dry weather require you to carry far less than activities in cold and wet weather. You may need to carry more water in hot weather, but you can cut down on clothing in particular. You need a lot of space to fit full waterproofs, warm layers, a spare hat and gloves, or even winter safety equipment in your rucksack.


Pack Size Top Tip

Laying out all the clothing, equipment, food and water you want your rucksack to carry should give you a pretty good idea on how much space you need. Many companies offer at least a 30-day return period (we offer 123 free returns) so it’s always a good idea to do a ‘test pack’ at home before deciding to keep your rucksack.

jumping over a bog in the Peak District with the Gnarl 15 rucksack

What litre backpack do I need?

Here’s a rough guide to what each size of rucksack is best suited to:

0-15 Litre Packs

<1 Day — Fast-paced activities like running and mountain biking or shorter hikes and scrambles

These types of packs are close-fitting and streamlined to allow you to move unrestricted. They often balance weight around the body and include hydration sleeves and ports for using with hydration bladders. This capacity is big enough for a bit of water, some snacks, minimal survival kit and light, packable layers.

15-30 Litre Packs

1 Day – Packs for hiking and scrambling in warm-cool conditions, fastpacking and summit packs

These packs have enough space for carrying waterproofs, warm layers, food, water and a small amount of safety equipment for a full day out. They will often have lots of pockets to keep things you need throughout the day handy, and prioritise being lightweight to reduce the overall weight you’re carrying. Some, like our Presta 25, are light and streamlined enough for using on the bike or for carrying ultralight mountain marathon kit.

30-45 Litre Packs

1-2 Days — Day packs for year-round use (including winter) or for overnight bivvies, bothy trips and wild camps

Hiking in winter or carrying lightweight camping kit requires a larger capacity and more support. These packs are generally better padded and have more substantial hip belts. This allows you to carry extra clothing, crampons and storm shelters in winter; or a sleeping bag, sleeping mat, cooking gear and a backpacking tent/bivvy bag for overnighters in spring-autumn. 45L packs are often tailored towards climbing/mountaineering with a streamlined shape and attachments for carrying gear.

45-65 Litre Packs

2+ Days – Multi-day backpacking trips for self-sufficient adventures

Designed specifically for backpacking, these packs will have plenty of padding, support and ways of adjusting the fit for carrying heavy loads in comfort. Some, like our Pacific Crest 65, have adjustable back systems, allowing you to tailor the lumbar support and strap positions to your own back length. Multiple pockets allow you to separate your equipment for better access. These packs have a big enough capacity to carry everything you need to stay self-sufficient for days. This size of pack is often favoured for Duke of Edinburgh and Scouts expeditions, and as well as for travelling backpacks.

65 Litre + Packs

7+ days – Expedition backpacking and carrying huge amounts of equipment

Serious rucksacks for serious trips. These packs share many characteristics with backpacking rucksacks in terms of padding, support and adjustment. But their larger capacities are designed for carrying huge amounts of equipment, or enough food to stay self-sufficient for days at a time.

Walking across the Kinder plateau in the snow

A rucksack for every eventuality

Rucksack technologies have progressed substantially over the last 40 years or so, and this has led to ever more specialised packs. We try to design our rucksacks to have as much cross-over use as possible, for multiple different activities. But you’ll probably find you will still need at least two rucksacks to cover your various different needs.

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