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.
Choosing what size backpack you 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.
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.
There are three key factors to consider when choosing a backpack size. Although, 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 morning’s walk will require far less food, water and equipment than a full day hike or even a multi-day backpacking trip.
What type of activity will you use it for? You’ll need more space for more activities like winter mountaineering than just for a run.
What weather conditions will you use it in? You may need to carry more water in hot weather, but you can cut down on the kit you’d need in winter.
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.