What size backpack do I need?

Choosing a backpack size? It’s about your journey. Day trip or expedition, find that perfect fit that shoulders the load and lets you stride freely forward.

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

Choosing what size backpack you need

1. What litre backpack do I need?
2. How is backpack volume measured?
3. How do I choose a backpack size?
4. How to pack a rucksack
5. Where can I get a custom designed 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 use 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 handy throughout the day, and they will prioritise being lightweight to reduce the overall weight you’re carrying. Some, like our Presta 25, are light and streamlined enough for use on the bike or for carrying an 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 a 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, as well as 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 capacity that is large enough to carry everything you need to stay self-sufficient for days. This size of the pack is often favoured for Duke of Edinburgh and Scouts expeditions, 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 regarding padding, support and adjustment. However, their larger capacities are designed to carry huge amounts of equipment or enough food to stay self-sufficient for days at a time.

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 it 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 of how much your rucksack can carry.

How do I choose a backpack size?

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.

How to pack your rucksack

Laying out all the clothing, equipment, food and water you want your rucksack to carry should give you a pretty good idea of how much space you need. We offer 123 day free returns, so you can check the pack for size at home before deciding to hit the trail. Better still, bring your kit into one of our stores and walk around with a loading pack.

Once you have settled on your size dive into more advice on packing your rucksack.

Design your own rucksack

If you need a specific size or shape, get in touch. We make custom rucksacks and luggage in our UK factory. Recent projects include an MRT medical pack.