When it comes to choosing the size of your backpack, you'll need to have in mind how long you'll be on your feet and what kit you'll need to carry considering your environment. If racing, you'll need to consider the length between each water/feed station and if the race organiser has set a mandatory kit list.
If you need to be self-sufficient for:
Up to 20k to 40k: 5 Litres
Decathlon's range of 5L packs come with a 1L water bladder. With six pockets there's also enough space to fit essentials such as: 150ml soft flask, packable jacket, keys, phone, gels, hat, gloves and a survival blanket.
40k to 80k: 10 Litres
The 10L capacity packs also come with a 1L water bladder but has a total of 10 pockets to help carry your essentials. The front six pockets enable you to carry: 2 x 500ml soft flasks, gels, bars and a collapsible cup. The rear pockets come in handy for clothing, head-torch and valuables such as keys & smartphone. There are also 2 elastic straps on either side of the bag also allow you to carry your 3-section running poles.
80k+: 15 Litres
The 15l backpack is the largest in the range, with a 2L water bladder, elastic pole straps and a total of 15 pockets. This pack really is designed for the ultra runner looking to carry the mandatory kit needed for such distances.
Light and fast up to 20 to 40k: 5L Vest
The 5L vest is a definite firm favorite with me as I like to carry as little weight as possible. Despite it's size it can fit a surprising amount of kit. In terms of hydration there a 2 pockets at the front for 2x500ml soft flasks. There are a further ten pockets that allow you to comfortably carry: packable jacket, keys, phone, gels, hat, gloves, head-torch, collapsible cup and survival blanket. Want to carry collapsible poles with you? No problem, you can attach a quiver to the back of the vest.
All of Decathlon's running backpacks have been tested and approved by both Men and Women. Not only that, apart from the 5L backpack, all of the packs come in three different sizes:
XS/S - 24 to 28 Inch (Measurement of waist at the belly button)
M/L - 28 to 32 Inch
XL - 33 inch 38 Inch
The 5L vest, 10L and 15L backpacks also have three adjustable chest straps to better adapt to the users shape.