Trail Running Vest Size: How Many Liters and How to Pack

How Many Liters: What Seven Guides Say

Here’s a fact that should change how you pack: in a 2024 study, fifteen trained trail runners ran on a treadmill in a vest weighted to 5% and then 10% of their body weight. Going from 0 to 5% had a moderate effect on speed at the effort thresholds; going from 5% to 10% had a very large one. [1] So the ideal vest isn’t the biggest one your budget allows.

To figure out what size to aim for, we compared the ranges in seven guides published by brands, retailers and coaches. [2] [3] [4] [5] [6] [7] [8] They overlap more than they contradict each other.

Only one guide recommends a single do-it-all pack: 8 liters, which covers most situations. [7] Another sees 5 to 8 liters as a good compromise for up to about ten hours of effort. [3]

Why the Ranges Differ: The Race Gear List

The gap between 8 and 15 liters for ultras isn’t a math error. Salomon says it plainly: capacity depends on how long your outings are and, in races, on how far apart the aid stations sit. [2] Glisshop adds weather, terrain, altitude and the required gear list. [4]

The required gear list is the one constraint you can’t negotiate. A race rulebook like the Zugspitz Ultra Trail’s states that every participant carries everything on the list for the whole event, even without using it, with systematic or random checks and a penalty that can go as far as disqualification. [9] Keep that in mind when you shop: the vest has to swallow the whole list, not most of it.

A bigger pack isn’t a safer one: it has to hold the race gear and your food, not fill itself up. Alltricks warns that a pack with room in it will take whatever you give it, and reminds runners to carry the required gear, not extras. [6]

What a Loaded Pack Weighs, and What It Costs

Water weighs about 1 kg (2.2 lb) per liter: Alltricks allows 1 to 1.5 L of water for a 5 to 8 L pack, and 1.5 to 2 L for a 10 to 15 L pack. [6] Add the jacket, the food and the emergency blanket: in trail and ultra races, required gear, water and food add 3 to 4 kg (6.6 to 8.8 lb) of load depending on distance and climate, according to the introduction of the study cited above. [1]

For a 72 kg (159 lb) runner, the average weight of the sample, 5% is about 3.6 kg (7.9 lb) and 10% about 7.2 kg (15.9 lb). The study involved fifteen men averaging 38 years old, on a treadmill, in a weighted vest: the speed at which effort thresholds appear drops as the load rises, and not in a straight line. [1] The authors also note that the weighted vest was uncomfortable, and recommend using real trail vests in later studies. [1] These are reference points, not a rulebook.

How to Pack: Water Up Front, Heavy Low, Safety Within Reach

Packing a trail vest is a lot like organizing a kitchen: the stuff you grab constantly goes where your hands land first. Salomon puts it this way: everything should be reachable without taking the pack off, food, water, extra layer, poles. [2] Campus gives the method: rarely used gear at the bottom, the most frequent at the top; REI adds that you should always put each item in the same place, so you find it without thinking when you’re tired. [3] [10]

1. Flasks in the chest pockets: you can grab them one-handed, check them at a glance and refill them at aid stations. [2] [10]
2. Bars, gels and purees in the side pockets or within easy reach. [3] [10]
3. The rain jacket, phone and emergency blanket accessible in the mountains, because safety isn’t up for negotiation. [3]
4. Gear you probably won’t use at the bottom of the back compartment. [3]
5. The phone in a waterproof bag or case, against rain and sweat; keys in a zipped pocket with a clip. [10]
6. A pocket kept for trash, so the wrapper from the last bar doesn’t end up on the ground. [10]

One detail on weight distribution: Outnorth notes that 2 L of water on the back makes the pack feel overloaded at the rear, and the reverse at the front. [5] Tuning the order by feel is part of breaking a vest in.

Do you wear the vest over or under your jacket? Campus doesn’t pick a side: over it, you can reach the pockets; under it, the vest stays dry and warm, as long as your jacket is roomy enough to fit over it. [3]

Adjusting the Vest So It Doesn’t Bounce

The choice between a running vest and an ordinary backpack gets settled in the first mile. Salomon explains that a vest sits high on the back and hugs the body to limit movement, whereas a multi-activity backpack, more versatile, moves around more, which can end up irritating you or even causing chafing. [2]

Vest size follows chest measurement, not volume. Alltricks advises measuring your chest, checking the brand’s size chart and, if you’re torn between two sizes, taking the smaller: an oversized pack bounces with every stride and rubs. [6] Run The Wild remarks for its part that a vest too big in volume moves around, but that if you doubt the capacity, it’s better to aim a little bigger, because a race vest gains little weight from extra capacity. [7] The two pieces of advice don’t contradict each other: pick a snug size and a generous capacity.

Glisshop sums it up: a loose vest bounces, a vest that’s too tight gets in the way when you’re working hard, and tightening systems help hold it even when it’s half full. [4] REI adds that vests have several elastic fasteners to limit bounce. [10] Alltricks advises testing it loaded, with full flasks, jacket and phone, and jumping in place to spot anything that bounces, then breaking it in on a long run: never debut new gear on race day. [6]

Flasks or a Water Bladder?

Campus sees in flasks easy refilling at aid stations and an easy check on what’s left; the bladder holds up to 3 liters but adds weight on the back. [3] Run The Wild also likes soft flasks: easier to clean, less prone to leaks, and they let you alternate water and sports drink. [7] With a bladder, Salomon recommends wedging it in place to limit movement. [2]

For drinking, Campus’s benchmarks are 400 to 600 mL per hour of effort, with a third 500 mL flask at the back when it’s hot and water points are far apart. [3] For a first race, our guide to trail running for beginners adds to these benchmarks.

The right vest is the one you forget about by the first mile. The rest is the trail waiting for you.

Nolwenn Bastide

Trail running, snow, water, nature and destinations

Nolwenn covers trail running, snow, water, nature and destinations for Outfarer. Curious about everything, from meteor showers to night-train schedules, she has a knack for digging up the one detail that changes everything, and nailing it in a single sentence.

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