Fit is an important factor when selecting the best pack for you. A hydration pack that fits well is more than just a matter of comfort; it's a performance enhancer. An improperly fitting pack might rub and chafe, causing skin irritation as well as soreness and all-around discomfort. Like any article of clothing, the fit of a hydration vest is personal, and you may find it worthwhile to compare a couple of choices before you pick your perfect pack.
How Should a Running Vest Fit?
Snug Fit: A well-fitting hydration pack will hug your body snugly without constricting arm movement or breathing. Regardless of your pack design, make sure it sits securely so that it doesn't bounce or rub while you're running.
No Rubbing: Pay attention to potential rubbing points. If any part of the pack consistently rubs against your skin, it might lead to discomfort during longer runs.
Arm Mobility: A well-fitting pack should allow free arm movement. Straps that dig into your underarms or restrict your swing can lead to fatigue and hinder your natural running motion. Test the pack by moving your arms in a running motion. Your arms should move freely without any restriction or discomfort.
Breathing: The pack shouldn't constrict your chest or restrict your breathing. Proper ventilation and a fit that accommodates deep breaths are essential for maintaining your endurance during long runs. Take deep breaths; the pack should expand and contract with your chest without restricting your lung capacity.
Stability: Regardless of design, a properly fitting pack should stay securely in place without bouncing excessively. This stability prevents friction and rubbing against your skin. Jump or jog in place to test the stability. The pack should remain in place without excessive bouncing.

Sizing and Gender-Specific Packs
When deciding what size running vest you should get, it is also important to understand the different sizing options that are available. While some may be fine with a one-size-fits-all pack, those with larger shoulders, shorter torsos, or feminine bodies may benefit from specific pack sizing. Once you understand the sizing options, you can then use the desired pack's sizing charts to identify the best size for you.
Universal Sizing: Some packs offer universal sizing, which means they are adjustable to fit a wide range of body shapes. While these packs provide versatility, they may not offer the tailored comfort of size-specific options.
Size-Specific Packs: Many hydration packs come in different sizes, similar to clothing. These options provide a more precise fit, reducing the chances of discomfort caused by excess material or improper adjustment.
Gender-Specific Packs: Some brands offer packs designed with gender-specific anatomical differences in mind. These packs consider chest and torso dimensions to provide a snug, supportive fit.