It's good to have options: The Rucker comes in 20L and 25L. The 20L is the best for most people and allows you to shoulder carry, which keeps the weight high and stable on your back. The 25L sits lower because of it's length allows you to distribute weight throughout your hips, especially when paired with our padded hip belt. It also comes with two side handles, making it a ruck PT machine. Also, consider your body size when choosing, but when in doubt go with the 20L because it's the most versa