Most ski poles have a shaft diameter of 16–18mm at the grip that tapers down to 12–14mm at the tip. This tapering gives you stiffness where you need it (near your hands) while keeping swing weight low at the tip, so your poles feel lighter and more responsive as you plant them turn after turn.
For most resort skiers, shaft diameter isn't something to stress over—it's rarely a primary buying factor. Where it does matter is if you're racing, where more aggressive tapering reduces aerodynamic drag, or if you're a weight-conscious backcountry skier counting every ounce.
One common mistake is assuming a thicker shaft means a stronger pole. The material (aluminum vs. carbon composite, for example) matters far more than diameter when it comes to durability. Focus on material, length, and grip comfort first, and consider shaft diameter only if you're fine-tuning for racing performance or minimizing swing weight.