Waterproof rating measures how much water pressure a fabric can handle before leaking, shown in millimeters (mm). Think of it as a tall column of water sitting on the fabric—5,000mm means it can withstand a 5-meter column before water pushes through. Higher numbers mean better protection.
For most resort skiers and riders, 10,000–15,000mm handles typical conditions just fine. If you're in the Pacific Northwest or other wet, coastal climates where you're skiing in heavy rain or wet snow, look for 15,000–20,000mm. Anything over 20,000mm is really for extreme conditions or professional use.
Don't just chase the highest number, though. Seam sealing and DWR (durable water repellent) coating matter just as much for staying dry. A 10,000mm pant with fully taped seams and fresh DWR will outperform a 20,000mm pant with poor construction. Also, if you ski in dry, interior climates like Utah or Colorado, you're probably fine on the lower end—no need to over-purchase.