There are many things that decide what size ski you should get. As much as the consumer would like to see an equation that answers this question precisely, there is no truely right answer.
With Junior skis, we have a saying, "A ski does not know how tall you are, just how heavy you are by standing on it." We use this expression a lot fitting your kids. The weight of your child is necessary to figure out what ski is going to flex appropriately for them. In extreme circumstances, if a ski is visibly too tall or short, we will size down/up appropriately. For the most part, perception is half the battle. A ski may appear slightly taller than the last ski they had, but the weight is the crucial factor.
Lets move on to adults...
There are many more factors that come into play when sizing for adults.
All skis are not the same. You will enjoy your skis more if you pick a ski that is specific to the terrain you ski the most. Which of these sounds most like you?
* All Mountain Skis - The mountain is your playground. From corduroy to chutes, from backcountry to bumps, nothing is left untouched.
* Park and Pipe Skis - Nothing like a 50 foot kicker to get the heart going. The rest of the mountain looks fun, but you spend your time working on your skills in the park.
* Powder Skis - The white room. You spend your time seeking the deepest, freshest, fluffiest pow lines. waking up at 3am to catch first tracks and chasing the storms is a regular occurrence.
* Carving Skis - nothing like untouched machine groomed corduroy to start your day. No need for the trees and bumps, you spend your time working on the perfect turns day in and day out.
Pick a ski that accurately matches your ability level. A beginner skier generally will find it to be a lot of work pressuring an expert ski. On the flip side, an expert skier will overpower a beginner ski.
The number of days you put in on the mountain will directly relate to how fast you improve. If you are an intermediate skier who plans to ski 30/40 days a year, you might want to purchase a ski a level up, so you aren't limited by a lesser ski.
After asking yourself these questions, and determining where you ski, how often you ski, and what level skier you are, we can make a general statement with the following:
Reasons to size your skis shorter, closer to your chin:
* You are a beginner or intermediate skier
* Your weight is lighter than average for your height
* You like to make short, quick turns
Reasons to size your skis longer, closer to the top of your head:
* You are skiing fast and aggressively
* You weigh more than average for your height
* You plan to do the majority of your skiing off the trail
Twin tip skis are sized a little differently. We tend to suggest going close to head high, because the effective edge of the ski is shorter, due to the turned up tail. Perception is half the battle here. Just because the ski appears tall, does not mean it will feel that way skiing.
Rockered skis follow a similar story. There are many different styles of rockered skis, from minimal tip rocker, to full reverse camber. SIZE YOUR SKIS LONGER! A rockered ski will feel like it skis very short. Do not get the right ski and realize you got the wrong size because you were unsure of length. Generally, if you are getting a rockered ski, it is common to go about +5 to +10cm taller than your preferred downhill ski.
I hope this cleared up some of your questions. As always, it comes down to your level of comfort.
-Schmitty @ Ski Haus