There are three categories when it comes to squat depth:
It’s anywhere from a legal powerlifting squat to “ass to grass” or ATG. A legal powerlifting squat is when the hip joint drops below the knee joint. The upper thigh should be slightly below parallel.
The upper thigh is parallel to the floor. The knees are at 80-85 degrees.
Anywhere from a 90-degree knee angle up to around 110 degrees.
Which is “correct”? That depends on your goal.
Depth and Muscle Size
For hypertrophy, a full squat is slightly more effective than a half squat and significantly more effective than a partial squat.
Muscle damage is one of the main stimuli for growth. It occurs when you lengthen a muscle fiber that’s producing force/tension. By using a greater range of motion, you’re causing more muscle damage by lengthening the muscle’s fibers more and doing it while they’re producing a high level of tension.
But a half squat can absolutely cause growth. If that’s all you can do, you can build your legs just fine. However, partial squats aren’t as effective as either previous range when your goal is building muscle.
Depth and Strength
You can use all three ranges. The shorter the range, the more weight you can use. The greater overload you create, the more strength you can build.
However, strength is normally gained in the range being trained. So if your goal is to get stronger on the full-range back squat, focus mostly on training the full range. Although, squatting a bit higher (parallel) might help you get stronger in the full range by allowing you to use 5-10 percent more weight. It’s close enough to have good strength transfer.
But not everyone needs to be strong in the full range. Sprinters and throwers, for example, often stick to half and even partial squats because their knee angle never goes below 90-degrees in their sport. Sure, a full squat should be used to develop their muscle mass, but once they have enough muscle for their sport, they can focus on getting stronger in the zone they require.
Not surprisingly, a partial squat leads to a greater improvement in jumping performance than a full squat because you overload the specific range of motion you need.
The depth you’re using depends on your objective. No depth is inherently wrong.