What is the weight distribution ratio of a back stance in Tae Kwon Do?

There are differences in weight distribution taught among various schools and Taekwondo organizations, so this is a detailed question that only your instructor can answer for you in your school.

There are some sparring and fixed stances that position the feet and posture very similar to the back stances, yet have a 50/50 weight distribution. Thus, these would not be considered "back stances." Only when the majority of your weight is shifted to the rear foot, does it become a back stance.

Most Taekwondo Instructors will identify a back stance with having a 60/40 weight distribution. Some will go so far as to say it should be 70/30, or even 80/20. In reality, a Taekwondo fighter can shift any amount from 51% to 89% to the rear leg, and be considered in a Back Stance. Your weight should not be stagnant, nor rigid to one specific number, but able to adjust to the circumstances. However, when first learning postures, instructors will teach a specific number of their preference to get all beginner students doing the same thing before becoming proficient enough to adjust accordingly.

If you reach 90% or more of your weight on your rear foot, it is typically considered a "rear-foot" stance which is also called a "cat stance" or "tiger stance." Placing all of your weight on the rear leg makes it a "one-legged stance" or a "crane stance."

Ask your instructor what weight distribution he/she uses as a teaching tool for learning the Back Stance posture. Over time, you will discover that this is more of a guide for beginners, and a middle ground for what should be a flexible and adjustable number. The whole purpose of a Back Stance is to become rooted and grounded enough to deliver a powerful defense, yet mobile enough that you don't get caught in a fixed position. The sparring stance is slightly more flexible, mobile, and less rigid in its demand for a specific number of weight distribution. Each stance serves its purpose.