It really depends on what you mean by center.
If you are referring to your center of gravity then it is generally in between the top of your pelvis and the bottom of your rib cage. This will vary slightly with different body types. Some people carry their weight/center of gravity higher or lower than others. This is generally speaking and implying that you are doing solo practice.
The center of gravity can also shift around the body depending on how you are engaged with another person. If standing it will remain close to the area specified above, however, the center can shift to other parts of the body if you are throwing or grappling.