When I measure the diameter is seems to be between 20 an 21mm
description from Zero Sports:
Zero/Sports Sway Bars will drastically reduce body roll without decreasing ride comfort. The Zero/Sports Sway Bars feature a unique lightweight tubular design. The hollow tube design gives the bars a stiffer response to twisting forces than a similar solid sway bar. These easy to install sway bars will improve handling, steering wheel response, and will reduce your cars natural tendency to under steer in corners.
If you use a solid core structure stabilizer, we insist you try our product, you will definitely feel the difference!
For maximum performance, we suggest using the front and rear as a set!