In Viet Nam, there are quite a lot of standard, documents and textbooks on Steel Structures [1, 3, 4, 5, 6, 7, 8]..., however, calculating construction site joints uses circular cap plate and bolts to Connecting steel tube members is only mentioned in the textbook Steel Structures for Civil and Industrial Construction [2]. In particular, calculating circular cap plate thickness is determined based on looking up the chart, this method is quite manual and inconvenient. Therefore, it is necessary for the author to establish a concise formula to determine circular cap plate thickness. Therefore, this paper has presented circular cap plate connections, proposed a formula, established look-up tables to determine circular flange thickness according to that formula and compared with the results in the chart; then present conclusions and recommendations.