Concrete-filled steel tube (CFST) with circular cross-section has several advantages compared to conventional reinforced concrete or empty steel tubes because three-axial compression on the concrete-filled section increases strength and strain capacity. Many studies have been conducted on CFST, and several researchers proposed experimental and theoretical formulas to estimate confinement effect and confined compressive strength of CFT cross-section. However, the previous formulas significantly differ due to complexity and uncertainty of the three-axial stable state of concrete-filled cross-sections. In this study, fuzzy logic was used to propose an alternative method to determine confinement effect of concrete filling and axial load capacity of CFST cross-section. This study focused on accurate estimation of confinement effect in CFST using a fuzzy-based model, which allows assessing the interaction between different parameters affecting confinement effect. The proposed fuzzy-based model was compared with current design codes and the results obtained from previous studies in terms of confinement effect and axial load capacity of CFST circular cross-section. In addition, it was shown that the proposed fuzzy-based model can accurately estimate confinement effect and axial load capacity of CFST circular cross-section. Finally, the design diagram was presented to estimate confinement effect of CFST for practical implementation.