Abstract Theoretical formulations are provided for the determination of stiffness of composite tubes. A three-dimensional laminate theory is used to determine the equivalent flexural stiffness 〈EI〉 for composite tubes. The same theory is also used to determine the load versus axial strain of the tubes. An approximate (more simplified) formulation is also presented. Values of the equivalent bending stiffness 〈EI〉 are compared between the two formulations. Experimental work was carried out on four composite tubes made of different lay-ups. The stiffness represented by the slope of the force-axial strains compares well between theoretical formulation and experimental results.