Research has been undertaken to characterise the in-plane and out-of-plane stiffness and strength properties of paperboards to enable data for use in determining constitutive parameters needed in finite element simulations. Paperboards with different bending stiffness were analysed, using five folding box boards of varying grammage (235, 255, 270, 315 and 340gsm). The stiffness and strength properties were determined at different relative humidity (RH), namely, 20, 50, 70 and 90%. As well as in-plane tensile test and out-of-plane tensile test, the short-span compression test (SCT) was carried out, together with bending stiffness test and double-notch shear test. The results revealed a linear relation between mechanical properties and moisture ratio for each paperboard. When the data were normalised with data for the standard climate (50% RH) and investigated as a function of moisture ratio, it was determined that the normalised mechanical properties for all paperboards coincided along one single line. Thus they could be expressed as a linear function of moisture ratio and two constants. It has been concluded that by knowing the structural properties for the preferred level of RH and the mechanical property for the standard climate, the mechanical properties of a paperboard could be obtained.