Sodium ion diffusion plays a key role within the charging and discharging mechanisms in a sodium ion battery. The mobility of sodium ions is important to allow full sodiation of hard carbon materials and enables faster charge and discharge rates for batteries. This investigation studies the diffusion of sodium ions by combining a physical characterisation technique and an electrochemical testing method for a hard carbon composite electrode. Untreated and sodiated hard carbon composite electrodes have been imaged on microscale using x-ray computed tomography to visualise volume changes, and simulate tortuosity and sodium ion concentration distribution by using the MATLAB plug-in TauFactor. Furthermore a comparison to calculated apparent diffusion coefficients determinded by galvanostatic intermittent titration technique was done. The results indicate a correlation of an increasing tortuosity due to the sodiation of hard carbon and the decrease in sodium ion mobility.