Fibrin formed from fibrinogen is the main component of thrombi. Clot structure is characterized by fiber thickness and pore size, which differs within a given clot and between individuals. Plasma clot architecture is largely determined by the quantity and quality of fibrinogen. Plasma fibrinogen concentrations are most commonly measured in citrated plasma using the Clauss method. However, several factors, including instrument type and reagent, may affect results. Other approaches to express the ability of fibrinogen to clot involve prothrombin time-derived or clottable protein assays, while fibrinogen antigen levels in clinical settings are measured using immunological or precipitation assays. Fibrin clot permeability (reflected by the Darcy constant, Ks) being proportional to a buffer volume percolating through a clot under a given hydrostatic pressure is now the most commonly used measure of clot structure. Low Ks values indicating tightly packed fibrin structure have been shown to be associated with venous and arterial thrombotic complications, while high Ks might contribute to bleeding disorders. The measurement of Ks, however, is not standardized and validated. This review summarizes the current knowledge on practical aspects of the measurement of fibrinogen levels and Ks in patients.