The pressure-volume relationship in real arteries is nonlinear. As a result, stiffness is pressure-dependent. Hereby, I propose a model-based pressure-independent and measurable characterization for such arteries. Literature shows that under normal conditions arterial pressure P, as measured in vitro or in vivo at the systolic pressure (S) and diastolic pressure (D), increases exponentially with the absolute or relative arterial volume V, lumen area or diameter (Eq.1&Figure). The pressure-independent exponent β (‘stiffness constant’) has diagnostic and prognostic significance. Defining arterial stiffness G_P by dP/dV, Eq.1 shows that G_P equals to βP plus a constant (Eq.2&Figure). Thus, β expresses the stiffness change per 1mmHg increase of pressure. Eq.2 predicts a linear relationship between repeatedly measured S and D, (Eq.3&Figure). Eq.2 shows that the change in S per 1mmHg increase in D (‘dS/dD’) is the mean relative increase in arterial stiffness during the systole G_S/G_D (Eq.3&Figure), i.e. ‘stiffening’, where dS/dD=1 for elastic arteries. The statistical estimate of dS/dD is given by SD_S/SD_D, where SD is the standard deviation (Figure). Using 24hABP data of 1,247 hypertensive patients (age 57±26 and 50% males) Eq.3 was confirmed with r=0.76±0.14 and dS/dD=1.4±0.3 with 90% of values in the range 1.0–2.0. Eq.2 was confirmed by Schillaci et al 2011 by measuring the diastolic pulse wave velocity (PWV_D) at different arm positions (since G_D∼PWV_D²), and Eq.3 was used to determine the systolic PWV or G_S. In conclusion, nonlinear arterial properties are strongly expressed during the cardiac cycle. Its modelling may be useful in interpreting stiffness measured by different methods.