The continuity equation for aortic valve area (AVA) is the echocardiographic gold standard for quantifying aortic stenosis severity, derived from the hydrodynamic principle of conservation of mass — stroke volume must be equal at the LVOT and at the aortic valve orifice. Developed as a clinical tool in the 1980s by Hakki and colleagues (simplified formula) and refined by Otto and colleagues, the continuity equation-derived AVA is incorporated into all major AS guidelines as the primary quantitative measure of stenosis severity. Accurate AVA calculation is essential because clinical decision-making (valve replacement timing) is heavily dependent on AVA ≤1.0 cm² as the threshold for severe stenosis.