Circle Occlusion and Flow Calculator Larger tube size: 20.0 Fr (d = 6.37 mm) Smaller tube size: 10.0 Fr (d = 3.18 mm) % Occlusion of larger tube: 25.0% Residual flow capacity: 56.3% Residual CSA: 20.5 mm² (≈ 14.5 Fr) Notes: d = diameter; CSA = cross-sectional area; French (Fr) denotes circumference in mm.