Is ISMB 400 (Indian) equivalent to W16×50 (American)?

Close but not identical. ISMB 400: depth 400 mm, width 140 mm, weight 61.5 kg/m, Ix ≈ 20,220 cm⁴, Sx ≈ 1,011 cm³. W16×50 (American): depth ~413 mm (16.3″), width 180 mm (7.07″), weight ~74 kg/m, Ix ≈ 27,140 cm⁴, Sx ≈ 1,315 cm³. They are in the same general family (intermediate I-beams ~400 mm deep) but W16×50 is heavier and stiffer. A true ISMB 400 equivalent in AISC catalogs is closer to W16×40 (weight 60 kg/m, Sx ≈ 1,048 cm³). When substituting profiles across standards, always compare moment of inertia and section modulus — not just depth. CalcSteel lets you load any profile from any standard and compare their properties side by side.