Empirical formulas are derived that describe neutron flux distributions in straight and bent cylindrical steel-walled ducts in symmetrical and asymmetrical geometries. Asymmetry is defined by the slant angle between the duct axis and a line passing through the center of the duct mouth and source. Streaming neutrons are divided into direct, albedo, and penetration components. The first two components are described by a function of the axial distance in units of the square root of the line-of-sight area. The last component is described by a function of the neutron flux distribution in the shield in the absence of the duct. Formulas are applicable to thermal, epithermal, intermediate, and fast neutrons, respectively, in the following range: (a) duct diameter is 5 to 20 cm, (b) duct length is up to 240 cm, (c) slant angle is 0 to 90 deg, (d) steel wall thickness is 0 to 1.0 cm, (e) bent angle is 45 to 90 deg, and (f) surrounding medium of the duct is water or ordinary concrete. Calculations by formulas agree with experimental data, in general, within an accuracy of ±30%.