The bending formulas are pretty straightforward but you did not give enough information to know which formula to apply. They all depend upon E, the Young’s modulus for the material (where steel is 29*10^6 in Imperial units) and the area moment which for:
1. A Rod is I =.25 * pi * r^4
2. While a hollow square tube is I= (a^4 –b^4)/12 (if rounded corners are ignored) where a is outside dimensions and b is inside dimensions both across the flats.
But you need to think about how the rod or tube is supported at the ends to select the bending formula. Simply supported means the beam can have deflection slope at the support and with a midpoint load the maximum deflection is (W*L^3)/(48 *E*I). Rigid support means the beam is held to allow no bending at the support (like it was cantilevered at both ends) and for that situation the maximum deflection is (W*L^3)/(96*E*I). You may need both formulas. Suppose the steel tubing base is welded then the top member is effectively rigidly supported and the bending length( L) is the distance between the supports. But if the round rail is supported by relatively small standoffs then it is simply supported and L is the center to center distance between standoffs.