How to calculate the weight of reinforcement/steel bars of different diameters?

For calculating the weight of bars (of uniform Cross-section) following formula can be used :
Weight = Cross-sectional area X Length X Density
            = [(π × D²)/4 X L X ρ  
Here, 
        π = 3.1416
        D = Diameter of bar (mm)
        L = Length (m)
        ρ = Density of mild steel 7850kg/m³

Weight = [(π × D²)/4 X L X ρ 
            = [(3.1416 ×  D²)/(4 × 1000 ×1000)] X L X 7850     [NOTE: 4 × 1000 ×1000 , Since                                                                                                                                diameter is in mm. So converting to m.]
           = D²  X L X 0.00616225
            = (D²  X L)/162               [NOTE: 0.00616225 = 1/162]

Just keep in mind that , while using the above formula, the unit for Diameter is millimeter (mm) while that of Length is meter(m).
and the unit of weight is kilogram (Kg).

If you have the length in ft, then the formula will 
Weight =  (D²  X L)/ 531.522 ~~  = (D²  X L)/ 532


And also note that for calculating weight of bars/ reinforcements of different materials (other than mild steel), the density part must be changed accordingly.