How to find out the unit weight of reinforcement bar per meter

This construction video briefly explains the formula (Derivation Of (D?/162) for detecting the unit weight of reinforcement bar per meter.

This formula is specifically useful for circular shape steel (the density of which is 7850kg/m3).

Formula:
Weight density = weight/volume
Weight = weight density x volume.
Volume of cylinder= ph d^2 /4
h denotes the height of cylinder bar. = 1m
d is diameter in mm
Weight of bar per meter = Area of steel bar X Density of steel x Length of bar