What is the method to calculate duct quantity of 24 g to 18 g?

First of all inorder to select guage for a GI duct there are SMACNA standards for guage selection.

0-450mm=24 guage (0.6mm)

451mm-900mm=22 guage (0.8mm)

901mm-1219mm=20 guage (1.0 mm)

1220mm-3048mm=18 guage (1.2mm)

Now if you want to calculate the duct quantity in sqm then,

A=2(W+H)xL  (Sq.m)

where

A=Area of the duct (sq.m)

W=width of the duct(m)

H=Height of the duct(m)

L=Length of the duct(m)

volume = Area * length of duct .It can be described in Sq.Mts  or Kgs.