size in metres (dimension 1) x size in metres (dimension 2) x no of sheets x paper weight divided by 1000 = weight in kilos
So for example if I was to ship 100 x 28pp A4 booklets that are printed on 130gsm the formula would be:
0.21m x 0.297m x 2800 x 130 / 1000 = 22.7 kg
Remember to allow a little for boxes! We would recommend adding 1 kg which is more than enough.
Have nice day