Add Unit Items

While i am entering my purchase or sales invoice i want to have an additional column where i can input no. of units . For example 25 kg bag x 10 nos. = 250 Kg.
please suggest

You can’t do that. You could define the inventory item unit as 25 kg bag. And, of course, you enter the number of units sold. But Manager does not support subsidiary calculations. It calculates extended prices.