As an example say you need potatoes for your menu. You purchase potatoes for $10 per bag and you need 300 grams of potatoes to make your menu. You need to make an assumption about how many grams (or Kg) of potatoes are in the bag to be able to calculate the cost for 1 gram (or Kg) of potatoes. Otherwise if you are purchasing a product in Kgs and are using 300 grams of that product, you do not need to make an assumption because the two units of measure are easily convertible from Kg to g.For example. If you have purchased a bag of onions for $7.50 and you 1.5 Kgs of onion to make your menu and you assume there are 2.5 Kgs of onions in each back. You take the price of the bag (7.50) and divide it by the amount of Kg you assume are in the bag. In this case, $7.5/2.5= $3 per Kg. Meaning "your price per kg" is $3 per Kg of onions