如何修改Access 2013中的Totals行

I'm trying to modify a total query so that if the amount of the sum surpasses 1000 for example it changes the value in 2 fields. for example, 1000 grams change to 1 kilogram, and vice versa. For example, if I have 20 items in grams, and the total of the sum of those 20 items is 1200 grams, how do I make it so that it automatically changes it to 1.2 kilograms. How do I make it detect that its over 1000 to convert it from grams to kilograms.I'm trying to modify a total query so that if t