Suppose you have 10 colonies on a plate. The dilution you plated was 1000000. So you will have 10000000.

Now if you plated 100 ul of your dilution. Your colonies are 10000000 /ml

So formula used is

Colonies on plate X dilution X amount plated.

Thank Randhir for your explaination .

But here my quation is that colony forming units per microlitre not mill liter.

can you explan this

what ever I said above divide it by 1000 and you will have in microlitre