A cloth / some cloths

He is measuring some cloth with a ruler.

Thanks, but if you check in your dictionary, "cloth" can be countable and uncountable. Why do we use it as uncountable?

Because it is being measured. I cannot account for all of the grammatical but useless sentences you produce, Vincent. I can only indicate which ones actually occur in normal communications.

A countable cloth is usually one which has been prepared (sized, shaped, bordered) for some particular use, as a cleaning cloth, or a wash cloth, or a loin cloth, perhaps.

Uncountable cloth is usually cloth on a big roll in the store, which you may pay for by the yard. Sometimes it's in a flat roll, called a bolt. How can there be a flat roll?