Storing floating point values for various units of measurement (kg, ml, mm, pl) in PostgreSQL

I have a table like the following

order_item
------------
serial      | id (PK)
integer     | order_id
????        | quantity
integer     | unit_of_measure_id (PK) --> ml, l, kg, mg, mm, m, etc.

I'm doing an application where I need to store the amount of material used in chemical analysis. Sometimes the materials are used in such a small amount as microliters, or sometimes more, like one or two liters.

Is it the numeric datatype good enough?

I have read that it is a good idea to convert the values to the smallest unit of measure. So if I have 10 liters, I have to pass to 10000000 microliters. But what if you need to use a unit of measurement even lower in the future? like picoliters? Inevitably, the application will have to use floating point numbers I think.

I hope you understand my question, my English is not very good. Thanks.