I have an old Stanley measurer which reads out in feet or metres, there's really nothing complicated in using it. You just multiply length by breadth.
I would suggest that square feet is better in use for households.
After many years, I price each job " as I find it ", ie, size, any furniture needing to be moved, degree of soiling, all of which amounts to the likely time taken to complete.
In domestics I believe my eyes, in estimating the area to be cleaned, but would never do that with commercial work, where a price, per square foot is still the expected format.