Wait, why do you even need to know this, surely you can just check the file size.
The formula given for plain text is hardly difficult, I'm sure you are capable of working a calculator... Why should we do it for you?
The simple formula will also only work for plain text (.txt file), it is impossible to work out the file size of it if it includes any sort of formatting, fonts, layout, compression.etc such as in a word document or PDF file.
Drives me mental when people just want a numerical answer and totally reject helpful advice that attempts to explain how it all actually works...