By default, in the Mac finder you can see the size of individual files but not folders (unless you click on “get info” for each one.

However it’s easy to set the option to see the folder size. Just go Finder, View, Show View Options, and select the “calculate all sizes” box.

Bear in mind that the [...]

This is pretty useful and I can never remember the exact syntax, so here it is for me (and now you) to remember.
In a Google spreadsheet, you can do an online currency conversion that looks up the exchange rate in real time, making my life much easier.
For example to get EUR to GBP just type
=googlefinance((”CURRENCY:EURGBP”))
(note: [...]

One of the things I often need to do is to calculate the number of work days between 2 dates in order to calculate billable days. There is a very useful function called NETWORKDAYS which does exactly this. It works the same in both Excel and Google Spreadsheets
The syntax is =NETWORKDAYS (start_date, end_date, [holidays])
example =NETWORKDAYS [...]