Xcode free space requirement

I am trying to update Xcode to 8.0. My OS is 10.11.6 El Capitan. However I am getting:

We could not complete your update
There is not enough disk space available to install the product.

I have got 14.22 GB free space. How much do I need?

UPDATE: I do not know the exact figure, though I did manage to update with 17.21 GB free space later on.

Solution 1:

You'll need about 20GB free to do the install when you consider the space needed for holding/swapping files with the .dmg during the install process.

I saw this in the Apple developer forums:

Have a look at About This Mac → Storage tab. You might see a purple segment labeled "Backups." That's the local Time Machine cache, and it doesn't show up in the Finder's free space display because it can theoretically be purged if that space is needed.

Update Sept 18, 2020: Since this answer was written, Xcode free space requirements have been increasing. As of Xcode 12, I needed at least 49GB of space to complete the installation.

Solution 2:

As of Sept 2020 Xcode now requires over 40 gbs of free space to install.

It finally started downloading for me today once I had 42gb of free space on my hard drive, as measured by the Get Info window. Cmd-I on your hard drive to open it.