how much memory is unity when installed?

i want to get unity but i need to make sure my mac has enough memory

If you mean hard drive space, something like 500-600 mb with the example project, or thereabouts, plus 300 for the installer while installing it, though you'll probably want to keep way more than that free to keep the OS running smoothly

If you mean RAM, then 1GB is probably on the low end, but manageable. 2GB if you want it running well

2015 Update

RAM SPACE

(Windos 7)

  • 4GB Low minimum
  • 8GB OK
  • 16GB Good for Unity 5 (big game)

HD SPACE

  • 6.5GB In my HD for Unity 4.6 + plugin and example project. All my project (10GB) exclude!
  • 4.7GB In my HD for Unity 5.1 + plugin and example project. All my project (1GB) exclude!
  • I think to reserve 10GB for Unity5 growing just in case.
  • My 128 is full. You need a 256GB SSD HD because you will need other programs after OS ***

At the same time if you make small assets and one 2D project you will need much less space.

(***Windows 10 exclude that probably will need more).