Simply a WIDGET for see how much space is avaiable on SD.
A little icon on your desktop shaped like an SD shows you the % of free space in your SD card.

When you click the widget you can open SD-memory and Internal-memory stats.

P.S.: the % in the widget is rounded.

!! IMPORTANT: this APP is old, in new smartphones doesn't work well, try it.
You can ADD the WIDGET from the widget section in your app drawer / long tap in the desktop (it depends from your Android version).