Even if you knew the exact amount of free space remaining on your memory card you could still only guess the approximate number of remaining frames you could add to the card. It is, in fact, exactly what your camera is already doing for you.

This is because even RAW files are compressed, albeit losslessly, and so the exact size of a file depends upon the content of the file. The more uniform the data in a frame is, the smaller the resulting file will be. The more varied the data in a frame is, the larger the resulting file will be. A photo of a uniformly lit gray wall will can be compressed into a much smaller file than a photo of a spectacular fall foliage scene that includes almost every variation of tone, shade, and color that the camera is capable of recording.

In real world practice, I just looked at several hundred photos I took late last week. All of the files were taken at full resolution and saved as .cr2 files. The file sizes ranged from 22,366 to 36,459 KB (average of 27,893 KB) for my 5616x3744 pixel (21MP) Canon 5DII, and from 20,147 to 27,861 KB (average of 24,442 KB) for my 5184x3456 (18MP) Canon 7D. Both save RAW files in 14-bit depth. So if I have room for 10 'average' photos on my 5DII, I might fit as many as 12 or as few as 7 more shots on the card, depending on the content of the photos and how far they compress.