I am using 4mm DATA TAPE (Dell) having Capcity of 36GB. I backup my data on it. Now I find how much space are free to take further backups.

Thanks

You can make an educated guess based on how much data has been backed up. Are you using data compression? Hardware based compression is a bit better than software based compression and it depends if any of the data you are backing up is already compressed via NTFS compression or if the file type can not be compressed.

So, if you are not using compression, look at the total of data that has been backed up and subtract it from the total tape capacity. If you are using compression, you will get an average of 1.4 to 1.7 to 1 for hardware compression and 1.2 to 1.4 to 1 for software based compression. Use this to calculate the estimated capacity of the tape.

Your backup software should tell you how much is remaining.