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I just bought Dell XPS 13 model with 16GB RAM, 512GB SSD and Ubuntu 18.04 preinstalled. When I click on Properties, it shows 425gb free space. May I know why?

Results of sudo parted -l :

Number Start End Size File system Name Flags 
 1 1049kB 786MB 786MB fat32 EFI System Partition boot, esp
 2 787MB 6156MB 5369MB fat32 Basic Data Partition msftres
 3 6156MB 512GB 506GB ext4 

Open the terminal and run sudo parted -l to show all the partitions that Ubuntu can see. The results of sudo parted -l show a table that looks like this.

$ sudo parted -l
Model: ATA ST9500420AS (scsi)
Disk /dev/sda: 500GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos

Number Start End Size Type File system Flags
 1 32.3kB 41.1MB 41.1MB primary fat16 diag
 2 41.9MB 15.8GB 15.7GB primary ntfs boot
 3 15.8GB 99.7GB 83.9GB primary ntfs
 4 99.7GB 500GB 400GB extended lba
 5 99.7GB 102GB 2147MB logical fat32 lba
 7 102GB 176GB 73.8GB logical ext4
 6 176GB 492GB 316GB logical ext4
 8 492GB 500GB 8389MB logical linux-swap(v1)

The size of each partition is listed in the 4th column. The partition's size can give you a clue about what it is used for. One of the partitions may contain software that was bundled by Dell along with the XPS 13 laptop.

Your results of sudo parted -l are as follows:

Number Start End Size File system Name Flags 
 1 1049kB 786MB 786MB fat32 EFI System Partition boot, esp
 2 787MB 6156MB 5369MB fat32 Basic Data Partition msftres
 3 6156MB 512GB 506GB ext4 

The above results show that 5.4GB are used as a Basic Data Partition and 0.8GB are used as a EFI System Partition for the boot loader. This accounts for some of the missing space. The rest of the space that isn't free space is taken up by the Ubuntu installation, installed packages, and saved user files in the user's home directory.

The next thing to do is to see what's using 81GB of the 506GB in your ext4 Ubuntu partition graphically. For new installations of Ubuntu 18.04 a swap file is created by default instead of a swap partition. For your system that has 16GB RAM the size of the swap file is at least 4GB. If hibernation is enabled the size of the swap file could be 20GB-32GB.

Open the built-in Disk Usage Analyzer by clicking on its icon in the Dash and click on the ext4 partition. Then wait a few minutes for Disk Usage Analyzer to make a pie chart showing what is taking up the remainder of the used space on the 506GB ext4 partition.

^{(Click image to enlarge)}

To find out what partitions you have and the available space on these open a terminal by pressing ALT+STRG+T

In the terminal type the diskfree command:

df -h

Have a look at the entries starting with /dev

If you want help resizing partitions, post the output here and I'll have a look at it and maybe make a suggestion.