In Kusto Query Language (KQL), you can convert gigabytes to petabytes by dividing the size in gigabytes by 1,024 (since 1 petabyte is equal to 1,024 terabytes and 1 terabyte is equal to 1,024 gigabytes).

To create a new column next to the “size” column, you can use the extend operator. Here’s how you can do it:

datatable(Database: string, SizeInBytes: long)
[
    "sub1", 2541304016738179270,
    "sub2", 2539224975164406590
]
| extend SizeInGB = SizeInBytes / (1024 * 1024 * 1024)  // Convert bytes to gigabytes
| extend SizeInPB = SizeInGB / 1024                      // Convert gigabytes to petabytes
| project Database, SizeInBytes, SizeInGB, SizeInPB

I have a kusto query that have two columns as shown below & size column is in Giga bytes. How can I change the Gigabytes into petabytes & create a new column next to “size” using “extend” function or a different way?

As your column is in Gb, You can use below KQL query:

let rithTable=datatable(Database:string, size_gb: long)
[
    "cho1", 2541304016738179270,
    "cho2", 2539224975164406590
];
rithTable
| extend sz_in_pg = size_gb / 1048576.0

Here,

1 TB = 1024 Gb

1 PB = 1024 Tb

So, 1 PB is 1024*1024 = 1048576 GB

Output:

Fiddle.