How to correctly estimate the transaction logs used in one INSERT operation for SQL Server?

Question

I plan to insert large amount of records (hundred thousands of rows containing blob PDF data) into a table (one time) and I would like to make sure the disk the database is on has enough space beforehand. I also know that while inserting, the transaction log also takes additional space. My question is how to correctly estimate how much space the transaction log will take? Is there a rule of thumb? Is transaction log going to take the same amount of space as the data (in the current batch) takes? Or it is not possible?

To narrow down the scope of this question,

• we are using simple recovery model
• let's not consider minimally logged operations

Thanks.