How to estimate log growth when providing DML

Hi there,

what is the way how to estimate log growth when providing UPDATE, INSERT,DELETE on table?

I have tested growth on test table which has 25 milion rows of 4 columns, each type int. Whole table has storage size 390 MB. There is no index on the table. My log file has 2 MB and is almost empty. My DB is in simple mode.

When I update one value for all rows

UPDATE TABLE
SET col1 = col1 + 10

then my log is going to 10000+ MB of allocated space and 8000+MB of used space. How it works? What happened that log went to 10000 of MBs? Then he evidentally released 8000 MB of log data. Could you please explain in detail what is happening when DML is done ?

Then I repeated test with full recovery option on DB and result is same like in simple flow.

For me its strange, that it is not enough to store original data in log table I am trying to update. (390 MB)

Thanks for help