Thank-you all for the answers. Now it is clear that "margins" are indirectly derived by defining the body area dimensions namely \textwidth and \textheight.
I did not know the layout package before. Now I managed to set everything properly.
What are the precise dimensions of the margins for an a4paper, report document without geometry?
They are in columns two and three of the table for oneside and twoside. (the 62pt allocated to the margins are split in a different way). The code shows how to get this values.
But, more important, you also wanted to keep the original layout of the cover while ...
You can use the layouts package to report the current page layout values (and also see what happens if you change them). You can use the changepage package's adjustwidth environment to temporarily alter the current left and right margins. Here is an example.
% marginsprob.tex SE 591067
I have answered this before somewhere but I couldn't find the duplicate so ...
The two options are doing very different things.
Classically TeX has no concept of paper size or margin, it just sets the size of the text block and the margins are the difference between that and whatever size paper you print on.
There is a nominal offset of 1in from the top left ...
You can try the following book-modified \chapter command. It looks at the value of the page and accordingly inserts the appropriate number of \clearpages (with an empty page style). It only does this for chapter 2 onward (to avoid issues with \part), but that can be changed.
% Taken from
This is a simple correction to be considered (i.e. if the book has less than two dozen chapters), and applied as one of the final touches, when all the material of the book is completed, after each chapter that finish on a even page.
It does not affect \part or other packages.