Multi Column table always slightly too wide

I have a table with a lot of data that I am displaying without telling it how wide I want it
My problem is that it is always slightly wider than the page
My page has a header, menu and footer the table is in the middle and it has margins either side of it
I think it is the margin that is being ignored when the table calculates its width.
I always have to scroll right to see the whole table except when my browser is big enough - nearly at full screen (1920)
If I change the browser width and reload the table it draws to a different size but always slightly too big

Any ideas?