What units do table headers use for width?

[Napster653]

I am creating a simple table passing my data and my headers to it. On my headers array I'm setting the width of each column.

I am using a default a4 page with default mm units, so the max page width is 210.0015555555555

However, if I set one of the columns to that width it doesn't fill the whole page. In column units that is ~280 'something'.

What is that 'something' and how can I convert mm to it?

    let headers_exc = [
        { 'id': 'id', 'name': 'id', 'prompt': ' ', 'width': 10, 'align': 'center', 'padding': 0 },
        { 'id': 'aaa', 'name': 'aaa', 'prompt': 'aaa', 'width': 160, 'align': 'left', 'padding': 0 },
        { 'id': 'si', 'name': 'si', 'prompt': 'SI', 'width': 40, 'align': 'center', 'padding': 0 },
        { 'id': 'no', 'name': 'no', 'prompt': 'NO', 'width': 40, 'align': 'center', 'padding': 0 }
    ];