Measuring a square room is pretty easy – the area of the room is the length times the width.
A room that isn't a simple square or rectangle is a little more complicated, but not much.
First of all, divide the room up so that it is made up of rectangular areas, as in the diagram below.
If there is a feature such as a bay window then measure all the way to the end of the bay area.
Once you have measured each length, suc (AxB)+(Ch as wall A or wall D, add a few inches/centimetres to the length to make sure you will have some overlap for safety.
You don't need much, just an inch or two. When you fit the underlay you will trim this extra amount off. Now, however many rectangles you have, just calculate the area for each and add them together – in our example the area is (AxB)+(CxD)+(ExF).