For round rug

I would measure from the door frame to the curtains. That will give you your diameter. Then, I'd measure exactly halfway along that measurement and put a piece of tape or other marking. Let's call that spot X. Then, from X, you can measure a half-diameter in any direction. That is the size of your rug. You probably do want to take a bit of string of the half-diameter length and draw a circle to see where the rug will be under your chair.

If the walls behind the chair are not the same size, you probably want to use the

shorter. For example, if one wall is 11 feet long and the other 10, you'd want to measure your diagonal from the edge of the 10 foot wall to 10 feet from the corner of the 11 foot wall.

A circle enscribed in a square (ie the run inside the square made by the two walls behind the chair and 2 imaginary walls for the rest of the square) will have a diameter that is the diagonal measurement of the square (top right corner to bottom left corner).

You can also do the whole thing with math with Pythagorus's theorem. (A^2 + B^2 = C^2)

Measure the shorter wall, square that (A^2), double it (this is B^2, but A=B), then take the square root for your diameter.

Pay attention to any obstacles, like baseboards, radiators or other things. You need to measure at the floor.

If you want a bigger rug than the one that fits into your corner, you need to find a diagonal further into your room to measure, projecting from the walls. For example, a point midway into the door to an equal distance along the wall with the window.

Since a circle enscribed in a square has the diameter of the diagonal of the square, as long as you have a square of floor space, you can figure out the diameter of the rug.

This is 200 cm wide long way I’m shooting for 200 cm diameter rug- thoughts?