Measuring Carpet for Landing and Stairs

This is the first time I have attempted this and was hoping someone could confirm that I am about right in terms of the amount of carpet I think I'll need.

Looking to order online and just get a fitter to come and do the work, something I have done for all the other rooms in the house but with stairs I just want to make sure I am not missing something obvious

Looking at the attached measurements I have come up with a requirements of 4m x 3.5m?

Your lucky to have a carpet fitter to fit carpet not supplied by them. Not the answer you want but for landing and stairs I would let a professional come and do the measurements and order the carpet any problems is then down to them. If you miss order and the fitter can’t complete they will still expect payment for their time.

Ive done all my carpets like this, order online and then get a fitter to install. They don't seem to mind as the room is already clean and within the hour its usually done, obviously longer for landing and stairs.