The answer is 20mm
An average of 6 feet. But you may wanna consider the height of the door's owner.
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You find a tape measure and take the measurement across the door, then the length of it, then if necessary the thickness.
height =2,10 m (210 cm) =width =0,80m (80cm)
1.5 meters
You should be able to turn it at an angle so that the horizontal 'width' is less than 28 inches. Another way, would be if the length of the sofa is less than the height of the door, you can stand it on end, slide part of it through, then slide it over so that the rest of it will go through the door.
There is no practical difference between height and length. When a tall person lies down does his height change into length? If you need to specify that the door needs to be 4X8 and you want to avoid confusion, you specify a four foot wide by eight foot high.