A door that opens into a room is a flat panel on a hinge, and as it swings it sweeps out a quarter circle on the floor. The radius of that circle is simply the width of the door leaf, the moving panel itself, not the hole in the wall. So a 80 cm door claims a quarter circle 80 cm across, and nothing tall or solid can live inside it if you want the door to open fully.
It is easy to miss in a small room. On a floor plan the furniture fits, because the door is drawn as a thin line. In real life the wardrobe is fine until someone tries to come in and the door stops against its corner. This guide lets you test that before you move anything heavy.
Below is a top-down plan of a small room. The green fan is the area the door sweeps. Drag the three pieces of furniture around (or focus one and use the arrow keys), change the door width, and open the door with the slider. A piece turns terracotta when the door would touch it.
You only need a tape measure and two minutes. Open the door and measure the leaf, the actual panel, edge to edge. Interior doors are often somewhere between 60 and 90 cm wide, but older homes and bathrooms vary, so measure rather than assume. Then note which side the hinges are on and which way it opens.
Add a few centimetres for the handle, which sticks out past the leaf and is the first thing to catch a wardrobe corner. If the door has a tall skirting board or a draught excluder at the bottom, measure from the floor you actually walk on.
A wardrobe or cupboard with hinged doors has the same problem in reverse: its doors need a quarter circle of free floor too, usually half the width of the cabinet per door. In a narrow room that arc can overlap the bed or the room door. Sliding doors need almost no floor space, which is why they are common in small bedrooms. If you are buying, check which kind it is before you check the colour.
You rarely need to buy anything. Try these in order, cheapest first:
A door that opens fully is the safe default, but it is not law. In a spare room that you walk through once a day, a door that stops at 70° against a chest of drawers may be a fine trade for the extra storage. In a bedroom you use every morning, or anywhere someone might need to get out quickly or get a wheelchair or pushchair through, keep the swing clear. Think about who uses the door, not only whether the floor plan works on paper.
Write down the leaf width and hinge side of each door in the room, draw the quarter circle on the floor with masking tape, and walk through it. Tape is free, it peels off, and it will tell you in one minute what a floor plan took an hour to guess.