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How do you read which way a door opens on a floor plan?

The quarter-circle arc by the door is the answer: it starts at the hinge side and sits in the room the door swings into. Here is the full reading — and why it decides whether the bed can go there.

The arc is the door's footprint on the floor

A door is drawn on a floor plan with three lines: the gap in the wall, a thin straight line (the leaf, drawn open at 90 degrees), and a quarter-circle arc from the leaf's free edge back to the wall. The arc is the patch of floor the leaf sweeps through when it opens.

That makes the arc more than a symbol. It is a reservation of floor space. Anything sitting inside the arc gets hit by the door: a chest of drawers, a radiator, a door from another room, the foot of a bed. That is why door arcs are the first thing an architect checks when furniture is going in — and the thing a buyer most often overlooks when a floor plan looks spacious.

The arc has the same radius as the leaf's width. An 82.5 cm wide door therefore needs 82.5 cm of clear floor, measured from the hinge all the way round, to open fully.

The four things the arc tells you

1. Which side the hinges are on

The arc always starts at the hinge side. That is the fixed point — the leaf turns about it, the way a compass turns about its point. Look at the arc's two ends: the end where the arc meets the wall at a sharp corner is the hinge side. The other end is the door's free edge, where the handle and lock sit.

2. Which room the door opens into

The arc sits on whichever side of the wall the door swings towards. If the arc lies in the hallway, the door opens out into the hallway. If it lies inside the bathroom, it opens into the bathroom.

3. How much floor is taken up

The quarter circle's area is the space that cannot be furnished. The sum for an 82.5 cm door: a quarter circle with a radius of 0.825 m has an area of (π × 0.825²) ÷ 4 = (3.1416 × 0.681) ÷ 4 = 0.53 m². A home with 8 internal doors ties up roughly 4 m² of floor to door swings alone. In a small flat that is noticeable.

4. Whether two doors clash

If two arcs overlap on the drawing, the two doors cannot be open at the same time. This typically happens in hallways and in narrow corridors with three or four doors. It is a real problem, not a detail: a door that strikes another door scratches both and wears the hinges crooked.

Left-hung or right-hung — and why it confuses people

When you order a new door, you need to state whether it is left- or right-hung. The common method: stand on the side the door opens towards you (the side where you can see the hinges). If the hinges are on the left, the door is left-hung; on the right, it is right-hung.

But be careful: different manufacturers and countries define this differently, and some count from the opposite side. That is why the safe way to order is always phrased as a description, never as a single word: "hinges on the right as seen from the hallway, door opens into the room." With that sentence nobody can misread you, whatever standard the supplier uses. Bring a photo too, if you can.

Reading a floor plan, you do not need the word at all — the arc says it all.

Inward or outward: what usually decides it

There are practical habits behind which way doors face in a home:

• Internal doors generally open into the smaller room — into the bedroom, into the bathroom — so the hallway stays clear. Otherwise an open door blocks the passage.
• Bathroom doors should open outward, if there is room. If someone collapses on the bathroom floor, an inward-opening door cannot be opened. In bathrooms that need to work with a walker or wheelchair, an outward-opening door is normally what is assumed — exactly what is required depends on the building code where the home stands, and should be confirmed with the authorities before work begins.
• Front doors in single-family houses often open inward in many places, partly because of weather and frame construction. Doors that serve as escape routes in public buildings, by contrast, open out in the direction of escape.
• A door opens towards the nearest wall, so it can stand open without jutting into the room. That is why the hinges are almost always closest to the corner the door sits in.

The doors that have no arc

Not every door is drawn with a quarter circle. Here is how to spot the others:

• Surface-mounted sliding door: the gap in the wall, and a thin rectangular panel drawn parallel to the wall beside the gap — often with a small arrow showing the slide direction. No arc.
• Pocket sliding door: the same, but the panel is drawn dashed, inside the wall itself, because it disappears into a pocket. The wall is thicker here, since it has to house the leaf plus lining on both sides.
• Folding door: a zigzag figure in the opening, showing how the leaves fold together.
• Double door: two leaves and two arcs, mirroring each other at the centre of the opening.
• Open passage: a plain break in the wall — no arc, no leaf, no cross-lines. Two wall ends with air between them.

By comparison, a window also has no arc, but the gap is "closed" by one to three thin cross-lines — the glass. The rule to remember: arc = door, cross-lines = window, nothing = passage.

How wide is the door, really?

On the drawing, the door's width is often given in the innermost dimension chain, or as a number referring to a door schedule. Watch out: there are three different widths in play, and they are constantly mixed up:

• The width of the wall opening — the gap in the wall, before the frame goes in. This is usually what the drawing's dimension chain shows.
• The width of the leaf — the panel hanging in the hinges. Internal leaves come in fixed standard widths — in Denmark 62.5, 72.5, 82.5 and 92.5 cm. Standards differ from country to country, so measure up if the door is to be replaced.
• The clear passage width — what you can actually carry a piece of furniture through when the door stands open. It is a few centimetres less than the leaf, because the frame's stop and the hinges steal a little, and because the door can rarely open a full 180 degrees.

If you are judging whether the sofa will fit through, it is the clear passage width you need — and the safest way to measure it is with a tape measure at the open door, not on the drawing.

How to fix a door that swings the wrong way on the drawing

If a door faces the wrong way on a drawing you made yourself — or you want to check what happens if a door were flipped — there are four states to choose between for the same opening: hinges at the left or right end, combined with opening into one room or the other.

In HouseSense you can choose which way a door opens, and the choice carries through both to the floor plan, where the arc moves, and to the 3D model and the walkthrough, where the leaf stands that way. That is handy when checking whether a door that today opens into a narrow corridor would work better the other way — because the arc shows straight away which floor space you gain and lose.

Flipping a door in real life, incidentally, is rarely hard when it is only about the hinge side: the frame is redrilled and the leaf is turned. If the door instead needs to open the other way through the wall, the frame's stop typically has to move, or the frame replaced. Ask a carpenter before assuming it is an afternoon's work.

Three things the arcs reveal on any floor plan

1. Walk through every door, one at a time, and see what sits in the arc. Radiators, sockets and chests of drawers in a door arc are a sign the plan was not thought through.
2. Look for arcs that overlap. Two doors in the same corner cannot be open at once.
3. See whether the arc blocks a passage. An open door that halves a corridor's width is felt every single day — even though it only takes up 0.5 m² on paper.