Exterior wall placement (Outside, Center, Inside)

Step by step

1. 1

   First: how rooms are measured (face to face)

   Every dimension on the plan is measured face to face — exactly what you'd get with a tape measure held against the wall surfaces. A room's interior size is the distance between the inner wall faces; the building's footprint is measured to the outer faces. The number you type when drawing sets one of those edges — and Exterior Wall Placement decides which edge it is.

2. 2

   Gross Area vs Usable Area

   Two readouts sit in the left panel: - Gross Area — the total footprint of the floor, measured to the outer faces of the exterior walls. It includes the wall thickness — this is the building's outside envelope. - Usable Area — the interior floor you can actually furnish, measured inner-face to inner-face, with the wall thickness removed. The gap between them is the floor space taken up by the walls themselves. Thicker walls → bigger gap.

3. 3

   Outside mode — the line is the inside

   In Outside mode (the default), the outline you draw is the inside face of the walls, and the thickness is added outward. So the number you typed is the inside clear dimension — what you'll actually live in — and the building grows outward by one wall thickness on every side. In the example below (a 16 ft × 12 ft room with 6.5-inch walls), the dimension chain reads 6.5″ — 16′ — 6.5″: your 16 ft is the interior, with a 6.5-inch wall added on each side. Usable Area = 192 sq ft (your drawn 16 × 12), while Gross Area = 224 sq ft (the larger outer footprint). Best when you're designing to interior room sizes.

   

4. 4

   Center mode — walls straddle the line

   In Center mode, the walls are centered on the outline — half the thickness falls inside the line, half outside. The drawn dimension is neither the pure inside nor the pure outside; it's the wall centerline. For the same 16 × 12 room, this lands between the other two modes: Gross Area = 207 sq ft, Usable Area = 177 sq ft. Common in architectural drafting, where walls are dimensioned to their centerlines.

   

5. 5

   Inside mode — the line is the outside

   In Inside mode, the outline is the outer envelope of the building, and the wall thickness is taken inward. So the number you typed is the outside footprint, and the interior shrinks by one wall thickness on every side. For the 16 × 12 room, the dimension chain now reads 6.5″ — 14′ 11″ — 6.5″: the 16 ft is the outside, leaving 14′11″ of interior. Gross Area = 192 sq ft (your drawn 16 × 12, the footprint) and Usable Area = 163 sq ft (the smaller interior). Best when you're matching an exterior footprint — say, a lot or a slab you've measured.

   

6. 6

   Putting it together

   Same 16 × 12 outline, same 6.5-inch walls — three readings: - Outside: Gross 224, Usable 192 (line = inside) - Center: Gross 207, Usable 177 (line = centerline) - Inside: Gross 192, Usable 163 (line = outside) Nothing about your geometry is wrong in any mode — you're just telling Room Sketch 3D which face your typed dimension refers to. Pick the mode that matches how you're thinking: interior room sizes → Outside; centerline drafting → Center; an exterior footprint → Inside. Change the wall thickness in Settings (or per wall) and you'll see the Gross/Usable gap grow or shrink accordingly.

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