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How to measure area on a floor plan or site plan

Flooring quotes, paint coverage, turf, paving, letting listings — sooner or later you need square metres or square feet off a drawing. Here's how to get them accurately from any plan, in your browser, without cutting the shape into rectangles by hand.

The old way, and why it undercounts

The manual method is to slice a room into rectangles, measure each, and add them up. It works for simple rooms and goes wrong everywhere else: bay windows, angled walls, L-shapes around a chimney breast, and any garden that isn't a rectangle. Every simplification you make loses (or invents) area.

A polygon tool removes the approximation: you click once on each corner of the real outline, and the enclosed area is computed exactly for that shape — Scale Rule uses the shoelace formula over your traced vertices, so 5 corners or 25 corners are equally fine.

Step by step

  1. Upload and calibrate. Open Scale Rule, drop in the plan (PDF, JPG or PNG), and calibrate against one known dimension — a labelled wall or a scale bar. Full details in the floor-plan guide. Area depends on the scale squared, so a careful calibration matters twice as much here.
  2. Pick the area tool and trace the outline. One click (or tap) per corner, working around the shape; finish with Enter or a double-tap. Vertex snapping helps you land exactly on wall junctions.
  3. Read the result. The area shows on the shape and in the measurements panel, in m² or ft² — toggle units any time. The panel's summary also totals every area you've drawn, useful for whole-floor or multi-room figures.
  4. Refine if needed. Drag any vertex to correct it, click a segment to insert a corner you missed, double-tap a vertex to remove it. The value updates live.
  5. Export. Label each area ("Kitchen", "Lawn"), then copy the list as TSV for a spreadsheet or Markdown for a quote email.

Inside face or outside face? For flooring, paint and furniture, trace the inside face of the walls — that's the usable floor, and it matches how GIA (gross internal area) is defined. Trace the outside face only when you genuinely want the building's footprint.

Worked example: flooring for an L-shaped room

Say the traced polygon over an L-shaped living room reads 23.4 m². Straight-lay laminate with ~10% cutting waste: 23.4 × 1.10 ≈ 25.7 m² → with packs covering 2.2 m² each, 25.7 ÷ 2.2 = 11.7, so order 12 packs. The same tracing gives you the perimeter too if you draw a path around the skirting line.

Gardens, plots and site plans

The method is identical on a site or location plan — calibrate against a labelled boundary or the scale bar, then trace the lawn, patio or plot. One caveat at very small scales (1:1250 location plans): a single pixel is roughly a quarter of a metre, so treat plot areas from location plans as rough estimates and prefer the largest- scale drawing you have. (More on scales in architectural scales explained.)

FAQ

How do I calculate the area of an L-shaped or irregular room?

You don't need to split it into rectangles — trace the room's outline as a single polygon, one click per corner, and the tool computes the exact area of that shape (Scale Rule uses the shoelace formula). Any number of corners works, including L-shapes, bays and angled walls.

How much extra should I add for flooring or turf?

Common practice is to add about 10% to the measured area for cutting waste on straight-lay flooring or turf, and 15% or more for patterned or diagonal layouts. Always follow the supplier's guidance for the specific product.

Should I measure to the walls' inside face or outside face?

It depends what the number is for. For flooring, paint or furniture, trace the inside face of the walls (the usable floor). Conventions like GIA (gross internal area) also use internal faces; gross external area (GEA) traces the outside. Be consistent, and say which you measured.

Trace it, don't slice it. Areas off any plan, free, in your browser — nothing uploads.

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