Short definition
A graphical bar or labelled dimension on a floor plan used to calibrate measurements against a known real-world length.
A scale bar is a graphical or labelled reference on a floor plan that tells you the relationship between distance on the page and distance on the floor. It can take three forms: a printed graphic bar with tick marks labelled in metres, a written notation like "1:100" or "1:50", or a dimension line on the drawing labelled with its real-world length (e.g. an internal wall marked 4.20m).
In a digital takeoff, the scale bar is what makes every other measurement possible. You set the scale once — click two points on the plan that correspond to a known real-world length, type that length in, and from then on every polygon you draw reports its area in true square metres.
Three things to watch for. First: PDFs of plans often display at a scale that differs from the printed scale — a 1:100 plan exported to a PDF and viewed at 100% zoom on a 96 dpi monitor is not actually 1:100 on screen. Always calibrate against a dimension on the plan, not the "1:100" label. Second: multi-page plans usually have a different scale on each page (a site plan at 1:200, floor plans at 1:100, details at 1:20) — calibrate each page separately. Third: when the plan has no scale bar and no labelled dimensions, the only fallback is a known fixture (a standard 820mm or 920mm doorway, a 600mm wide kitchen cabinet) — accept that the takeoff will be approximate until a real measurement is taken on site.
In AreaSum, scale calibration is the first step on every page. You click two points along a labelled dimension (a wall length, a door width, an explicit dimension line), enter the real-world length, and the page is locked to that scale until you change it.