A reference line converts image pixels into real-world distance. Use the longest clear dimension you can match precisely, enter the correct unit and check the outline before relying on the area.
1. Choose a trustworthy known distance
Look for a printed room width, wall length, opening or outside dimension whose two endpoints can also be located on the main floor plan. A longer straight distance usually reduces the effect of placing an endpoint a few pixels away from the correct position.
Convert the printed value before entering it. Architectural dimensions such as 13200 or 9200 are commonly millimetres, so enter 13.2 m or 9.2 m. Do not use a total that spans a different drawing, a site-plan inset or a dimension whose endpoints are unclear.
- Prefer a long horizontal or vertical dimension with clear endpoint ticks.
- Use the endpoints on the floor-plan drawing, not the distant printed number or dimension line.
- Confirm whether the value is in millimetres, metres, feet or inches before entering it.
2. Add the reference line in Simple mode
Open the completed result and leave the editor in Simple mode. In Set the plan scale, choose Add reference line. Drag from the first matching endpoint on the floor plan to the second. The line snaps horizontally or vertically when it is close to those directions.
Zoom in before drawing when wall faces or dimension ticks are close together. Place both endpoints using the same boundary convention—for example, inner wall face to inner wall face—used by the printed dimension.
3. Enter the real length
After drawing, enter the known length beside the reference line. FloorSQ immediately calculates metres per pixel and updates every included outline area. A valid reference line takes priority over an automatically detected reference room.
If the result changes sharply, recheck the unit and both endpoints. One reference line assumes the image has not been stretched differently across its width and height, so a distorted screenshot can still produce an inaccurate area.
metres per pixel = known real length ÷ reference-line pixel lengthestimated area = selected outline pixel area × metres per pixel × metres per pixel4. Review the outline and cross-check the scale
Check that the blue outline includes the intended rooms and excludes unrelated plans, large blank regions and site-plan details. Adjust the outline where it does not follow the boundary you want to measure.
When another printed distance is available, compare it with the first one. In Space Planner you can add a measurement ruler as a cross-check. A meaningful disagreement often indicates image stretching, the wrong endpoints or dimensions taken from different drawing scales.