Choose a reliable reference and establish units
Choose a length with identifiable endpoints, supported by an original dimension or a reliable physical measurement. Do not estimate it from typical furniture sizes, the on-screen display or image DPI.
Decide whether drawing units will represent millimetres, centimetres or metres. Interpret the measured and target lengths consistently. Changing a unit label is not, on its own, evidence that geometry has been resized.
Divide target length by measured length
For an illustrative example, suppose the measured segment is 250 drawing units and should represent 1000 millimetres, with one drawing unit intended to mean one millimetre. The factor is 1000 ÷ 250 = 4. Scale the relevant geometry uniformly and measure the segment again.
Keep related geometry, labels and leaders together when appropriate. Editing a number alone does not resize its associated shape. AutoCAD also provides a reference-length option in SCALE; consult the instructions for your version when using it.
Check another location and direction
Matching one reference does not validate the entire drawing. Measure another independent length and check both horizontal and vertical directions where possible. A mismatch may indicate source stretching, a tilted photograph, paper distortion or incorrectly selected endpoints.
Uniform scaling does not remove local distortion or perspective error. Repeatedly changing the overall scale cannot resolve different errors in different regions. Prepare a better source or reconstruct the affected geometry from reliable dimensions.
Review text, geometry and plotting separately
Recognized dimension labels are editable text; do not assume that they are associative CAD dimensions. Editing their displayed values may leave the geometry unchanged. Similarly, changing a plot scale does not correct model-space lengths.
Keep an unscaled copy and record the reference used. Before delivery, measure critical geometry again and check its labels and intended units. Complete the required design review before using the drawing for manufacturing or construction.
- Measure a known reference and establish consistent units.
- Calculate a factor or use reference scaling.
- Remeasure the reference and another independent dimension.
- Check labels, geometry and output scale settings.