Because CAD mistakes are sometimes made when creating drawings, the scale factor and/or paper size can occasionally be incorrect.
How do I know the scale is incorrect?
You’ll first need to verify the scale. Once know a drawing’s scale is incorrect, you can then move on to tracking down where the problem is.
Incorrect paper sizes
If you’ve verified that the real world paper size is different from the paper size shown in the drawing’s title block, use the below steps to correct it:
Determine how many standard paper sizes the real world paper size is out by
(EG: the difference between A0 & A2 is two standard paper sizes).
Determine which “direction” you need to make the change to account for the difference in paper size.
(EG: if your real world paper size is A0, and the paper size in the drawing’s title block is A1, you need to decrease the scale factor by one standard paper size).
If you’re decreasing the scale factor, multiply by 0.7071 for each standard paper size you are moving
(EG: if the scale shown in the drawing’s title block is 1:100 @ A1, and your real world paper size is A0, then 100 * 0.7071 = 70.71, gives a new scale of 1:70.71 @ A0).
If you’re increasing the scale factor, multiply by 1.4142 for each standard paper size you are moving
(EG: if the scale shown in the drawings title block is 1:100 @ A0, and your real world paper size is A1, then 100 * 1.4142 = 141.42, gives a new scale of 1:141.42 @ A1).
Difference in paper sizes, expressed as percentages
The table below shows the percentage difference between standard paper sizes:
Difference in paper sizes, expressed as decimals
The table below gives a decimal figure, which can be used for increasing or decreasing the scale factor shown in a drawing’s title block, to account for incorrect paper sizes:
Incorrect scale factors
Because scales are made up of two parts (paper size & scale factor), it’s possible for either (or both) of these parts to be incorrect.
If you have verified the real world paper size is correct, or verified it’s incorrect and followed the steps above to fix it, and you still find a known measurement on the drawing is wrong, then it’s likely the scale factor is incorrect.
To correct a scale factor:
Find a known measurement on the drawing (EG: a scale bar, grid line etc).
Measure a linear measurement over your known measurement.
Adjust the number within the scale field until you linear measurement equals the length of your known measurement.
When to request clarification from your client
If either the paper size or the scale factor is incorrect, while it’s often possible to correct them, it’s a good idea to also request clarification from your client to ensure your assumptions/corrections relating to a drawing’s scale are correct.
How to: Itemise measurements.