Hi FDP - thanks for sending the file. I cannot find any error in the conversion there aside from the display rounding issue mentioned above.

Your line that is labeled "This should be 6.35mm" starts out at 0.25 inches, and when switching units to mm if you select the line it does indeed read out as 6.35mm .

Your rectangle that is labeled "This should be 170mm" starts out at 6.69 x 6.69 inches (which you can double check in Rhino using the BoundingBox command in there), and after conversion to mm it is not 170mm but that's because 6.69 inches does not convert to exactly 170 mm, 6.69 inches actually converts to 169.926 millimeters.

6.69 x 25.4 = 169.926


If you convert from inches to mm in Rhino and then use BoundingBox in Rhino you'll see the exact same result as in MoI - your rectangle gets a millimeters size of 169.926 x 169.926 in Rhino as well.

It looks like your size in inches is rounded slightly when it was originally entered? If you wanted a size of 170 mm the equivalent in inches is not a nice number with limited digits, it's something like: 6.6929133858267716.... etc

So your original value in inches is truncated and didn't get close enough to what you'd need to get the equivalent of 170mm. You maybe should create it in mm to start with, or while you're in inches you can enter in values with mm at the end which will then do a higher accuracy conversion for you, like if you draw the rectangle in unit inches enter in both 170mm in the text field for width and height and you'll then get what you wanted.

How was it that you originally created the rectangle, did you type in 169.926 as the values for width and height?

Anyway, that's why the rectangle does not convert to what you were expecting, it's because it's not actually at that size.

- Michael