I have a line which snaps to guidelines, with a width of 1.25 cm.  I've added a "dimension" line with arrows at both ends, but when it's been snapped to the guidelines, it's width is 1.329 cm.

I've tested this on some lines of x width, and when I add arrows their widths increase.

Is there any way to prevent this weird behaviour?

I´d be nice to attache a sample file.

Otherwise you can try LPE MeasureSegments which I find very convenient:

Sorry, I thought this must be a "known" issue as it's so easy to reproduce - a sample is attached.

The "LPE Measure Segments" you mentioned looks interesting, although I could only find an animated gif show casing it.

Is this an extension?  Where would I find it?

Cheers, Max

It is common for markers (arrowheads etc.) to cover the end of the line to hide the corners (or "cap") of the stroke.

Hi Tyler, I've noticed that.  I was just a bit surprised that the "reported" length of such a line changes when adding the arrows. 

It means I should never specify a Width or Height of such a line and use it in a tech drawing, as it would not really be correct.

Cheers, Max

The Preferences>Tools allow for the visual or geometric bounding-box, if that helps.

Perfect, many thanks Tyler!

Why am I not surprised Inkscape has the answer to this - I really like using it. 🙂

Cheers, Max