Length measurements of freehand selections

Hi all,

I have a similar kind of a problem in measuring the length of the curved line.

  1. When I use a free hand tool & draw a curved line & click measure (ctrl +M) gives length. Other wise it gives zero. If there are more than one such line in an image & try calculating measure it gives zero.
    The first two measure are just when I draw them & click measure separately. Then 3,4 are those that I select these lines using magic wand tool & click measure. Why the length is 0.0? The 5 measurement is just by selecting the line tool & click measure. Also why other measure like area is looking different?
    Here is the screen shot:

  2. My second question is that I want to know how this measurements works (I mean mathematically, say want to know what formula they use). Where do I get such information?

Thanks in advance,


The Wand Tool is an area selection tool. Area selections do not have a defined length.

  • To measure the length of the circumference:

    • Use Edit > Selection > Area to Line and then measure
  • To measure the length of the center line:

    • Use Edit > Selection > Create Mask
    • then Plugins > Skeleton > Skeletonize (2D/3D)
    • then Analyze > Skeleton > Analyze Skeleton

The Area measurement of line selections is determined by the line width that can be configured by double-clicking on the respective tool button.

ImageJ is open source software. You can get the algorithms by browsing the source code.


Thanks for the reply Jan Eglinger. I found another module “Measure_Roi” which does a similar thing. Here it is, http://www.optinav.com/Measure-Roi.htm

If the Feret’s diameter really is what you want to measure (that’s what Measure_Roi is doing, according to its description), you can simply use the built-in measurements, no installation of an outdated plugin necessary.

My impression was that you want to measure the length along the curved line, hence my suggestions in the previous post.