The discrepancy in measurements happens when **your object has been scaled in object mode, but the transformation has not been [applied](https://docs.blender.org/manual/en/dev/editors/3dview/object/editing/transform/clear_apply.html?highlight=apply%20scale#apply)**.

As an example, look at this perfectly regular cube (2mx2mx2m) in **edit mode**:


[![enter image description here][1]][1]

What is wrong with it?


Look at the **scale** section on the panel in **object mode**:

[![enter image description here][2]][2]

If you select your object in **object mode** and press <kbd>Ctrl</kbd> <kbd>A</kbd> and select **Apply Scale**


[![enter image description here][3]][3]


The scale of the object will reset to 1,1,1

[![enter image description here][4]][4]

And the measurements in **edit mode** will reflect the real measurements of the object.

[![enter image description here][5]][5]


----------


Without applying transformations, you can also choose to display local or global measurements:

[![enter image description here][6]][6]

The measurements in **Local** units are **Relative to the Scale**, and show that the cube is 2x2x2, whereas the **Global** units show the **Absolute** measurements, revealing that the cube is no longer a cube, but a cuboid that is 4m tall and 2 meters wide.


[![enter image description here][7]][7]





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**Why should you care for local or global measurements?**
There are many operations that use the scale and rotation of the object and not its absolute dimensions 

Quote from the [blender manual](https://docs.blender.org/manual/en/dev/editors/3dview/object/editing/transform/clear_apply.html?highlight=apply%20scale#apply)

>Applying transform values essentially resets the values of object’s location, rotation or scale, while visually keeping the object data in-place.

>The object origin point is moved to the global origin, to rotation is cleared and scale values are set to 1.

>For simple cases you won't notice any difference the 3D View or rendered output, however modifiers and constraints may depend on object transformation.

Beveling, UV unwrapping, physics simulations, booleans, and many other functions will use the objects relative dimensions and will give unexpected results if the transformations have not been applied.


Please read: https://blender.stackexchange.com/questions/7298/why-is-it-important-to-apply-transformation-to-an-objects-data

  [1]: https://i.sstatic.net/Awbg4.png
  [2]: https://i.sstatic.net/dDiZI.png
  [3]: https://i.sstatic.net/2gIIx.png
  [4]: https://i.sstatic.net/SCHaG.png
  [5]: https://i.sstatic.net/u5COQ.png
  [6]: https://i.sstatic.net/kjrjn.png
  [7]:https://i.sstatic.net/dIFWo.gif