This post is mainly my answer to the corresponding question.
The main rule for accurate measurements is to define correct CRS for your layers and project. This is somewhat wide theme to talk about so I won't ;-)
Lets just discuss what you should do when the project CRS can't be easily used for measurement (like Web Mercator) and you want to use Measure tool to calculate distance or area. Go Settings -> Project Properties, in CRS tab enable on the fly transformation; in General tab in Measure tool menu choose ellipsoid for distance calculations. See picture:
When you start measurements with the Measure tool move your cursor over the Measure window - an information about measurement settings will pop up just like that:
Now you can make your measurements and be sure that they are accurate. NOTE that you should always check Measure tool settings like it shown at the picture above because Ellipsoid settings for the Measure tool are not always stored or displayed in Project settings correctly (QGIS 2.6.1).
When you start measurements with the Measure tool move your cursor over the Measure window - an information about measurement settings will pop up just like that:
Now you can make your measurements and be sure that they are accurate. NOTE that you should always check Measure tool settings like it shown at the picture above because Ellipsoid settings for the Measure tool are not always stored or displayed in Project settings correctly (QGIS 2.6.1).
