Please allow me to quote the book "QCAD - An Introduction to Computer- Aided Design (CAD)" page 234:
"The Scale of Isometric Projections
One of the advantages of isometric projections compared to other three-dimensional projections is that the scale along all three axes of the Cartesian coordinate system is identical. If a cube is projected, all edges have the same length in the isometric projection. However, the length in the projection is not identical to the real length or original length as shown in the orthogonal projection. The projected lengths in an isometric projection are about 0.81647 times the true length. The exact scale factor can be calculated as the cosine of 35 degrees and 16 minutes.
Traditionally, isometric projections are often scaled 1:1 because they are easier to produce in real scale. An isometric projection that is scaled to 100% of the true scale is called an Isometric Drawing. With QCAD, you can produce an isometric drawing by scaling the isometric projection with a factor of 1.224779 or 1 / cos(35°16')."
Note: QCAD >3.0 will have an option for "True scale" (as opposed to "True projection" as default). "True scale" produces a traditional isometric drawing.