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 1.22477