If one uses Inkscape he has snapping modes 'Snap to nodes', 'Snap to path crossings' and'Snap to grids' available at the same time. As well it can seen as a plane filled with equal equilateral triangles. Copy 1 is rotated counterclockwise 60 degrees. NOTE1: It's a bunch of equally spaced vertical lines and 2 copies of that bunch.

The height of the ellipse is 1 original XY plane unit divided by sqrt(3). The blue ellipse still has width = 1 unit in the original XY-plane units because just that direction length is not distorted in isometric projection. The horizontally oriented blue ellipse in the next image is the isometric projection of 1 unit diameter XY plane circle: It can only be approximated.Ī practical way to construct the isometric image of an 1 unit diameter XY-plane circle in isometric grid is to draw the shown black ellipse and scale it to 1/sqrt(2) = 70,7% It's not exact, but probably accurate enough for art purposes. One cannot form it by adding together fractions of 1 unit. Making the grid denser does not help (see NOTE2), because sqrt(2) is an irrational number.

Unfortunately that ellipse will not snap to the grid.

You wanted an ellipse which presents a XY-plane circle which has diameter = 1 unit. Unfortunately its diameter is 1,41 units, exactly 1 unit multiplied by sqrt(2) where the sqrt means square root. It's the one which is easy to draw into the grid, because it snaps. The black ellipse presents a XY-plane circle. When measured in the image above, the lengths of the green and red lines are not 1 in the original XY-plane units, they are the isometric projections of 1 unit long sides of a square. There's as faint blue the ordinary isometric grid (see NOTE1) which actually contains the isometric images of tiled 1 unit x 1 unit x 1 unit cubes.

Isometric grid illustrator