A way of drawing a 3D object where the two horizontal axes are both set at 30 degrees to the horizontal and vertical lines stay vertical, so measurements along all three axes stay true to scale.
An isometric drawing shows the front, the side and the top of an object in a single picture, so a reader sees the whole form without turning the page.
It is a pictorial drawing, which means it looks like the object rather than being a flat view of one face.
Nothing is drawn smaller for being further away, so an isometric drawing can be measured in a way a photograph cannot.
Looking three dimensional and measuring true at the same time is why flat pack furniture sheets and product assembly guides are drawn isometrically.
Reach for it in your design folder when the reader needs to understand the shape and still check a size.
The 30 Degree Rule Is the Whole Trick
Vertical edges stay vertical on the page, exactly as they would in a front view.
Every horizontal edge is drawn at 30 degrees to the horizontal, sloping up to the right for one direction and up to the left for the other.
The three axes therefore sit 120 degrees apart on the paper, which is what gives the drawing its even, balanced look.
A 30/60 set square slid along a ruler gives you both 30 degree directions without measuring a single angle.
Start at the nearest vertical corner of the object and build outwards along the three axes from that one point.
Edges that are parallel on the real object stay parallel on the paper, so no line on an isometric drawing ever converges.
Isometric Grid Paper Does the Angles for You
Isometric grid paper is printed with lines or dots running vertically, 30 degrees left and 30 degrees right.
Choose what one grid step stands for, say 10 mm, and a 60 mm edge becomes six steps counted off the grid.
Counting steps is quicker than measuring and you physically cannot drift off the 30 degree angle.
Draw the crate first in grid steps, then cut the real shape out of it, the same habit you use in freehand sketching.
On plain paper, rule two faint 30 degree construction lines through your starting corner and work from those instead.
Tip
Fix your grid step before the first line: one step to 10 mm keeps the arithmetic easy for most product work.
Draw the whole crate lightly, including the hidden back corner, then darken only the edges you would really see.
Put the nearest vertical corner near the middle of the sheet so the object does not run off an edge.
Dimensions Stay True Along the Three Axes
Any length measured along one of the three axes is drawn at full scale, so a 40 mm edge is 40 mm on the paper whichever axis it runs on.
That is what lets somebody lay a ruler on your drawing and read off a real dimension.
Lengths that do not follow an axis, such as a diagonal across a face, come out wrong and must never be measured.
A true isometric projection shortens every axis to about 0.82 of full size, but in school and in industry the axes are drawn full size and the result is called an isometric drawing.
Angles are not true either: a 90 degree corner on the real object appears as 60 or 120 degrees on the page.
Note
Measure along the three axes only, never across a face and never through the object.
Write the scale, such as 1:2, somewhere on the drawing so nobody reads a half size drawing as full size.
Add a few key dimensions in figures as well, because a printed page may be resized.
Circles Become Ellipses, and That Is the Fiddly Part
A circle sitting on an isometric face appears as an ellipse, because that face is tilted away from you.
Draw the square the circle would fit inside first, which becomes a rhombus on the isometric face.
Fit the ellipse inside that rhombus so it touches the middle of each of the four sides.
The long axis of the ellipse runs across the rhombus at right angles to the axis of the hole or cylinder.
A cylinder is two ellipses joined by two straight lines along the axis, with the far half of the back ellipse hidden.
Isometric Flattens Depth, and It Does So on Purpose
Because nothing converges, a long object such as a 2 metre bench can look as though it flares wider at the far end.
There is no vanishing point and no foreshortening, so the drawing never gives the feeling of standing in front of the object.
Curved organic forms, like a moulded chair shell, are awkward to construct on isometric axes and usually read better in perspective.
Two different objects can produce the same isometric outline, which is exactly how the impossible Penrose triangle is drawn.
Only three faces ever show, so the back, the underside and the far side need a second isometric view or a set of flat views.
Common Mistake
Sloping a horizontal edge at 45 degrees instead of 30 gives an oblique drawing, not an isometric one.
Making far edges shorter than near ones breaks the whole point of isometric, which is that lengths stay true.
Choosing Isometric over the Alternatives
Choose isometric when the reader has to see the shape and check sizes together, for example a housing you plan to 3D print.
Choose it for an exploded view, because parts pulled apart along a shared axis stay parallel and line up cleanly.
Choose perspective instead when you are selling the look of the product to a client.
Choose a set of flat orthographic views instead when a workshop has to make the part and needs every dimension on the page.
Active recall
What angle do the two horizontal axes sit at, and what happens to vertical edges?
Which lines on an isometric drawing can be measured with a ruler, and which cannot?
How do you construct a circle on an isometric face?
Name one kind of object that suits isometric badly, and say why.
Why do flat pack assembly instructions use isometric rather than perspective?