Guides

How to dimension an isometric drawing

An isometric sketch shows the shape. Dimensions turn it into something you can build. The rules are the ones you’d use on a flat drawing, plus one more: every dimension lies in the plane of the face it measures.

The four parts of a dimension

• Extension lines, also called witness lines, carry an edge out past the part so the size doesn’t sit on top of the drawing. Leave a small gap where they leave the part, and let them run a little past the dimension line.
• The dimension line runs between the extension lines, parallel to the edge it measures.
• End marks show where it stops: arrowheads, or short slanted ticks. Ticks are quicker by hand and stay clear at small sizes.
• The number. Write the size along the line or level on the page (see below), and note the units once for the whole sheet.

Rules for isometric dimensions

1. Keep each dimension in its face’s plane. Decide which face the edge belongs to. The extension lines follow that face’s other direction, and the dimension line runs parallel to the edge. On the block above, the bottom edge gets vertical extension lines, the height takes its extension lines from the depth direction, and the depth on top takes them from the width direction.
2. Place dimensions off the part. Keep them outside the outline where you can, with smaller sizes closest to the part and overall sizes outside them, so extension lines never cross a dimension line.
3. Give each size once. If the width is on the front, don’t repeat it at the back. Repeated sizes drift apart when the design changes.
4. Dimension edges you can see. Don’t dimension to hidden edges. If a feature is out of sight, give it its own view.
5. Measure along the three axes only. Only lines parallel to the isometric directions show true length. A slope, chamfer or angled cut is foreshortened, so locate its two ends with dimensions along the axes instead of measuring along it.
6. Holes show as ellipses. Mark the center with short center lines along the axes, locate it from two edges, and give the diameter in a note with a leader, such as ⌀8.

Aligned or unidirectional text

Aligned text runs along the dimension line and tilts with it, the way the numbers on the block above are written. Unidirectional text stays level on the page whichever way the line runs, so every number reads without turning your head. Drafting practice accepts both. On a sketch, pick one and keep it for the whole drawing. Aligned is usually faster by hand, because you write along the line you just drew.

A worked example

Take the block in the figure: 50 mm wide, 30 mm deep and 30 mm tall.

1. Draw the block. Start at the front corner: the vertical edge, then the two bottom edges along the lattice, then close the top.
2. Width. From both ends of the bottom-right edge, drop extension lines straight down. Draw the dimension line between them, tick both ends, and write 50 along it.
3. Height. From the bottom and top of the left edge, run extension lines out along the depth direction, away from the block. Dimension between them and write 30.
4. Depth. On the top face, carry both ends of the right-hand top edge out along the width direction, and dimension it 30.
5. Units. Write “All sizes in mm” once, in a corner.

Three dimensions, one per axis, and the block is fully defined. On a real part, the features come next: holes, slots and cutouts, each located from an edge you’ve already dimensioned.