Dimension Anatomy and Placement

A dimension connects a number to a specific feature

A drawing shows the shape. Its dimensions tell the reader how big the features are and where they belong. Read the stated value; do not measure the picture.

An original plate example labels the dimension line, extension lines, visible gap, arrowheads, dimension text and a leader pointing to a hole. — Thin dimensioning lines keep the heavier part outline easy to follow. These teaching sketches illustrate dimensioning choices; they are not complete manufacturing drawings.

Dimension line: Runs in the direction being measured. Its arrowheads mark the extent of the dimension.
Extension line: Projects an edge, center or other point out to the dimension. Leave a visible gap at the part and continue a little beyond the last related dimension line.
Arrowheads and text: The arrows identify the endpoints; the text gives the value and any needed symbol. Use a consistent arrow style and readable text size.
Leader: A thin line connecting a note or callout to its feature. A hole leader ends with an arrow on the circle, directed toward its center.

Keep the dimension line separate from the part outline. A visible edge or centerline is not a substitute for a dimension line. For line types and views, see the Engineering Graphics collection’s Reading Drawings group.

Put dimensions outside the view, with the shortest nearest the part

Keep the outline clear. Put short dimensions close to the view and longer dimensions farther out. This also reduces the number of extension lines that cross dimension lines.

Two good and bad comparisons: moving dimensions outside a tabbed plate clears its outline; putting the short dimension inside the overall dimension eliminates a crossing. — Good placement makes both the shape and the dimension’s endpoints easier to find.

A dimension inside the view can be appropriate when it is clearer there or avoids an excessively long leader. The aim is clarity, not an empty interior at any cost.

Leave room for the lines, arrows and text

Use at least 10 mm from the part outline to the first dimension line and 6 mm between successive parallel dimension lines as layout guides. Those are distances on the drawing sheet, regardless of the part’s scale. The familiar inch drafting guides are about 3/8 in and 1/4 in; they are rounded guides, not exact conversions.

Good layout leaves 10 mm to the first dimension and 6 mm to the next, with visible extension-line gaps and overruns. Bad layout crowds both dimension lines against the outline. — Check the final printed size. A readable CAD screen does not guarantee a readable reduced print.

Keep the gap at the part visible and extend the extension line slightly beyond the dimension line. About 2 mm of overrun is a useful drafting target. Increase spacing when the annotation needs more room.

Which parts are standards, and which are layout guides?

ASME Y14.5 governs dimensioning conventions; its spacing guidance allows for legibility and reproduction. The current edition governs new ASME work unless the drawing or contract specifies another edition. ASME currently lists Y14.5-2018 (R2024). These pages explain conventions in plain words rather than reproducing the standard. For an existing drawing, read its stated standard and revision.

Keep text readable from the bottom of the sheet

Use unidirectional text for the ordinary linear dimensions here: the numbers read from the bottom even when the dimension line is vertical. Leave clear space around the number. On a horizontal line, the text may sit above it or in a break in it.

Good and bad examples compare horizontal readable text with rotated text, outside arrows for a narrow dimension with crowded arrows, and staggered values with a tight column of numbers. — Fit the value and arrows between the extension lines when there is room. Move them outside when there is not; do not shrink the lettering to squeeze it in.

Stagger values on several parallel dimension lines so they are easier to separate. Keep lettering free of part outlines, extension lines and leaders.

Why older drawings may look different

Aligned text rotates with the dimension line. You may meet it on older drawings and in other drafting systems. “Old” and “new” alone are not enough to identify the applicable rule. These examples use the ordinary ASME orthographic convention; specialized coordinate dimensioning has additional presentation rules.

Rearrange crossing lines before accepting clutter; Practice

Rearrange crossing lines before accepting clutter

Move dimensions or notes to avoid crossing dimension lines, extension lines and leaders wherever practical. Keep a leader short, with a clear arrow target and a short horizontal landing next to the note.

Good leaders reach their own holes without crossing; bad leaders cross. A second comparison shows a radial leader hitting a circle clearly versus a tangent leader grazing it. — A reader should be able to follow each line from value to feature without guessing.

If extension lines must cross, normally leave them continuous. Do not introduce gaps that make their endpoints ambiguous. A break near an arrowhead can preserve its readability.

Next: choose the information the drawing needs. Practice placement