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.

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.

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.

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.

Supporting detail: Addendum 1 — Which parts are standards, and which are layout guides?.

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.

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

Supporting detail: Addendum 2 — Why older drawings may look different.

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.

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

Ask what information is necessary to make this part

Start with the geometry, not with the dimension tool. Break the part into familiar forms: blocks, cylinders, holes, steps, slots and arcs.

Size dimensions

Describe the feature itself: a block’s length, width and thickness; a hole’s diameter; a slot’s width and length.

Location dimensions

Place that feature relative to other features or a chosen origin. A hole diameter alone does not say where the hole goes.

Locate a round hole by its center, not by a tangent edge that moves when the diameter changes. Choose reference surfaces that make sense for how the part fits and is checked. A formal datum has a specific meaning in geometric tolerancing; an arbitrary sketch origin is not automatically a datum.

Dimension a feature in the view that shows its shape

Contour dimensioning keeps a feature’s dimensions with the view where its outline is clearest. Put a hole size on a view showing its circle. Put a step’s height where the step is visible. Use a section or another suitable view for an internal feature.

For the beginner drawings here, do not dimension to hidden lines: choose a view that exposes the feature. Keep the dimensions of one feature together when practical. See the Reading Drawings group in Engineering Graphics for projection, section views and view selection.

Give each independent requirement once

Completeness means every feature is defined. It does not mean every possible distance is dimensioned. A feature may need several dimensions, but the same size or location requirement should not be repeated in another view or restated by a closed chain.

A closed chain gives all the segments and their total as controlling dimensions. Remove one link, or make a useful derived value a reference dimension. Which link to remove depends on what the part must do.

Supporting detail: Addendum 3 — Why nominal arithmetic is not enough.

Parentheses mark information, not an extra acceptance requirement

A reference dimension is shown in parentheses, such as (1.50). It helps the reader without imposing another manufacturing or inspection requirement. Use one when the derived value is useful; leave it out when it only adds clutter.

In the two-hole example, .80 and 2.30 can control the centers while (1.50) reports their nominal spacing. Parentheses do not mean that the physical distance cannot vary.

A basic dimension, shown in a rectangular box, is different: it supplies a theoretically exact value for a geometric tolerance scheme. It does not demand a physically perfect part. See Tolerances and Fits before interpreting limits or fits.

Find a redundant dimension Count the required dimensions

Read the units before reading the numbers

Find the drawing’s unit note or title block first. On an inch drawing, .625 means 0.625 inch, not 625 inches and not millimeters. The unit need not be repeated beside every value when the drawing states it.

Decimal-inch dimensions conventionally omit the leading zero below one inch. Keep trailing zeros: .50 and .500 have the same nominal size, but may select different rows in the drawing’s general tolerance block. There is no universal “three decimals means ±.005” rule; read the actual block and any local tolerance.

An angle still needs its angle notation: 90°. Mixed units must be identified. For units and precision, use Measurement Language; for tolerance values, limits and fits, use Tolerances and Fits.

Use a diameter for a full circle and a radius for an arc

⌀ before a value means diameter. R before a value means radius. Thus ⌀.60 describes a full circular diameter of .60, while R.30 describes an arc radius of .30. The values are related, but the callouts describe different kinds of feature.

n X means the requirement applies in n places. A repeated size still needs an unambiguous pattern: center coordinates, spacing, or a fully defined bolt circle. A sketch that merely looks evenly spaced is not enough.

A round-ended slot needs size, location and direction

For a straight slot with semicircular ends, give its width and either its overall length or the distance between the end centers. Make the chosen meaning clear. Then locate the slot and establish its direction relative to the part.

Do not control the width, end radius, center spacing and overall length all at once. That repeats the relationships already built into the semicircular-ended shape. If a slot is not semicircular-ended, this shortcut does not apply.

Read a hole callout from the main hole to the added recess

The first size usually describes the main hole. Read the remaining symbols with the values immediately following them. All examples below are invented decimal-inch teaching values.

Read the leader and section view to identify the entry side. A hole callout supplies size and end condition; it still needs location dimensions. A countersink angle is not automatically 90°.

A spotface provides a local flat seat

A spotface provides a flat circular seating surface around a hole. Its modern symbol puts SF inside the counterbore-shaped mark. Older drawings may use the counterbore mark with a note or context identifying the spotface.

A spotface may have a stated depth or remaining material thickness. If neither is specified, the ASME convention is the minimum depth needed to clean up the specified diameter. Do not invent a depth from the sketch.

Supporting detail: Addendum 4 — Reading old and new symbol styles.