【2026 Mechanical Drawings】Rules for dimension marking in mechanical drawings
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I. Basic Principles 1. The true size of the components shall be based on the dimension values indicated in the drawings, regardless of the size of the drawings or the accuracy of the drawing. 2. In the drawings (including technical requirements and other specifications), when dimensions are given in mm, there is no need to indicate the code or name of the unit of measurement; if other units are used, the corresponding code or name of the unit of measurement must be specified. 3. The dimensions indicated in the drawing represent the final finished dimensions of the component shown in that drawing; otherwise, separate clarification shall be provided. 4. Each dimension of a component is generally indicated only once, and it should be marked on the drawing that shows the structure most clearly.
II. Dimension composition As shown in the figure above, a complete dimension generally should include the dimension number, the dimension line, the dimension boundary lines, and arrows or slashes indicating the ends of the dimension line. 1. The numerical value of a linear dimension should generally be indicated above the dimension line; it is also allowed to be indicated at an interruption of the dimension line. 2. Dimension lines shall be drawn as thin solid lines; they cannot be replaced by other line types, and generally shall not overlap with other lines or be drawn on their extensions. When marking linear dimensions, the dimension line must be parallel to the line segment being measured ； When there are several mutually parallel dimension lines, the larger dimension should be indicated outside the smaller one to prevent the dimension lines from intersecting the dimension boundaries. When marking diameter or radius dimensions on a circle or arc, the dimension line should generally pass through the center of the circle or its extension. The ends of dimension lines come in two forms, as shown in the figure: arrows are suitable for all types of drawings, and in the figure b represents the width of the thick solid line ； The slant lines are drawn as thin solid lines, and h in the figure represents the font height. The ends of the dimension lines for the diameter of a circle, the radius of an arc, and angles should be drawn as arrows. When using the slanted format, the dimension lines and dimension boundaries must be perpendicular to each other. 3. Dimension lines shall be drawn as thin solid lines, and shall originate from the contour lines, axes, or lines of symmetry of the figure. Contour lines, axes, or lines of symmetry can also be used as dimensioning boundaries. Dimension lines should generally be perpendicular to the dimension lines and extend about 2 mm beyond their ends.
III. Examples of dimensioning ●●● Direction of dimension numbers Linear dimensions: The dimension numbers should be written in the direction shown in the upper left diagram, and efforts should be made to avoid marking dimensions within the area indicated by 30 in the diagram; if this is not possible, the numbers can be marked as shown in the upper right diagram. Second method: When it will not cause confusion, for dimensions that are not horizontal, the numbers can be written horizontally at the break in the dimension line, as shown in the two diagrams below. In a single diagram, the same method should be used as much as possible; generally, the first method should be employed for labeling.
●●●The dimension lines for angles should be drawn radially, with the dimension lines forming arcs whose center is the vertex of the angle. Dimension numbers should always be written horizontally, usually at the break in the dimension line; if necessary, they can also be marked as shown in the figure on the right.
●●●The diameter of a circle should generally be indicated according to these two examples.
●●●Arc: The radius dimension of an arc should generally be indicated as shown in these two examples.
●●●When it is not possible to mark the center of a large arc within the drawing area, it can be indicated as shown in the figure on the left； When it is not necessary to indicate the center position, it can be marked as shown in the figure on the right.
●●●Small size: As indicated in the legend above, when there is not enough space, the arrows can be drawn outside, or small dots can be used to replace the two arrows； The dimension numbers can also be written outside or in leader annotations. The small sizes of circles and arcs can be indicated with the two rows of example illustrations.
●●●Sphere: To indicate the dimensions of a sphere, as shown in the two diagrams on the left, “S” should be added before φ or R. It can be omitted if it does not cause confusion, such as the spherical surface at the right end in the figure above.
●●●Chord length and arc length: When marking the chord length and arc length, as shown in these two examples, the boundary lines should be parallel to the perpendicular bisector of the chord. When marking the arc length scale, the dimension line should be an arc, and the symbol “^” should be placed above the dimension value.
●●●For symmetrical parts drawn only half or more than half, the dimensions 84 and 64 should have their dimension lines extending slightly beyond the line of symmetry or the boundary line at the break point; arrows are placed only at the ends of the dimension lines, while two parallel thin solid lines perpendicular to it are drawn at each end of the line of symmetry (the symmetry symbol).