Dimensioning best practices for mechanical and architectural drawings

Just adding dimensions in a drawing is not always sufficient and if you want the machinist, architect or stakeholders to read your drawing exactly the same way every time then you should follow the dimensioning best practices as outlined in this article.

The source of most of these mechanical best practices (but not all) is ASME Y14.5 M – 2018.

For most of the Architectural drawing best practices, the source is National CAD Standards (NCS, USA).

Although there are tens of hundreds of guidelines for dimensioning drawings properly, in this article, I have selected only some of the most popular ones that frequently appear in a drawing.

Mechanical Drawings

This list contains 36 best practices related to mechanical drawings.

These best practices are added with illustrations wherever required.

Best Practice 1

Do not duplicate dimensions or add dimensions giving the same information in two different ways, except for the dual dimensioning or when you are adding a basic dimension.

Don’t add any dimension which is not required to make a part.

Here dimensions marked in the red box are not required and should be omitted.

Best Practice 2

Add dimensions so that no calculation is required to infer any dimension.

Also, add dimensions with the correct scale so that no scale conversion is required to infer the dimension.

Best Practice 3

Dimensions should be added to the view which shows the shape of the part clearly.

Best Practice 4

Don’t place dimensions directly on the object view unless it makes the drawing clearer.

Best Practice 5

Avoid dimension with reference to the hidden lines of the drawing view and place it with respect to center lines.

Best Practice 6

The largest dimension value should be placed furthest from the object so that smaller dimensions can be placed relatively closer to the object making the overall dimensioning clear.

Best Practice 7

One dimension should be attached to only one view of the drawing, don’t attach one dimension to multiple views using extension lines.

Best Practice 8

Dimension your drawing as per the manufacturing process used.

Best Practice 9

Never cross the dimension lines and avoid crossing extension lines as well, you can however cross extension lines if at all necessary.

When extension lines cross no break is required at the point of crossing.

Best Practice 10

Generally, a circle should be dimensioned by its diameter and an arc by it’s radius.

The prefix ⌀ should be added before the diameter value like ⌀18 and the R prefix before the radius like R36.

Best Practice 11

The radius dimension should contain only one arrowhead and it should touch the arc.

Best Practice 12

Use jogged extension lines for the radius dimension if the center of the arc falls outside the sheet or if it is in another view.

Best Practice 13

Lines in the drawing should not be used as dimension or extension lines.

Best Practice 14

Extension lines should not directly start from the part and there should be a gap of nearly 1/16″ from the object.

Extension lines should be extended nearly 1/8″ beyond dimension lines.

Best Practice 15

Add extension lines on the nearest point of view to be dimensioned.

Best Practice 16

Centerline can be used as an extension line but it should retain the properties of a centerline and not of an extension line.

Don’t use the centerline from one view for another view as well.

Best Practice 17

Dimension numbers should be placed at the centre of the dimension line except where dimensions are stacked.

In the case of stacked dimensions, they can be distributed over the dimension lines to maximize clarity.

Best Practice 18

Align dimensions in one line and group dimension lines as much as possible instead of staggering them.

Best Practice 19

Text and other notes should be placed horizontally in the drawing and not inclined to any angle or in a verticle direction.

Best Practice 20

Dimension text size should be 1/8″ for whole numbers and for fractions it should be 1/4″ for every number.

Best Practice 21

Never show hidden lines in the isometric drawing views but show the tangent lines.

Show hidden lines in orthographic view.

Best Practice 22

Leader lines should slope at angles close to 30, 45 or 60 degrees but they can be made at any angle except horizontally and vertically.

The landing of the leader line should be a straight line and not curved.

Best Practice 23

Use leader line with a note to show chamfer distance and angle.

Best Practice 24

When your drawing contains several rough features like fillets or chamfers of the same size then instead of dimensioning all the features add dimension to one and specify the number of times it is repeated in the drawing using the X symbol.

You can also specify these dimensions using a note in the drawing as well.

The “Typical” abbreviation is also widely used for these types of features but the use of the “Typical” abbreviation is not recognized by ASME Y14.5 – 2018.

So, try avoiding TYP or Typical notation with several similar dimensions as it says nothing about the number of times a feature is repeated and hence it can be in some cases interpreted in more than one way.

Best Practice 25

Place the first dimension nearly 1/2″ (∼12.7 mm) away from the object and add all stacked dimensions nearly 3/8″ (∼9.6 mm) from the nearest dimension.

Best Practice 26

Location dimension of holes and cylinders should be added in views that show them as circles and the location should be labelled from the centre line.

Best Practice 27

Dimension of holes and cylinders should be added in longitudinal view.

Best Practice 28

The dimension for a hole created with boring, drilling or other manufacturing processes should be mentioned by a note connected to a leader line where the arrow of the leader is pointed towards the center of the hole.

The manufacturing process can be added in the drawing as a note and not necessarily accompanied by the hole dimension.

Best Practice 29

Surface finish marks should be placed on the edge of the finished surface.

The finish marks should be omitted on holes or cylinders where a note specifies the machining operation which created the feature.

Best Practice 30

In metric drawing if the number is less than 1 include leading zero like 0.32 but in imperial drawing omit leading zero like .25″.

Show trailing zeros in imperial drawings like .320″ and 2.70″ and don’t show trailing zeroes in metric drawings like 0.84 and 3.4

Best Practice 31

Don’t include unit with all dimensions instead add a note specifying the unit of the drawing like “Unless otherwise specified, all dimensions are in mm“.

Add unit in the drawing only when it is clearly necessary like labelling size of a component which is in a different unit as an example 1/2” hose.

Best Practice 32

Don’t assume that parts in a drawing are symmetrical even if they look alike.

Add centerline or dimension both symmetrical parts with proper dimensions or abbreviations.

Use note if there are several symmetrical parts in the drawing.

Best Practice 33

If the view is overcrowded with different dimensions, two and more separate drawings with the same view can be produced.

Best Practice 34

To avoid several dimensions in a drawing use Ordinate dimensioning where the location of features like a hole, slot and shaft is defined from zero point in the drawing.

In this case, no arrow is required in the extension line.

In the following image, the lower-left point is the origin or point zero for both horizontal and vertical direction and all other dimensions are measured with respect to that point.

Best Practice 35

Symbols of hole features like counterbore, countersink or spotface should precede the feature diameter symbol and the depth symbol is added after feature diameter.

Best Practice 36

The height of the keyseat and keyway itself is not dimensioned rather it is dimensioned with respect to the shaft or hole.

For keyseat, dimension is from the bottom of the keyseat to the other end of the shaft and for keyway, it is from the top of the keyway to the bottom of the hole.

The width of the keyway or keyseat is added additionally.

Architectural Drawings

This list contains 20 best practices related to architectural drawings.

These best practices are added with illustrations wherever required.

Best Practice 1

Use architectural tick as arrow style as it’s one of the most popular ones used on drawings and easily recognizable.

Though there is no set standard when it comes to using arrow style and as per National Cad Standards (USA), you can use either architectural tick or even solid arrow that is popularly used in mechanical drawings.

Best Practice 2

Be consistent when using arrow style and only use one arrow style throughout the drawing.

Also, make sure ticks of architectural tick is in the same direction on the dimension line.

Best Practice 3

Do not add too many or too few dimensions to the drawings.

Add more than sufficient dimensions in the drawing for clarity but don’t over-dimension your drawing leaving no room for adjustments on the site.

On the other hand, don’t add very few dimensions leaving most of the planning to guesswork.

Best Practice 4

You can add a chain of dimensions as well as an overall dimension in your drawing.

In mechanical drawings, only the required number of dimensions are added, and usually, overall dimension is avoided as it tends to mess up the tolerance.

This is not the case with architectural drawings as there is no concern of tolerances affecting the overall dimension, so add overall dimensions in architectural drawings.

Best Practice 5

Don’t add very tight tolerances in architectural drawings where it has no practical use.

Architectural drawings are usually not made with very tight tolerances so maximum precision floats around ¼” and in some commercial buildings it can go up to 1/8”.

The dimension fractions should not be added in increments less than 1/16” as per National Cad standards.

Best Practice 6

Dimension text is placed above the dimensions line in architectural drawings not in-between dimension lines like mechanical drawings.

Best Practice 7

The outermost dimension furthest away from the drawing is the overall dimension followed by the dimension of critical components like wall locations and distance of doors and windows.

Lastly, minor details like fixture and cabinetry distances are added in the third line of the dimension which is nearest to the drawing.

Best Practice 8

In drawings use sans serif fonts to improve clarity, this is as per the National Cad Standard recommendation.

Serifs are fonts with overhangs that may not look clean on an architectural drawing and sans serif are fonts without serifs or overhangs.

In the following image yellow highlights are the overhangs that make a font “serif”.

Best Practice 9

As per National Cad Standard, minimum dimension text size should not be less than 3/32” (2.5 mm) in CAD drawings and it should not be less than 1/8” (3.2 mm) on hand-drafted drawings.

Best Practice 10

All dimensioning notations should be in uppercase and avoid italics, underline or bold fonts in dimensioning.

Best Practice 11

For dimensions more than an inch you should add it in the form of feet and inch both and not only in inches.

If, however, local building codes mandate adding dimensions in inches then you can add it in inches only.

For example, add 1’4” as dimension, not 16” even though they are the same.

Best Practice 12

Columns, fixtures (like toilet, sink etc) and openings like doors and windows are dimensioned from their centre.

Columns are often the first place where dimensioning should start in a drawing.

Best Practice 13

Try avoiding crossing dimension or extension lines but when it is unavoidable make a loop at the point of crossing.

Best Practice 14

D or Ø symbol is used as a prefix for diameter and R is used as a prefix for Radius.

We usually use dimeter for the circle and radius for dimensioning an arc.

Best Practice 15

Use dimension text that is aligned to the dimension line.

Aligned dimension text should face the bottom and right side and hence the reading direction is from left to right and bottom to top.

The dimension text is placed above and on the left side of the dimension line.

Best Practice 16

Exterior dimensions are added from the exterior face of the stud to the centre mark of openings like doors or windows.

Interior dimensions are added with respect to the wall centres.

We ignore the finish material thickness on the walls as it’s usually not accurate or uniform.

For masonry walls, we dimension from edge to edge for the wall as well as for the openings like Doors and windows.

Best Practice 17

Add the dimensions of exterior drawing elements like windows, doors outside the drawing and interior dimensions inside the drawing.

Best Practice 18

As per NCS, there should be a minimum of 1/16” (1.6 mm) gap between drawing and extension line, minimum 9/16” (14.5 mm) inch length of first extension line and minimum 3/8” (10 mm) for baseline spacing.

The baseline spacing for all parallel dimensions should remain consistent with a minimum gap of 3/8” (10 mm).

Best Practice 19

Try to keep drawing clean with organized dimensions, try not to clutter your drawings with randomly placed dimensions and keep them grouped for clarity.

Best Practice 20

Slopes are added with slope notation as shown in the image below.

Here for every 10-unit horizontal distance, the slope rises 4 units in height.

Summary

So, how many of these best practices were new to you?

If you have a best practice that you think should be included in this list then share it in the comments down below.