C-3.2 Use Scale Rulers to Determine Actual Dimensions from a Piping Diagram
In the piping trades, most of the drawings you work from are not shown at their actual size. Buildings, mechanical rooms, and piping systems are far too large to fit on a page, so designers reduce the size of the drawing while keeping every part in proportion. To understand these drawings and to correctly determine the real-world lengths of pipe and the locations of fittings, you must be able to use a scale ruler.
A scale ruler allows you to read measurements that have been reduced by a certain ratio, such as 1/4″ = 1′-0″ in imperial units or 1:50 in metric. When you understand how these ratios work, you can accurately interpret piping diagrams, estimate material lengths, and verify dimensions on both imperial and metric plans.
This section will teach you how to read and use architect’s (imperial) scales and metric scales, how to convert scale measurements into real-world dimensions, and how to avoid common mistakes when measuring from drawings. By the end, you will be able to confidently obtain approximate pipe lengths directly from a drawing whenever written dimensions are not provided.
Scale drawings are accurate and convenient visual representations made and used by engineers, architects, and people in the construction trades. The accuracy is achieved because the drawing is proportional to the real object. The convenience comes from the reduced size of the drawing. It is large enough to provide the desired detail but small enough to be manageable.
The flexibility to draw proportionally in different sizes is provided by the use of scales. Since piping systems are large compared to piping drawings, we will only be concerned with reduction scales. Reduction scales make the drawing smaller than the object. The kinds of rules we will typically use to make scaled drawings are the architect’s scale and the metric scale, both shown in Figure 3.
Proportional Scaling
The scale of the drawing is always written on the drawing. The scale is the ratio of the size of the drawing to the object. For drawings smaller than the object, the ratio will be that of a smaller distance to a larger one.
Architect’s scales use ratios of inches to a foot. The most common architect’s scale used is [latex]\frac{1}{4}\text{"}[/latex] to the foot, written on drawings as:
[latex]\quad\text{Scale } \frac{1}{4}\text{"} = 1\text{'}-0\text{"}[/latex]
This means that a line [latex]\frac{1}{4}\text{"}[/latex] long on the drawing represents an object that is 1 foot long. At the same scale, a line [latex]1\frac{1}{2}\text{"}[/latex] long represents an object 6′ long, because [latex]1\frac{1}{2}\text{"}[/latex] contains six quarter-inches.
Metric scale ratios use the same units in both ratio terms, resulting in an expression of how many times smaller than the object the drawing is. For example, the standard metric scale ratio that approximately corresponds to [latex]\frac{1}{4}\text{"} = 1\text{'}-0\text{"}[/latex] is written on drawings as:
[latex]\quad\text{Scale 1:50}[/latex]
This means that the object is actually 50 times as large as what’s shown on the drawing, so 50 mm on the object is represented by 1 mm on the drawing. For another example, 30 mm on the drawing represents [latex]50 \times 30[/latex]
[latex]= 1,500 \text{ mm (or 1.5 metres) on the object}[/latex].
Table 1 lists the scale ratios commonly used for building plans and construction drawings in both metric and the equivalent architectural scale ratios.
|
Type of Drawing |
Common Metric Ratios |
Use |
Imperial Equivalents and Ratios |
|
|
Site Plan |
1:500 1:200 |
To locate the building, the services and reference points on the site. |
1″ = 40′-0″ [latex]\frac{1}{16}\text{"}[/latex] = 1′-0″ |
(1:480) (1:192) |
|
Sketch Plans General Locations Drawings |
1:200 1:100 1:50 |
To show the overall design of the building. To indicate the juxtaposition of the rooms and locate the positions of piping systems and components. |
[latex]\frac{1}{16}\text{"}[/latex] = 1′-0″ [latex]\frac{1}{8}\text{"}[/latex] = 1′-0″ [latex]\frac{1}{4}\text{"}[/latex] = 1′-0″ |
(1:192) (1:96) (1:48) |
|
|
1:20 |
To show the detail of system components and assemblies. |
[latex]\frac{1}{2}\text{"}[/latex] = 1′-0″ |
(1:24) |
|
Construction |
1:10 |
1″ = 1′-0″ |
(1:12) |
|
|
Details |
1:5 |
3″ = 1′-0″ |
(1:4) |
|
|
|
1:1 |
Full size |
(1:1) |
|
Architect’s (Imperial) Scales
Traditional architectural measurements of length are written very precisely in feet and inches using the appropriate symbols for feet and inches, separated by a hyphen (e.g., [latex]4\text{'}-3\frac{1}{2}\text{" and } 7\text{'}-0\text{"}[/latex]). This is the way that all imperial measurements should be written on construction drawings.
Listed below are the scales found on the architect’s triangular scale rule.
- [latex]\frac{3}{32}\text{"} = 1\text{'}-0\text{"}[/latex] 7. [latex]1\text{"} = 1\text{'}-0\text{"}[/latex]
- [latex]\frac{3}{16}\text{"} = 1\text{'}-0\text{"}[/latex] 8. [latex]\frac{1}{2}\text{"} = 1\text{'}-0\text{"}[/latex]
- [latex]\frac{1}{8}\text{"} = 1\text{'}-0\text{"}[/latex] 9. [latex]1\frac{1}{2}\text{"} = 1\text{'}-0\text{"}[/latex]
- [latex]\frac{1}{4}\text{"} = 1\text{'}-0\text{"}[/latex] 10. [latex]3\text{"} = 1\text{'}-0\text{"}[/latex]
- [latex]\frac{3}{4}\text{"} = 1\text{'}-0\text{"}[/latex] 11. [latex]1\text{"} = 1\text{'}-0\text{" (full size - use the scale labelled "16")}[/latex]
- [latex]\frac{3}{8}\text{"} = 1\text{'}-0\text{"}[/latex]
Figure 4 shows the ends of one face of an architect’s imperial triangular scale rule. There are two edges on each face, and each edge contains two scales that run in opposite directions. At each end of an edge, a number or fraction indicates the distance in inches that represents 1 foot. The top edge is in eighths of an inch from left to right and in quarters of an inch from right to left. Note that on a full rule the 1/8″ scale from 0 to the right end represents 95 feet and the 1/4″ scale from 0 to the left end represents 47 feet.
Watch the following video by MTCopeland (2020) on YouTube titled “How to Use an Architect Scale Ruler” [3:40].
If you are using a printed copy, you can scan the QR code with your digital device to go directly to the video: How to Use an Architect Scale Ruler
At each end, between the zero and the number indicating scale, the length representing 1 foot is subdivided into 6, 12, 24 or more parts to indicate inches and, in some scales, fractions of an inch. For example, each of the six marks on the [latex]\frac{1}{8}\text{"}[/latex] scale represents 2 inches, while each mark equals a quarter of an inch on the 1″ foot-reduction scale and 1 inch on the [latex]\frac{1}{4}\text{"}[/latex] scale.
Now look at the [latex]1\frac{1}{2}\text{"}[/latex] scale in Figure 5. The subdivided foot unit is divided into inches and fractions of an inch. Reading left from the zero, notice the figures “3”, “6” and “9,” which represent measurements of 3″, 6″ and 9″. From the zero to the first long mark represents 1 inch. Between the zero and the 1-inch mark there are four lines, each of which represents one quarter of an inch.
Piping drawings usually use a [latex]\frac{1}{8}\text{"}[/latex] scale for larger buildings, a [latex]\frac{1}{4}\text{"}[/latex] scale for smaller buildings and houses and a [latex]\frac{1}{2}\text{"}[/latex] scale for details.
Each drawing will state in the title box the scale that is used. Sometimes when special details are given, the scale is placed directly under the detail.
To draw or measure a length to scale, first find the edge of the rule containing the scale. One end of the length will rest exactly on one of the foot marks of the scale, and the other end should rest either on the zero marker or somewhere on the inch subdivision of the scale. The length can then be marked and drawn or read off from a drawing.
Figures 6 and 7 demonstrate this manner of reading dimensions from four of the ratios on the architect’s scale.
Metric Scales
A triangular metric scale is similar to the architectural scale in that it has six edges, but it has only one scale ratio per edge. The ratio is marked at the left end of the scale. For example, the scale of 1:50 means that 1 mm on the drawing represents 50 mm on the object. This means that the object is 50 times larger than the drawing of it. An object 450 mm long would be represented by a line 9 mm long [latex](\frac{450\text{ mm}}{50})[/latex].
Figure 8 shows one of the three sides of a metric scale. The scale labelled 1:50 is read from left to right, from 0 to 15 m. The scale labelled 1:2 can also be read from left to right (0 to 600 mm) by turning the scale around.
Watch the following video by Steve Bailes (2012) on YouTube titled “Metric Scale” [2:07].
If you are using a printed copy, you can scan the QR code with your digital device to go directly to the video: Metric Scale
If the ratio is 1:1, it means that 1 mm on the drawing represents 1 mm. In other words, the object in the drawing is being drawn to its actual size.
You will notice that all the edges on a metric scale are marked with spaces that are 1 mm apart, similar to a metric tape measure. The difference is that each edge is marked off or labelled according to a different ratio, so that proportionate lengths are read directly from the scale. This eliminates the need to calculate dimensions.
Figure 9 shows common metric scales for comparison. Notice that all the scales shown are labelled in metres and that 0.5 m = 500 mm. All the scales in Figure 9 are marked at the scaled position of 250 mm.
Obtain Dimensions From Drawings
The best way to get exact dimensions from drawings is to use the explicit dimensions (in millimetres or in feet and inches) written between the dimension lines. Any measurements that you need should be somewhere on the drawings. Drawings normally only give each dimension once. If there are a number of parallel lengths, only one will have a measurement. To find the dimension you need, you may need to refer to other views, or you may have to add or subtract other dimensions.
Measuring lines of a drawing to determine the measurement is not an accurate way to extract dimensions. This is because the drawing is only a representation and may not be exact. Photocopies and reproductions of drawings also may not be to the scale of the original.
The scale of the drawing and lack of attention in measuring can lead to inaccuracies. For example, if the scale of a drawing is [latex]\frac{1}{8}\text{"} = 1\text{'} - 0\text{"}[/latex], an error of [latex]\frac{1}{32}\text{"}[/latex] in measuring the plan amounts to 3″ of error in the actual object measured. Detail drawings permit more exact precision because they are proportionately larger. However, details often require more exactness and usually contain any needed dimensions.
When accuracy is not required and approximate dimensions are adequate, measuring plans is a quick method of taking off material for estimating the cost of a job. In such cases, about 10% is usually added for cut-off and waste allowance.
If you use the scale of the drawing, it will be simple to read off the measurements. However, in the field you will often need approximate measurements and the only measuring tool at hand will be a measuring tape.
A steel pocket tape measure has a movable hook on the end that allows accurate measuring, either when butted against a surface or when hooked on the end of an object (Figure 11). The end of the flexible tape itself is shortened to allow for the hook.
To avoid any error, and to place the tape flat on the drawing, use a convenient unit mark, such as 100 mm as the starting point for measuring, as in Figure 12.
The distance between the dimension lines is 40 mm. Since the scale is 1:50, the centre-to-centre length is:
[latex]\quad40 \text{ mm} \times 50 = 2,000\text{ mm}[/latex]
If the scale had been 1:100 the length would be [latex]40 \text{ mm} \times 100 = 4,000\text{ mm}[/latex].
Determining approximate piping lengths from imperial scale drawings follows the same procedure. An imperial tape measure is used in the same way to find a length on the drawing in inches. As an example, suppose the length is [latex]7\frac{1}{2}\text{"}[/latex]. To find the length represented, use the scale of the drawing.
In every case, if you divide the length on the drawing by the scale fraction or number, you will be calculating the length in feet. Since dividing by a fraction is the same as multiplying by its reciprocal, you multiply the drawn length in inches by:
- 4 when the scale is [latex]\frac{1}{4}\text{"} = 1\text{'} - 0\text{"}[/latex]
- 8 when the scale is [latex]\frac{1}{8}\text{"} = 1\text{'} - 0\text{"}[/latex]
- 2 when the scale is [latex]\frac{1}{2}\text{"} = 1\text{'} - 0\text{"}[/latex]
|
Scale |
[latex]\frac{3}{32}[/latex] |
[latex]\frac{1}{8}[/latex] |
[latex]\frac{3}{16}[/latex] |
[latex]\frac{1}{4}[/latex] |
[latex]\frac{3}{8}[/latex] |
[latex]\frac{1}{2}[/latex] |
[latex]\frac{3}{4}[/latex] |
[latex]1[/latex] |
[latex]1\frac{1}{2}\space (\frac{3}{2})[/latex] |
[latex]3[/latex] |
|
Reciprocal |
[latex]\frac{32}{3}[/latex] |
[latex]8[/latex] |
[latex]\frac{16}{3}[/latex] |
[latex]4[/latex] |
[latex]\frac{8}{3}[/latex] |
[latex]2[/latex] |
[latex]\frac{4}{3}[/latex] |
[latex]1[/latex] |
[latex]\frac{2}{3}[/latex] |
[latex]\frac{1}{3}[/latex] |
The [latex]7\frac{1}{2}\text{"}[/latex] length on a drawing scaled [latex]\frac{1}{4}\text{"} = 1\text{'} - 0\text{"}[/latex] would represent:
[latex]\quad7\frac{1}{2}\text{"} \times 4 = 30\text{'}[/latex]
On a scale of [latex]\frac{3}{8}\text{"} = 1\text{'} - 0\text{"}[/latex], the same [latex]7\frac{1}{2}\text{"}[/latex] drawing length represents:
[latex]\quad7\frac{1}{2}\text{"} \times \frac{8}{3} = 20\text{'}[/latex]
Figure 13 is part of a piping run drawn to the scale of [latex]\frac{1}{8}\text{"} = 1\text{'} - 0\text{"}[/latex]. The total length of the run is:
[latex]\quad4\frac{1}{4}\text{"} \times 8 = 34\text{'}[/latex].
Self-Test C-3.2: Use Scale Rulers to Determine Actual Dimensions from a Piping Diagram
Complete Self-Test C-3.2 and check your answers.
If you are using a printed copy, please find Self-Test C-3.2 and Answer Key at the end of this section. If you prefer, you can scan the QR code with your digital device to go directly to the interactive Self-Test.
References
Bailes, S. (2012). Metric scale [Video]. YouTube. https://www.youtube.com/watch?v=w6__oQQI3oE
BCcampus. (n.d.). Playlist: Tools and equipment videos. BCcampus MediaSpace. https://media.bccampus.ca/playlist/details/0_3g8xp22x/categoryId/175673 Playlist Details – Trades Access Common Core Line C: Tools and Equipment Videos – BCcampus
BC Industry Training Authority. (2019). Piping trades apprenticeship program: Use Tools and Equipment—Level 1 harmonized [Binder]. Crown Publications, Queen’s Printer for British Columbia. https://www.crownpub.bc.ca/Product/Details/7960000261_S
- Plumber: Competency C-2 Interpret Drawings and Specifications
- Steamfitter: Competency C-2 Interpret Drawings and Specifications
- Sprinkler Fitter: Competency C-2 Interpret Drawings and Specifications
Camosun College. (2019). Line D: Tools and Equipment—Competency D-3: Competency D-3: Read Drawings and Specifications (Rev. ed.) [Learning guide]. BCcampus. https://collection.bccampus.ca/textbook/yxJ3AwTa/
Camosun College. (2015). Trades Access Common Core Competency D-3: Competency D-3: Read Drawings and Specifications. Victoria, B.C.: Crown Publications. Download for free from the B.C. Open Textbook Collection (https://open.bccampus.ca/browse-ourcollection/find-open-textbooks/).
MTCopeland. (2020). How to use an architect scale ruler [Video]. YouTube. https://www.youtube.com/watch?v=-0g7BwFhWW8&t=5s
Media Attributions
All figures are sourced from Industry Training Authority (2019) and/or Camosun College (2019) and are used under the Creative Commons Attribution 4.0 (CC BY 4.0) licence unless otherwise noted. Images copyrighted by the BC Industry Training Authority are licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 (CC BY-NC-SA 4.0) licence.
- Figure 1 Acropolis, Propylaea: reconstruction elevation from the west, by Mnesicles on Flickr is used under the CC BY-NC 2.0 license.
- Figure 2 House Numeric Labels , by Cmdrjameson (adapted original by İnfoCan) on Wikimedia Commons is used under the CC BY-SA 3.0 license.
- Figure 10 Detailed 2D Steel Structure CAD Drawing [Model: Nano Banana 2] is from Easy-Peasy.AI and is used under a CC BY 4.0 license.
Parts used to connect sections of pipe, such as elbows or couplings. (Section C-3.2)
A tool used to measure drawings that are not full size. (Section C-3.2)
A measurement (like length, width, or height) shown on a drawing. (Section C-3.2)
A special ruler used to measure drawings in inches and feet, often used in building plans. (Section C-3.2)
A picture of something that is made smaller or larger but keeps the same shape and proportions. (Section C-3.2)
When all parts of a drawing match the real object in the correct size relationship. (Section C-3.2)
A scale that makes a drawing smaller than the real object. (Section C-3.2)
The relationship between the size of a drawing and the actual size of the object. (Section C-3.2)
A section on a drawing that shows important information like the scale and drawing name. (Section C-3.2)
A three-sided ruler used to measure drawings that are not full size. Each side of the triangle has two edges, giving you six different scale ratios to work with. (Section C-3.2)
A measuring system that uses millimetres and metres to read drawings. (Section C-3.2)
