The subtitle is "Mistakes, dimension lines, symbols, and scales.
A simple answer is that dimensions are read on construction blueprints by reading the scale in the title block. Then draw the dimension line between the extension lines and read the written number, and not a ruler measurement. Calculate the smaller dimensions and compare them to the overall dimension; read the notes and symbols (such as TYP., O.C., and CL) to understand exactly what each dimension represents.
One of those things you can't learn to read on the first day, but you can figure out by the 3rd month, is reading the dimensions on the construction blueprints. The first time you unroll a set of drawings, the lines cross over the lines, the numbers are in strange places, and the little symbols appear to have a significance.
If you've heard this before, you're not alone. Contractors, project owners and new estimators ask us this question on an almost daily basis, so here is the walkthrough we would provide a new team member at Design Estimation.
Why blueprint dimensions are so important
Dimensions are used to understand the size, length, height and distance from each other of all things in a building. They are a key component of every wall, window, beam, and slab. A misread dimension can result in ordering the wrong amount of material, cutting a framing member short, or pouring a footing in the wrong place. Those errors cost you money and time.
It's even more important for estimators. Quantities for concrete, drywall, flooring, roofing and lumber all come straight from the plans. Takeoff off, bid off.
Use Scale: Start with the title block.
Don't read any dimension without first referring to the title block, typically in the bottom right corner of the sheet. It displays the project name, sheet number, revision date and the scale.
The scale is the ratio of the drawing to the actual building. One of the standard architectural scales is 1/4" = 1'-0", which means that for every quarter inch on the paper, there is a 1' in reality. Engineering scales (e.g., 1" = 20') are frequently used on site plans. When it has detail views, a single sheet may contain more than one scale—check when in doubt.
Be aware that printed and scanned copies may shrink or expand. Hence, you always can rely on written dimensions over ruler measurements.
Extend lines and use dimension lines and arrows.
The vast majority of dimensions are constructed using a relatively small number of components:
Extension lines are extended from the edge of the object and indicate exactly what is being measured.
• The number is placed on dimension lines, which are located between the extension lines.
• The ends of the dimension line are indicated with arrowheads, ticks or dots.
• The real size of the text is called dimension text.
Inspect the location of the extension lines. In walls they may refer to the face of the stud, the face of the finish, or the center line. That little factor can make a difference of several inches in a measurement.
Interpretation of feet and inches.
Size is typically expressed in feet and inches in the USA. That is, 12'-6" represents twelve feet, six inches and a single " represents feet and a double " represents inches. Fractions also are present, such as 48", and 3'-4 1/2" is actually 3 feet 4 and a half inches.
Metric drawings involve the use of millimeters, and the unit is typically indicated on the drawing sheet. Never assume. Review general notes to make sure you don't confuse units.
Overall, partial and baseline dimensions.
When a dimension string is added to a drawing, it can include multiple dimension strings:
• The overall dimensions indicate the overall length or width of the building.
• Partial (chain) dimensions divide the total into parts, e.g., room widths and window spacing.
• The baseline dimensions measure from a starting point.
It is good practice to sum up the parts of the dimensions and check with the whole. If they don't match, indicate it, ask for clarification and continue. It is inexpensive to find a discrepancy on paper. It can't be found on site.
The grid lines and centerlines.
In larger projects, columns, walls and structural elements are designated by letters and numbers in numbered and lettered bubbles on the grid lines. Often dimensions are taken from one grid line to another. A dashed line that has alternating long and short dashes is generally a centerline, and the dimension will extend to the center of the object, such as a wall or column, not to the face of the object. Search for CL, or centerline.
Elevations, sections and vertical dimensions
Horizontal measurements are indicated in floor plans. Elevations and sections are required for height. Elevations represent exterior views of the building, including floor, roof, window and door heights. Sections cut through the building to expose the thickness of the walls, ceiling height and the depth of the foundations.
Markers such as EL may also be present. A height above a designated reference point, usually finished floor or site datum is referred to as 100'-0".
For detailed drawing conventions throughout the United States, the National CAD Standard is a good reference as to how drawings are organized and labeled.
Here are some of the most common abbreviations you need to be familiar with.
• The dimension repeats in similar locations (typical).
• On center (O.C.) indicates the distance between the center of the studs, e.g., 16" O.C.
• R.O. is a rough opening—a hole in the frame for a window or door.
• Unless otherwise stated, it is U.N.O.—unless otherwise.
• Approximate, or "to be confirmed in the field," is indicated by ±.
• N.T.S. is an acronym that means not to scale. If it is necessary to measure from these drawings, do not do so, but use the written numbers.
The key to minimizing dimension mistakes is to keep these tips in mind.
• Read first the general notes and legends.
• Do not scale from a drawing that has a scale dimension.
• When architectural, structural and MEP drawings are sometimes conflicting, cross-check.
• Beware of revision clouds and always use the current revision.
• Use dimensions emphasizing as you go to prevent omissions.
• Ask questions early. Weeks of rework can be saved by asking a quick question about information.
From dimensions to accurate estimates
The starting point of a quantity takeoff is reading dimensions. Once you are comfortable interpreting lengths, areas and heights, you can determine the amount of concrete, steel, framing, flooring, paint or roofing required for a project and apply labor and material pricing. It's labor-intensive, and even seasoned contractors can miss hours on large and complicated plans.
We can help with that. Design estimation also offers professional construction estimation services, which involve meticulous blueprint reading and quantity take-off followed by cost estimation. Your drawings are reviewed, dimensions are checked and we provide you with clear, organized estimates to feel confident in bidding and focus on winning & constructing projects.
Frequently asked questions
The most convenient method of reading dimensions on blueprints?
Verify the scale in the title block and measure by reading the numbers on the dimension lines, rather than measuring with a ruler, between extension lines.
What does 1/4" = 1'-0" mean on a blueprint?
All dimensions on the drawing are 1/4" to 1' in the actual building.
Having a mix of dimensions on blueprints that don't add up... what?
Mismatches may occur due to rounding or due to errors in revision or in drafting. If in doubt, mark them with a flag and ask what they are. Use flags and ask questions before estimating or building.
May blueprint readings and takeoffs be outsourced?
Yes. A lot of contractors will employ professional estimators to get a precise takeoff and accurate cost estimate more quickly.
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