Scales and dimensions

A floor of Kingsway Tower is about forty meters long. A drawing sheet is less than a meter wide. To fit the building onto paper, every drawing is made at a reduced size called its scale, and to build from the drawing you need to know two things: what scale it is, and where to find the real measurements. This lesson covers both, and the most important rule of all: build from the dimensions written on the drawing, not from measuring the paper.

TRD-102 covered ratios, proportions and scale drawings as math. Here the focus is on how scales and dimensions appear on real construction drawings and how tradespeople use them.

What a scale means

A scale is a ratio between the size on the drawing and the size of the real thing. American construction drawings normally use inch-foot scales. At 1/4 in = 1 ft 0 in, written on drawings as 1/4" = 1'-0", each quarter inch on paper represents one foot in the building. At 1/8" = 1'-0", each eighth of an inch represents a foot, so the drawing is half as large. The larger the fraction, the larger the drawing and the more detail it can show.

Typical scales on an American building project look like this. The same pattern holds on most jobs, though each firm has its preferences. Each architectural scale can also be written as a ratio: 1/4" = 1'-0" is 1:48, because one foot is 48 quarter inches.

Site plans are drawn by civil engineers with an engineering scale, in which one inch represents a number of feet: 10, 20, 30, 40, 50 or 60. Distances on a site plan are often in feet and decimal feet rather than feet and inches. Always check the units note.

Scale rules and scale bars

An architect's scale has eleven scales on its three-sided body: 3/32, 1/8, 3/16, 1/4, 3/8, 1/2, 3/4, 1, 1-1/2 and 3 inches to the foot, plus a full-size inch rule. Each scale is read from zero, with the subdivided end giving inches. An engineer's scale has the 10, 20, 30, 40, 50 and 60 divisions used on site plans. Many sheets also carry a graphic scale bar, a small ruler printed on the drawing itself, marked in feet.

The scale bar has one big advantage. Drawings are often printed at half size to make them easier to carry, or viewed on a tablet at whatever size the screen makes them. A half-size print of a 1/8" = 1'-0" drawing is effectively 1/16" = 1'-0", and the written scale is now wrong. The scale bar shrinks with the print, so it stays correct. If a sheet has no scale bar and you are not sure of the print size, do not measure it at all.

The golden rule: do not scale the drawing

Almost every drawing set carries a general note that says something like: do not scale drawings; use written dimensions; report discrepancies to the consultant before proceeding. There are good reasons for the rule.

• Paper stretches and shrinks with humidity, and prints are not always exactly the stated size.
• Drawings are sometimes edited without redrawing everything to scale, so a line may sit in the wrong place even though its dimension is correct.
• Many items are drawn with symbols that are not their true size, such as fixtures, outlets and valves.
• Some views are marked NTS, not to scale, because they show arrangement rather than size, like riser diagrams and single-line diagrams.

Scaling is still useful for a quick estimate or to locate something that has no dimension, such as a light fixture in the middle of a ceiling. When you do scale, treat the result as approximate and confirm it before you commit to anything permanent, such as cutting a hole through a slab. If a dimension you need is missing, ask; do not build from a scaled measurement for anything that matters.

Reading dimensions

Dimensions are the written measurements on a drawing. A dimension has four parts:

• Extension lines, thin lines that extend from the feature being measured, with a small gap so they do not touch the object.
• The dimension line, a thin line between the extension lines, parallel to the measurement.
• Terminators at each end of the dimension line: arrowheads, slashes called ticks, or dots. Architectural drawings usually use ticks.
• The dimension text, written above or in a break in the dimension line.

On American drawings, dimensions are written in feet and inches: 12'-6" is twelve feet six inches, and the dash between feet and inches is always there, even for 12'-0" or 0'-8". Small dimensions may be written in inches alone, such as 8". Elevations are written in feet and inches or in decimal feet, such as 100'-0" or 112.75'.

Strings, overalls and reference points

On plans, dimensions are usually grouped in strings: a row of chained dimensions that add up to an overall dimension shown on a second line. The strings tell you the size of each piece; the overall checks the total. A good habit is to add up every string you use. If the pieces do not add up to the overall, there is an error somewhere, and you report it before anyone builds to the wrong number.

Every dimension measures from somewhere. On architectural plans, check whether a dimension goes to the face of a wall, the center line of a wall, the face of a stud or block, or the face of the finished surface. General notes usually say which convention the drawings follow, for example that dimensions are to the face of stud or face of concrete unless noted otherwise. The difference can be 5/8 or 3/4 in, which is enough to put a toilet flange in the wrong place.

Grid lines, which lesson 5 covers in detail, are the master reference. Structural columns are located on grid lines, and most other dimensions run from a grid line. When you lay out work, measure from the nearest grid line rather than chaining from a wall that may itself be slightly off. That way errors do not build up from one dimension to the next.

Typical, similar and other words that save space

Drawings use short notes to avoid repeating the same dimension many times. TYP or typical means the dimension or detail applies to every similar condition, so one dimension on one bathroom may govern all of them. SIM or similar means the condition is like another one shown, but not necessarily identical, so check what differs. EQ on a dimension means the spaces are equal, for example EQ EQ EQ across three windows, and you divide the overall by the number of spaces. UNO or U.N.O. means unless noted otherwise, which tells you a general rule has exceptions written elsewhere on the drawings. Clear or CLR means the dimension is the open space between two surfaces, not to their centers.

These words are small, but they carry a lot of meaning. A TYP note on a detail can apply to dozens of locations, and missing it means doing the same thing wrong many times.

Tolerances

A drawing gives the intended size, but no one builds to the exact sixteenth. Specifications and standards set tolerances, the amount a finished dimension may vary. Concrete slabs, for example, have flatness and level tolerances in the concrete specification, and partition layouts have their own. Tradespeople need to know the tolerance for their own work and to respect the work of others: a pipe sleeve set an inch off may be acceptable to the plumber but may land in the middle of a wall the framer has to move.

Scale and dimension checks in code

This program does two everyday jobs: it converts a measurement taken on a print to a real length for a given scale, allowing for half-size prints, and it checks that a string of dimensions closes to the overall.

def real_length(paper, scale, print_factor=1.0):
    """paper length x scale; print_factor 0.5 means a half-size print."""
    return paper * scale / print_factor

def check_string(pieces, overall):
    total = sum(pieces)
    diff = overall - total
    return total, diff

# a measurement on paper
print(f"Full-size print: {real_length(4.5, 48):.0f} in")
print(f"Half-size print: {real_length(2.25, 48, 0.5):.0f} in")

total, diff = check_string([48, 36, 96, 36, 78], 300)
print(f"String total {total} vs overall 300: difference {diff} in")
print("String closes" if diff == 0 else "Report the discrepancy before building")

Full-size print: 216 in
Half-size print: 216 in
String total 294 vs overall 300: difference 6 in
Report the discrepancy before building

The paper measurement on the half-size print is half as long, and dividing by the print factor brings it back to the same real length. Writing the string in a single unit, millimeters or inches, makes the check a simple sum.

Measuring on a tablet

Plan-viewing apps add a measuring tool that many apprentices love, and it is worth knowing how it works before you trust it. The app does not know the scale of a drawing on its own. Someone has to calibrate each sheet by picking two points a known distance apart, usually the ends of a long written dimension or the grid lines, and typing in that distance. If the calibration is done carefully on a long dimension, the measuring tool is accurate enough for estimating. If it was done on a short dimension, or on a sheet whose views are at different scales, every measurement on that sheet is off.

Treat a tablet measurement the same way as a measurement with a scale rule on paper: good for an estimate, never a substitute for a written dimension. Before relying on it, check it against a dimension you can read on the same view. If the tool says a grid bay is the size the drawing says it is, the calibration is probably good for that view.

Converting between scales

Sometimes you need to compare two views drawn at different scales, such as a 1/8" = 1'-0" floor plan and a 1/4" = 1'-0" enlarged plan of the same restroom. A feature on the enlarged plan is drawn twice as large as on the floor plan, because a quarter inch is twice an eighth. The real size is the same; only the drawing size changes. The same reasoning lets you predict how large something will appear on another sheet: a 3'-0" door is 3/8 in wide at 1/8" = 1'-0", 3/4 in at 1/4" = 1'-0" and 2-1/4 in at 3/4" = 1'-0".

Knowing roughly how large common items should look at each scale is a useful sanity check. If a door looks far too wide on a plan, either the scale is not what you think, the print is not full size, or the plan is drawn at a different scale from the one in the title block.