Imperial Feet and Inches to SI Metric

8/8/2019 Imperial Feet and Inches to SI Metric

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Drawing in Metric Units

The drawing unit type typically used in the U.S.A. is known in the industry as

"Imperial" units (feet and inches). This type of measurement system is used onlyin the United States and Burma. Other countries use the "SystemInternational" (or "SI") for drawing units. This is based on the metric systemwhere all dimensions are measured in millimeters. Since a millimeter is verysmall (about 1/32"), dimensions of buildings are shown in whole numbers with nosuffix, such as mm after them. As an example, a dimension of 1'-0" would benoted in SI units as simply 300. Some parts of Europe still use the old Europeanmetric standard which uses centimeters and decimals of a centimeter. 1'-0" inthat system would be noted as 30.00cm. The United States is slowly "inching"toward using the SI system of measurement, but because our industrial base is

so entrenched in the Imperial system, it will take many years to accomplish. TheU.S. Government has mandated that all drawings made for federal constructionprojects use both systems beginning in 1993. AutoCAD is flexible in that onemay begin using "Architectural" units of measurement, and switch to "Decimal"units later.

There are three possible methods of creating metric (SI) type drawings:

1. The first method is to draw the building in metric system right from the start,and plot in metric system. This is the simplest approach. To do this, you set

up the units as decimal units, select mm as the dimension for plotting in theplot dialogue box, and select a metric plot scale and a metric sheet size.Note that in plotting in paper space, use the metric plot scale directly as afractional zoom xp factor. For instance, if you want to plot a viewport at 1:50scale, switch to model space, make the viewport you want to set at a scalecurrent, and change the paper space zoom factor by typing the following:z1/50xp

Metric plot scales are as follows (with roughly equivalent Imperial plotscales in parentheses):

1:1 (Full Size)

1:2 (Half Size)

1:5 (3"= 1'-0")

1:10 (1 1/2"=1'-0")

1:20 (3/4"=1'-0")

Page 1 of 5Drawing in Metric Units

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1:25 (1/2"=1'-0")

1:50 (1/4"=1'-0")

1:100 (1/8"=1'-0")

1:200 (1/16"=1'-0")

1:250 (1"=20'-0")

1:500 (1"=40'-0")

1:1000 (1"=80'-0")

Standard metric drawing sheet sizes in millimeters are as follows:

A4: 297x210 (11.7"x8.3")

A3: 420x297 (16.5"x11.7")

A2: 594x420 (23.4"x16.5")

A1: 841x594 (33.1"x23.4")

A0: 1189x841 (46.8"x33.1")

2. The second method is to draw the building using the Imperial System (feetand inches), using architectural units, with the main unit of measurementbeing inches. Turn variable DIMALT on, set DIMALTF at 25.4, and setDIMALTD to 0. Then when you do your dimensioning using thedimensioning program in AutoCAD, you will automatically get the dimensionin feet and inches, followed by the calculated metric dimension inmillimeters, rounded off to the nearest whole millimeter. An example of adimension might be the following:

3'-6: [1067]

When noting the drawing, always indicate sizes in both Imperial units

followed by SI units. An example of a note would be the following:


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This approach and is usually called "SOFT METRIC."

3. The third method would be to draw the building in the Imperial System, withwhich you are familiar, and then convert the entire drawing to the metric

system. In AutoCAD, this process is not straightforward, nor is it intuitive.Thus it is usually called "HARD METRIC" conversion. You follow thesesteps:

Create the drawing using Architectural Units. Set "Units" variable to "Decimal" UPDATE all dimensions that were drawn by typing DIM

UPDATE and then using a crossing window to select the entiredrawing with all dimensions in the window.

"Scale" (increase in size) the entire drawing model (in Model Space)

from base point 0,0 at a scale factor of 25.4 Change "Units" precision to 0 (no places after decimal point) UPDATE all dimensions that were drawn again. Select mm units when plotting, not inches Plot to some standard metric scale. Change notes to reflect proper metric dimensions, areas, forces,

volumes, etc. Some hard converted material dimensions are roundedoff to even millimeters, such as ceiling tile, which is usually 600x1200(approximately 24" x 48"), and plywood sheets which are 1200x2400(approximately 4'x8' sheet size), and brick (90x57x190) but others are

the precise equivalent of the Imperial size, such as actual drywallthickness (12.7 mm and 15.9 mm). When noting a material dimensionin millimeters, use the number with a decimal fraction, if fraction is non-zero, then a space, then the abbreviation mm (no periods). Like this:


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Pressure (as we would usually measure in pounds per square foot, or psf) ismeasured in Pa (Pascals). To convert PSF to Pa, multiply by 47.8803. Toconvert PSI to Pa, multiply by 6.89476.

Temperature is measured in ?C (degrees Celcius), or ?K (degrees Kelvin). Toconvert degrees Farenheit to degrees Celcius, multiply by 5/9(?F-32).

Liquid volume (which we measure in gallons) is measured in L (liters, or 1 000cm;). To convert gallons to L, multiply by 3.78541.

Heat energy (which we measure in Btu, or British Thermal Units) is measured inJ (Joules). To convert Btu to J, multiply by 1055.056.

Some Imperial measurements may have to continue in use, for example,pennyweight for measuring nails, such as a 10d nail, which is 76.2 mm long (3").The meaning of the pennyweight is not convertable, since it related to the original

cost in England of 100 nails of that size.

Another example is a "square" of roofing, which is 100 square feet. Most roofingis designated in pounds of weight per square. For instance, standard roofing feltused for a first course under an asphalt shingle roof on a house is called "15#roofing felt." That means that the material weighs 15 pounds per 100 square feetof area on the roof. However, a square would be equal to 9.29 m5 of roofing, andthe felt weight would be measured in kg, so you might logically say that youwanted 6.8 kg roofing felt, but the standard measurement area in the metricsystem is m5, so you would have to be more specific and say you wanted "0.73kg/m5 roofing felt." Needless to say, this measurement is not normally used.

One of the difficult things to do is to be able to use the degree symbol and theexponent sign in your AutoCAD notes when expressing temperature or squaremeters. To make a degree symbol, type %%d in the DTEXT command, beforetyping the C. That will show in your drawing as degrees C.

If you want a raised exponent to appear in the text line, you have to place it theremanually, and scale the size down a little, so m5 is not read as m2. I recommendthat you do this once, make a block out of it, call it m2, write that block to a file,which you can then copy to your hard drive, either in the student directory, or theACAD directory. Then when you want to use the m5 expression, you simplyplace the cursor where you want it to appear, make the current layer the ANOlayer, type insertm2. That will place the blockwhich looks like m5 on your note, thus creating a very professional and metricallycorrect look.

Note that most metric unit abbreviations are lower case, with the exception of N(Newton), J (Joule), Pa (Pascal), A (Ampere -- same as in English), W (Watt --same as in English), because these are all derived from a famous scientist'sname, and L (liter), which would be confused with the number 1 if used inlowercase. Never use a period after the abbreviation.




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Note that commas are never used in metric numbers as thousands separators.Instead, a space is used for this purpose. That is because a comma is"commonly" used in Europe as a decimal point. Therefore, where we would write10,000.00, in metric would be written 10 000.00. It is acceptable to use theperiod as a decimal point in the SI system.

For decimal numbers less than one, always use a zero before the decimal point.



Imperial Feet and Inches to SI Metric

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