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 only in the United States and Burma. Other countries use the "System International" (or "SI") for drawing units. This is based on the metric system where all dimensions are measured in millimeters. Since a millimeter is very small (about 1/32"), dimensions of buildings are shown in whole numbers with no suffix, such as mm after them. As an example, a dimension of 1'-0" would be noted in SI units as simply 300. Some parts of Europe still use the old European metric standard which uses centimeters and decimals of a centimeter. 1'-0" in that 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. The U.S. Government has mandated that all drawings made for federal construction projects use both systems beginning in 1993. AutoCAD is flexible in that one may begin using "Architectural" units of measurement, and switch to "Decimal" units later.

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

- 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 the plot 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 a fractional zoom xp factor. For instance, if you want to plot a viewport at 1:50 scale, switch to model space, make the viewport you want to set at a scale current, and change the paper space zoom factor by typing the following: z<RET>1/50xp<RET>
- The second method is to draw the building using the Imperial System (feet and inches), using architectural units, with the main unit of measurement being inches. Turn variable DIMALT on, set DIMALTF at 25.4, and set DIMALTD to 0. Then when you do your dimensioning using the dimensioning program in AutoCAD, you will automatically get the dimension in feet and inches, followed by the calculated metric dimension in millimeters, rounded off to the nearest whole millimeter. An example of a dimension might be the following:
- The third method would be to draw the building in the Imperial System, with which 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 these steps:
- Create the drawing using Architectural Units.
- Set "Units" variable to "Decimal"
- UPDATE all dimensions that were drawn by typing DIM <RET> UPDATE <RET> and then using a crossing window to select the entire drawing 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 rounded off 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 drywall thickness (12.7 mm and 15.9 mm). When noting a material dimension in millimeters, use the number with a decimal fraction, if fraction is non-zero, then a space, then the abbreviation mm (no periods). Like this:

Metric plot scales are as follows (with roughly equivalent Imperial plot scales 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")

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")

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:

<------- 2x4 [50x100] STUDS @ 16" [40] O.C.

This approach and is usually
called "SOFT METRIC."

Interior area is measured in m5 (square meter), site area is measured in ha (hectares -- 1 ha=10 000 m5). To convert SF to m5, multiply by 0.09290304. To convert acres to ha, multiply by 0.404687.

Mass (what we usually loosely call "weight") is measured in kg (kilograms) or metric tons (a metric ton is 1 000 kg). To convert Lb to kg, multiply by 0.453592.

Force (for example, the amount of "force" required to open a door, which we usually measure in pounds) is measured in N (Newtons). To convert Lb to N, multiply by 4.44822. To convert LBF to N/m, multiply by 14.5939.

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

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

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

Heat energy (which we measure in Btu, or British Thermal Units) is measured in J (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 roofing is designated in pounds of weight per square. For instance, standard roofing felt used 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 feet of area on the roof. However, a square would be equal to 9.29 m5 of roofing, and the felt weight would be measured in kg, so you might logically say that you wanted 6.8 kg roofing felt, but the standard measurement area in the metric system is m5, so you would have to be more specific and say you wanted "0.73 kg/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 the exponent sign in your AutoCAD notes when expressing temperature or square meters. To make a degree symbol, type %%d in the DTEXT command, before typing 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 there manually, and scale the size down a little, so m5 is not read as m2. I recommend that 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 the ACAD directory. Then when you want to use the m5 expression, you simply place the cursor where you want it to appear, make the current layer the ANO layer, type insert<RET>m2<RET><RET><RET><RET>. That will place the block which looks like m5 on your note, thus creating a very professional and metrically correct 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's name, and L (liter), which would be confused with the number 1 if used in lowercase. Never use a period after the abbreviation.

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 write 10,000.00, in metric would be written 10 000.00. It is acceptable to use the period as a decimal point in the SI system.

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