The density of aluminum is about 0.1 lb/in3 [2,700 kg/m3] and density values of aluminum alloys don't vary much because alloying elements make up a small portion of the composition. Most of aluminum alloys are within 5% of this number. This is about one-third the density of carbon and stainless steels.
Since density is the weighted average of the densities of the elements that constitute the alloy, the 5xxx and 6xxx series alloys are the lightest because magnesium is the lightest of the main alloying elements. The actual density of wrought alloys is usually close to the nominal value, but the density of castings is about 95% to 100% of the theoretical density of the cast alloy due to porosity.
A system of four-digit numerical designations is used to identify aluminum and aluminum alloys. The first digit indicates the alloy group as follows:
Wrought Aluminum alloys grouped by major alloying elements
The last two digits identify the aluminum alloy or indicate the aluminum purity. The second digit indicates modifications of the original alloy or impurity limits.
Cast Aluminum alloys grouped by major alloying elements
The second two digits identify the aluminum alloy or indicate the aluminum purity. The last digit, which is separated from the others by a decimal point, indicates the product form: that is, castings or ingot.