Surface Mount Packages Explained

PART 1: Discreet Devices

Surface-mount technology (SMT) discreet devices such as resistors, capacitors, etc. commonly use a four-digit package code such as 0402, 0603, 1206 to indicate their size on datasheets & for ordering. 

The two pairs of digits may represent the English units OR MAY represent the rounded unit in tens of mil (milli-inch) for a metric component!

e.g. 0603 may be describing

[06]     [03]
.06"   x .03" 
length   width

when the actual dimensions are 1.6mm x 0.8mm which is .0630" x .0315"

A metric version specification of the same would be 1608M where the pairs of digits represent the number of tenths of a mm. In the US, English style (milli-inches) units is far more prevalent and usually assumed. Always refer to a manufacturer's datasheet for actual device dimensions & tolerances.

Topline has a handly conversion page if you want to know the actual dimensions that correspond to the rounded English-unit description code. 

PART 2: ICs

Have you seen the terms SOIC, TSOP, SOT23, PLCC, QFP and other alphabet soup in reference to surface mount integrated circuit packages and have trouble keeping them straight? Topline to the rescue again - their SMT nomenclature guide is a nice reference. It describes most surface mount package types for ICs.