A decoder receives luma motion vector information for plural luma motion vectors for a macroblock (e.g., a 4:2:0 macroblock). The decoder derives a chroma motion vector for each of the plural luma motion vectors by performing at least one calculation on the luma motion vector information, maintaining a 1:1 ratio of chroma motion vectors to luma motion vectors for the macroblock. For example, the decoder receives four luma (frame or field) motion vectors for a macroblock and derives four chroma motion vectors for the macroblock. The deriving can comprise sub-sampling and/or rounding (e.g., using a field-based rounding table).