YUV 4:2:0 planar looks like this:
----------------------
| Y | Cb|Cr |
----------------------
where:
Y = width x height pixels (bytes)
Cb = Y / 4 pixels (bytes)
Cr = Y / 4 pixels (bytes)
Total num pixels (bytes) = width * height * 3 / 2
This is how pixels are placed in 4:2:0 sub-sampling:
As you can see, each chroma value is shared between 4 luma-pixels.