From To V mV kV MV
V 1 1000 0.001 1e-06
mV 0.001 1 1e-06 0
kV 1000 1e+06 1 0.001
MV 1e+06 1e+09 1000 1

Each cell shows the factor to convert 1 unit of the row header into the column header.

Highlighted diagonal cells represent identity conversions (1:1).