It should have international acceptance.
It should be of a convenient size.
It should be accepted by general conference of the measurement and units.
The S.I system is a decimal system with each component a multiple of 10. E.g 1kg=103 gram
The S.I units are coherent, that is there is only one unit for each quantity , and units of different quantities are combined without conversion factors.
The S. I system is a rational system.