They are sold by 25G, 50G etc... that's metric enough for me!!
OurKid1 if you are using a different yarn than the one specified in the pattern, you could find that you don't have enough yarn to finish the project - so I always check the length of yarn in a ball if using something different.
Lengths of yarn in a 50g ball, for instance, can vary quite a lot.
eg Rowan Moordale comes in 100g balls and has 230m in a ball
Patons Smoothie comes in 100g balls and has 200 metres in a ball.
Sirdar Snuggly has 165 metres in a 50g ball
etc
Things you find stressful that other people don't notice.