How do you calculate cement consumption for MM thick ceiling plastering?

Calculate the area you need to treat.Refer to the product you'll be usings,coverage estimate(s). It's simple math really but somewhat tedious. LOL.Metric calculations I'm not used to,

but let's say you have some plaster and it weighs 10 lbs.And let's say a pound can be spread over a 3 sq ft area @ 1mm thick.Thats 30 sq ft of coverage for that 10 lbs of plaster.So you might need 30 lbs for that ceiling,assuming it's area is 90 sq ft. There are numerous ways to calculate coverage area. And all of them only require simple math skills.

Though I wouldn't recommend putting cement, on your ceiling generally.Your driveway yes.Your ceiling no.Gypsum plaster would be more suitable, and you can find it everywhere,premixed and ready to apply in 5 gallon buckets.Good luck.