Calculations in metric are easy, it is a case of thinking in terms of tens, hundreds, thousands, etc.
In metric things can be measured in one unit only, and there are different units that can be used. For example, the size of an object can be measured in centimetres or millimetres. But if you choose to use cm for one dimension, then it makes sense to use cm for the other dimensions too rather than mm.
In metric there are no vulgar fractions, everything is decimal. This makes the mental arithmetic easy for a lot of situations, such as calculating the area of a wall (for wallpapering, for example). You measure the height and length, in centimetres, divide by 100 to get metres, multiply the two figures to get square metres; e.g. suppose you measure the height and it is 250 centimetres, and the length of the wall is 410 cm. You could say this is 2.5 metres and 4.1 metres, or just round down to 4. Then multiply 2.5 by 4, and this is easy, it works out as 10, which means the wall needs 10 square metres of wallpaper to cover it, but because wallpapering always requires more than you use, and the wall is 10 cm longer, add on 20% to get 12 square metres. If the rolls of paper are each 75 cm wide, then divide 12 by 0.75, which is the same as multiplying by 1.33, which gives a figure of 16, which is the length of the wallpaper required, i.e. 16 metres, for that wall. Add in what you need for the other walls and then you get the total amount of wallpaper you would need.
Other calculations you might want to do might be more complex, and there are calculators and computers to help us, of course. Not everyone can do mental arithmetic, and computers and calculators are ideally suited for calculations in decimal numbers. Also, using a calculator is good for larger figures and makes your calculations more accurate.