Apart from lengths (feet vs. metres) I don't really find myself needing to calculate between the different units. I couldn't tell you how many litres in a 4 pint bottle of milk (or a pint of beer :)) but all other milk (or beer) is sold with those units, so I can easily compare.
I mean, we are supposed to be metric - but there are plenty of imperial measures in use (height, is another obvious example).
Strangely enough, whilst I'm fine with lengths and distances in metres, the minute someone tries to tell me how tall they are in metres and centimetres, I'm completely thrown. It's the one conversion I pathetically still really struggle with (UK -> Poland in this case).
I had the very same problem, only in the opposite direction, living in Poland for the first 27 years of my life and moving to the UK just over a year ago.
Just bear in mind that 6ft is approximately 180 cm, anything above is above, and anything lower is lower. This gives you a first approximation with only remembering one number. Also, 4in is about 10cm and is easy to add/subtract both ways. This might not be very exact, but it will give you a quick idea.
Apart from lengths (feet vs. metres) I don't really find myself needing to calculate between the different units. I couldn't tell you how many litres in a 4 pint bottle of milk (or a pint of beer :)) but all other milk (or beer) is sold with those units, so I can easily compare.
I mean, we are supposed to be metric - but there are plenty of imperial measures in use (height, is another obvious example).