Lobster wrote:

I'm about to lay a floor screed (approx 3m x 1.8m area) using a 3:1 mix
of sharp sand:cement, using a mixer. I can easily come up with the
volume of wet mixture I need; but where can I find out the number of
bags of cement and sharp sand I need to pick up from Wickes?!!

Sand has a bulk density of between 1.4 and 1.6 kg/l (T/m^3)As you are
making a mortar, the higher value will apply. The cement fills in the
interstices, so make no allowance for that or the water. Once you have
calculated how many kilos of sand, decide on your mix. Calculating the
cement should then be a doddle.

John Schmitt