Noseall - I'm new, and you've been posting for years - so you're in a better position to advise what to do -- it seems to me that the screed estimator on this site is wrong - and badly so.
I've seen a fair bit of contradictory stuff on this site, and from other estimating tools on the web, so I've resorted to weighing cement powder! -- and it seems to me that it weighs about 1.3kg per litre -- or 1.3 tonnes/cu m -- which agrees with the Blue Circle and Lafarge data sheets. (e.g.
http://www.lafarge.co.uk/CementDatasheet/Mastercrete.pdf ) Of course cement powder can get compacted, but I'm trying to weigh it as it usually comes out of a bag.
I'm also assuming that a cube of sand weighs 1.6 tonnes - is that near enough? In other words sand weighs a fair bit more than cement.
You have posted elsewhere that a tonne of sand, for a 4:1 mix, needs about 8 bags cement -- and if I can quote another poster - "Plstering" - he says "I base most my mixes 4:1 6/7bags per tonne m8, if that helps"..
You and he seem to be correct to me, according to my weighings of sand and cement - it equates to about 5:1 by weight. (Skip these calcs if you're bored already :- if cement is 1.3 t /cu m then 1 tonne is 0.75 cu m - therefore eg 8 bags (200kg) is a fifth of 0.75 = 0.15 cu m. If you want 4 times as much sand by vol you need 0.6 cu m of sand - which should weigh a tonne, if a cube weighs 1.6 tonnes. 200kg cement is therefore a quarter of the volume of 1 tonne of sand [hence a 4:1 mix], obviously 5:1 by weight)
But the estimator sticky on this site has a 4:1 mix as 3:1 by weight ! -- it says 14 bags cement per tonne of sand ! - and the densities at the top of the estimator say that cement weighs more than sand - by a big margin - This seems just plain incorrect, but I don't want to attack a sticky without double checking..
The Source4me estimator - which lots of people seem to be trusting - looks identical to the one on this site - same bizarre densities..
By the way - 3:1 by weight gives 2.4:1 by vol (at correct densities) - that's a dangerously bad floor ...
Could I be correct?