Ribulose 1,5 bisphosphate is carboxylated and cleaved by rubisco to produce 2 molecules of 3 phosphoglycerate.
Phosphoglycerate kinase (using ATP) and 1,3 bisphosphoglycerate dehydrogenase (using NADPH) produce glyceraldehyde 3-phosphate - this can then be converted into glucose via gluconeogenic enzymes.
Stage 3 is the regeneration of our original acceptor molecule ribulose 1,5 bisphosphate
For the net formation of one molecule of glyceraldehyde 3 phosphate, we need to fix 3 molecules of CO2
So:
3 molecules of ribulose 1,5 bisphosphate (3x 5 carbons = 15 carbons total) accept 3 molecules of CO2 to generate 6 molecules of 3 phosphoglycerate (3 x 6 carbons = 18 carbons total)
These 6 molecules of 3 phosphoglycerate are reduced to 6 molecules of glyceraldehyde 3 phosphate = 18 carbons total.
ONE of these molecules of glyceraldehyde 3 phosphate can now be syphoned off into glucose synthesis = 3 carbons total
The remaining 5 molecules of glyceraldehyde 3 phosphate are used to regenerate the 3 molecules of ribulose 1,5 bisphosphate we had at the start of the cycle.
This means that 3 complete turns of the cycle result in the net formation of one molecule of glyceraldehyde 3 phosphate,
So 6 turns of the Calvin cycle are needed to make one molecule of glucose