The Calvin cycle consumes 18 ATP molecules to produce one glucose molecule. Glucose contains six carbon atoms, so the cycle must fix six molecules of carbon dioxide. Each fixed carbon requires three ATP overall: two ATP help convert 3-phosphoglycerate into glyceraldehyde-3-phosphate, and one ATP helps regenerate the RuBP acceptor.
Six turns therefore require 6 × 3 = 18 ATP. The same accounting gives 12 NADPH, because two NADPH are used per carbon dioxide fixed. Six carbon dioxide molecules produce 12 three-carbon G3P molecules in the accounting scheme; two G3P molecules provide the carbon skeleton for one glucose, while the rest are rearranged to regenerate six RuBP molecules.
The 18-ATP figure describes the conventional stoichiometry for carbohydrate production in the Calvin cycle. It refers to ATP consumed by the cycle, not the total ATP made by the light reactions or the energy cost of every downstream step used to convert triose phosphates into a particular cellular product. A frequent mistake is choosing 36, which would correspond to two glucose molecules under the same standard calculation.