Six molecules of carbon dioxide are needed to produce one molecule of glucose in the overall photosynthesis equation. The balanced equation is 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂, so six carbon atoms from six carbon-dioxide molecules become the six carbon atoms in glucose.
This carbon accounting occurs during the Calvin cycle, the light-independent stage of photosynthesis. Carbon dioxide is attached to a five-carbon compound called ribulose-1,5-bisphosphate, or RuBP. Through several enzyme-controlled reactions, the carbon is rearranged into molecules that can be used to build carbohydrates.
A common mix-up is thinking that plants directly manufacture one glucose molecule every time six carbon-dioxide molecules enter the cycle. In living cells, the Calvin cycle produces smaller three-carbon molecules that can later be combined or converted into glucose, sucrose, starch, and other compounds. The equation is therefore an overall summary, not a literal one-step reaction.
The oxygen shown in the equation comes primarily from splitting water during the light-dependent reactions, not from the carbon dioxide.