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How is the photosynthesis equation balanced?

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combustion — chemical reaction Log burning in a fire. Burning wood is an example of a chemical reaction in which wood in the presence of heat and oxygen is transformed into carbon dioxide, water vapor, and ash.

To balance the chemical equation for photosynthesis, you must ensure that the number of atoms for each element is equal on both the reactant and product sides, in accordance with the law of conservation of mass.1

The unbalanced reaction is:
${CO}_{2} + H_{2} O \to C_{6} H_{12} O_{6} + O_{2}$

To balance it, follow these steps:

Count the atoms: Initially, the left side has 1 carbon (C), 2 hydrogen (H), and 3 oxygen (O) atoms. The right side has 6 C, 12 H, and 8 O atoms.1
Adjust coefficients: Change the coefficient of ${CO}_{2}$ to 6 to balance the carbon atoms. Then, change the coefficient of $H_{2} O$ to 6 to balance the hydrogen atoms. This results in 18 oxygen atoms on the left.1
Finalize: To balance the oxygen atoms on the right, set the coefficient of $O_{2}$ to 6.

The final balanced equation is:
$6 {CO}_{2} + 6 H_{2} O \to C_{6} H_{12} O_{6} + 6 O_{2}$ 23

This equation represents the conversion of six carbon dioxide molecules and six water molecules into one sugar molecule and six oxygen molecules.23