From USPTO and Reaxys ® reaction data, a heuristic-based balancing algorithm, namely, ChemBalancer was developed to investigate if a reaction is balanced based on atom equality of each element and electron balance at the two sides of the reaction. ChemBalancer also attempted to complete the imbalanced reactions by adding molecules from reagents, solvents and a list of most frequently appeared small molecules in reactions. The remaining incomplete reactions were classified as either LHS or RHS species insufficient reactions. ChemBalancer identified 3.3% and 7.2% originally complete/balanced reactions from USPTO and Reaxys ® respectively, and further completed 14.3% and 36.9% of the total reactions by adding possible missing molecules to the reactions.