Eighteen computer structural models of mid-rise office building were analyzed to study the factors affecting the material usage and CO_2 emission. Section dimension and steel ratio were designed in accordance with the Taiwan building codes. The concrete and steel usage were calculated and then converted into the CO_2 emission with a Taiwan-based carbon databank. The analysis results showed that the material usage and carbon emission increase as the story number, seismic zone, span change, dead load, and live load increase. A rapid estimation equation is developed by including these factors. Comparison with actual residence buildings shows that the developed equation slightly underestimates the CO_2 emission. Modified equation by using the conservative design case result instead of the prototype result shows conservative estimation. Both equations have an average error lower than 5% and therefore can be considered reasonable.