Increasing density (D) of ensiled corn forage by proper harvest dry matter, chop length and silo packing results in decreased presence of oxygen which enhances bacterial fermentation of corn silage (CS). This research measured the effect of CS D and calculated gas filled porosity (P) (Holmes and Muck, 2006) on selected fermentation characteristics. From December, 2012 to July, 2013, 46 dairy operations in the Midwest United States ranging in size from approximately 300 to 4,500 cows (62,500 total) participated in a study to evaluate the effect of silo density (kg/meter³) of ensiled CS on the pH, lactic acid (LA), total volatile fatty acids (VFA), ammonia (NH₃), and ethanol (ETOH), all % of CS DM, of the corresponding corn silage. A minimum of at least 6 core samples from the face of CS bunkers or drive-over piles was consolidated and a composite sample sent to a commercial lab (Rock River Laboratories, Watertown, Wisconsin) for analysis using wet chemistry methods. Some dairies were sampled more than once as they changed sources of CS. The average D was 292.6 kg/meter³ (range 174.6–350.8). The REG procedure of SAS was used to analyze the relationship between D/P and the dependent variables. There were negative relationships between D and pH (R² = 0.31, P < 0.001), NH₃ (R² = 0.25, P < 0.001), and ETOH (R² = 0.14, P = 0.003). There were positive relationships between D and LA (R² = 0.26, P < 0.001), VFA (R² = 0.17, P < 0.05). Gas filled P was calculated as an alternative measurement of CS D with 0.4 or lower P recommended for proper fermentation. The CS calculated P average was 0.32 (range 0.06–0.69). Regression analysis indicated a negative relationship between P and LA (R² = 0.23, P < 0.001) and VFA (R² = 0.18, P = 0.001). A positive relationship between P and pH (R² = 0.30, P < 0.001) was observed. These results suggest that increased CS bunker/pile D measured by silage face core sampling is strongly correlated with improved fermentation characteristics of low pH, NH_3, ETOH and high VFA and LA concentrations. Gas filled P is an alternative measurement that was correlated with the increased fermentation acids measured and a decreased pH. This work demonstrates the importance of high D on improving the fermentation quality of CS.