The density of butter is:

At room temperature, butter softens to a spreadable consistency. At 32 to 35 °C (90 to 95 °F), butter melts to a thin liquid consistency. When refrigerated, butter maintains a firm solid. Butter has a density of 911 g/L (1514 oz/US pt). Its colour ranges from deep yellow to almost white, but is often pale yellow. Second, because butter is less thick than water, it floats. Butter has a density of roughly 0.9 g/mL compared to water's 1 g/mL. This tiny discrepancy is sufficient to cause the butter to float.