Melted butter has a density of about 0.91 g/ml, which means 100 grams of butter fills approximately 110 ml. Butter is less dense than water because it's mostly fat.
Solid butter is slightly denser than melted butter, but recipes that call for butter in ml usually mean melted. Butter is also commonly measured in sticks (1 US stick = 113 g). Use our gram to ml converter for accurate butter measurements.