Density calculator

Everything runs from rho equals mass over volume, rearranged for whichever side is blank. Unit factors are the exact treaty definitions, so a pound is 0.45359237 kg and a US gallon is 3.785411784 L, with ordinary floating point beyond that. Specific gravity is against pure water at 4 degrees C, 1000 kg per cubic metre; seawater is a touch denser, so the float verdict is for fresh water and real objects that trap air or sit partly above the surface can behave differently.