Density Calculator

Density equals mass divided by volume: ρ = m / V. Measure the mass of the object (in grams or kilograms) and its volume (in cm³, mL, or m³), then divide. For example, a 100-gram object that takes up 50 cm³ has a density of 2.0 g/cm³.

SI standard is kilograms per cubic meter (kg/m³). More commonly for everyday work, density is given in grams per cubic centimeter (g/cm³) or grams per milliliter (g/mL) — these are equivalent. US customary uses pounds per cubic foot (lb/ft³). Water is 1 g/cm³ or 1,000 kg/m³ or 62.4 lb/ft³.

Pure water at 4°C has a density of exactly 1.000 g/cm³ (which equals 1,000 kg/m³ or 62.43 lb/ft³). This is by definition — the gram was originally defined as the mass of 1 cm³ of water at this temperature. Water's density decreases slightly at higher temperatures (0.997 at 25°C, 0.958 at 100°C).

Use water displacement (Archimedes' principle). Fill a graduated cylinder with water, note the initial level, fully submerge the object, and note the new water level. The volume of the object equals the difference between the two levels. For example, if water rises from 50 mL to 73 mL when you submerge a rock, the rock's volume is 23 mL.

It comes down to density. An object floats in a fluid if its density is LESS than the fluid's density. Wood (0.5-0.8 g/cm³) floats in water (1.0 g/cm³) because wood is less dense. Iron (7.87 g/cm³) sinks. A boat made of iron floats because it contains air — the AVERAGE density of the boat (steel + air) is less than water.

Density has units (g/cm³, kg/m³, etc.) and is an absolute measure. Specific gravity is a unitless ratio: the density of a substance divided by the density of water at 4°C. So water has specific gravity of 1.0 by definition, and a substance with density 2.5 g/cm³ has specific gravity 2.5. They contain the same information; specific gravity just skips the units.

Most substances expand when heated and contract when cooled. Heating decreases density (same mass in larger volume); cooling increases density. Water is unusual — it's most dense at 4°C, NOT at its freezing point. This is why ice floats and why deep lakes maintain a 4°C bottom layer that supports aquatic life through winter.

Gases require knowing pressure and temperature. The simplest formula uses the ideal gas law: ρ = (P × M) / (R × T), where P is pressure, M is molar mass, R is the gas constant (8.314 J/mol·K), and T is temperature in Kelvin. At standard conditions (0°C, 1 atm), air has a density of about 1.293 kg/m³.

Osmium (22.59 g/cm³) and iridium (22.56 g/cm³) are the densest naturally occurring elements at standard conditions. White dwarf stars and neutron stars are far denser (millions to trillions of times denser than osmium), but those aren't really 'substances' in the chemical sense. The densest material made artificially in significant quantities is osmium tetroxide.

No — density is always positive. Mass is positive (no negative mass exists in classical physics), and volume is positive, so their ratio (density) must be positive. The minimum possible density is the density of a perfect vacuum: zero. Some theoretical or experimental contexts use 'negative effective density' in metamaterials, but that's a specialized engineering term, not true negative density.