Density Calculator

Density (p) equals mass (m) divided by volume (V): p = m/V. The SI unit for density is kilograms per cubic meter (kg/m3), though grams per cubic centimeter (g/cm3) is commonly used in chemistry. To find mass, multiply density by volume (m = p x V). To find volume, divide mass by density (V = m/p).

An object will float if its density is less than the density of the fluid it's placed in. For water (density = 1000 kg/m3 or 1 g/cm3), anything with lower density floats. Wood (500-900 kg/m3), ice (917 kg/m3), and most plastics float. Metals like steel (7850 kg/m3) sink unless shaped to displace enough water (like a ship).

Specific gravity (SG) is the ratio of a substance's density to the density of a reference substance, usually water at 4C (1000 kg/m3). Since it's a ratio, specific gravity is dimensionless (no units). Water has SG = 1.0, substances that float have SG < 1, and substances that sink have SG > 1. Specific gravity numerically equals density in g/cm3.

Ice floats because it's less dense than liquid water. Water is unusual - most substances are denser as solids than liquids. When water freezes, hydrogen bonds form a crystalline structure with more space between molecules, making ice about 9% less dense (917 kg/m3) than liquid water (1000 kg/m3). This is crucial for aquatic life in winter.

Common densities (kg/m3): Air 1.225, Wood (pine) 550, Ice 917, Water 1000, Plastic (PVC) 1400, Concrete 2400, Aluminum 2700, Steel 7850, Copper 8960, Lead 11340, Gold 19300. Gases are much less dense than liquids and solids. Metals tend to be denser than non-metals.

Temperature significantly affects density. Most substances expand when heated, increasing volume while mass stays constant, thus decreasing density. Water is unusual - it's densest at 4C, becoming less dense both when heated AND when cooled below 4C. This is why ice forms on top of lakes. Gases are highly affected by temperature changes.

Density is an absolute measurement with units (kg/m3, g/cm3, etc.), while specific gravity is a relative, dimensionless ratio comparing a substance to water. Specific gravity equals density in g/cm3 because water's density is 1 g/cm3. Both indicate whether something floats (< water) or sinks (> water).

Ships float due to buoyancy, not low density. A solid steel block sinks because its density (7850 kg/m3) exceeds water's. But a hollow steel ship displaces a large volume of water while its average density (steel + air inside) is less than water. The buoyant force equals the weight of displaced water. If this force exceeds the ship's weight, it floats.