Specific Gravity Calculator
Specific gravity compares how dense a substance is relative to water. Enter a density, a mass and volume, or work backward from a known specific gravity. Pick a substance preset, set the water reference temperature, and read off the ratio, the absolute density in several units, the API gravity, and whether it floats or sinks.
Formula
Worked example
Aluminium has a density of 2700 kg/m³. Against water at 4 °C (1000 kg/m³), SG = 2700 ÷ 1000 = 2.7, so it is 2.7 times as dense as water and sinks. Working backward, SG 2.7 × 1000 = 2700 kg/m³.
What specific gravity measures
Specific gravity is the ratio of a material density to the density of a reference substance, which for liquids and solids is almost always pure water (1000 kg/m³ at 4 °C, or equivalently 1 g/cm³). Because it divides one density by another, the units cancel and specific gravity is a pure, dimensionless number. A specific gravity of 2.7 simply means the material is 2.7 times as heavy as an equal volume of water, which is why it is such a convenient way to compare materials regardless of the measurement system you happen to be using. Enter a density directly, choose a substance preset, or give a mass and a volume and let the calculator find the density first.
Water temperature and the reference density
Water is densest at 4 °C, where it reaches 1000 kg/m³, and that is the classic reference. It matters in precise work because water expands as it warms: by 20 °C, the common laboratory reference, it has fallen to about 998.2 kg/m³, and near boiling it is around 958 kg/m³. This calculator lets you set the water reference temperature, then uses the Tanaka density-temperature fit to compute the exact reference density before dividing. For everyday float-or-sink questions the 4 °C value is fine, but for hydrometer readings, brewing, or chemical storage the temperature setting keeps the ratio honest.
Floating, sinking, density and API gravity
The reference value of 1 is the dividing line for buoyancy in fresh water. Anything with a specific gravity below 1, such as cork, ice, most oils, and dry softwoods, is less dense than water and floats, while anything above 1 sinks. The calculator also reports the absolute density in kg/m³, g/cm³, lb/ft³, and lb/gal so you can take the number straight into engineering or storage work, and it shows API gravity, the petroleum industry scale defined as 141.5 divided by specific gravity minus 131.5. Light crude oils sit above 10 °API and float on water; heavier oils fall below 10 °API and sink. Switch the Solve for menu to start from a known specific gravity and recover the density instead.
Specific gravity of common materials
| Material | Density (kg/m³) | Specific gravity | In water |
|---|---|---|---|
| Cork | 240 | 0.24 | Floats |
| Gasoline | 710 | 0.71 | Floats |
| Ethanol | 789 | 0.79 | Floats |
| Ice | 917 | 0.92 | Floats |
| Water | 1000 | 1 | Neutral |
| Seawater | 1025 | 1.03 | Sinks |
| Aluminium | 2700 | 2.7 | Sinks |
| Iron | 7870 | 7.87 | Sinks |
| Lead | 11340 | 11.34 | Sinks |
| Gold | 19300 | 19.3 | Sinks |
Approximate values relative to water at 4 °C (1000 kg/m³).
Frequently asked questions
Does specific gravity have units?
No. Specific gravity is a ratio of two densities, so the units cancel and it is a pure dimensionless number. That is why a value of 2.7 is the same whether you measured the original density in kg/m³, g/cm³, lb/ft³, or lb/gal.
What density of water should I use?
The standard reference is pure water at 4 °C, where it reaches its maximum density of 1000 kg/m³ (1 g/cm³). Many labs instead use 20 °C, where water is about 998.2 kg/m³. This calculator lets you set the reference temperature and adjusts the water density automatically, so your specific gravity stays accurate.
Can I work out density from a known specific gravity?
Yes. Set the Solve for menu to "Density (from specific gravity)" and enter the specific gravity. The calculator multiplies it by the water reference density to return the absolute density in kg/m³, g/cm³, lb/ft³, and lb/gal.
What is API gravity and how does it relate?
API gravity is a petroleum scale defined as 141.5 ÷ specific gravity − 131.5, measured at 60 °F. It inverts the trend of specific gravity, so a higher API number means a lighter, less dense oil. Liquids above 10 °API float on water; below 10 °API they sink.
How are specific gravity and density different?
Density is an absolute quantity with units, such as 2700 kg/m³. Specific gravity is that density divided by water density, giving a unitless ratio (2.7). They carry the same information, but specific gravity makes it easy to compare materials and to judge whether something floats or sinks.