Short answer
Density is mass per unit volume. Specific gravity, also called relative density, compares a substance’s density with a reference density. Density has units, such as grams per cubic centimeter. Specific gravity is a ratio, so the matching units cancel. 1 2
For liquids and solids, the reference is commonly water under stated conditions. A specific gravity of 2 therefore means twice the chosen reference density; it does not mean “2 grams” or “twice Earth’s gravity.” The reference and its conditions matter. 2 3
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At a glance
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| Question | Density | Specific gravity |
|---|---|---|
| Basic calculation | Mass ÷ volume. | Sample density ÷ reference density. |
| What does it express? | Mass in a given volume. | Density relative to a reference. |
| Units | For example, kg/m³ or g/cm³. | Dimensionless; no density unit remains. |
| Requires a reference substance? | No. | Yes; commonly water for liquids and solids. |
| Do measurement conditions matter? | Yes, including temperature. | Yes, for both the sample and reference. |
| Why can their numbers look alike? | Density in g/cm³ may be close to the relative-density number when water is the reference. | Water’s density is approximately 1 g/cm³ under common conditions. |
Sources: density definition and units 1; relative-density definition 2; water’s changing density and hydrometers 3.
What each thing is
Density
Density describes how much mass occupies a given volume. A sample with a mass of 500 grams and a volume of 200 cubic centimeters has a density of 2.5 g/cm³. The calculation divides mass by volume; it does not compare the sample with water. 1
The word mass matters. Density here means mass density, the quantity represented by the familiar formula ρ = m/V. A large object can have more mass than a small object while having lower density, because its mass is distributed across a larger volume. 1
Specific gravity
Specific gravity is a density ratio. IUPAC’s terminology entry uses relative density and identifies specific gravity as the older name. Its definition compares a density with a reference, usually water at 4°C. In practice, a reported value needs enough context to identify the reference conditions used. 2
Using an illustrative reference density of 1.0 g/cm³, the 2.5 g/cm³ sample has a specific gravity of 2.5. Express both densities in kg/m³ instead, and 2,500 divided by 1,000 still gives 2.5. Changing compatible units does not change the ratio. 1 2
Key differences
Density is an absolute mass-to-volume quantity; specific gravity is a comparison. To recover density from specific gravity, multiply by the applicable reference density. Without knowing that reference, a bare relative-density number is incomplete information. 1 2
Their units behave differently. A density of 2.5 g/cm³ is the same as 2,500 kg/m³. The corresponding specific gravity remains the same because both numerator and denominator change together. Writing “specific gravity: 2.5 g/cm³” mixes the ratio with the units of density. 1 2
Neither number automatically identifies a material. A density measurement describes a property, not a unique chemical identity. The sample’s composition, temperature, and how its volume was defined must be considered before interpreting it. Water alone demonstrates how a familiar material can have different measured densities under different conditions. 1 3
How to tell them apart
Look for a mass-per-volume unit. A value in g/mL, g/cm³, or kg/m³ is a density. A dimensionless comparison labeled SG or relative density needs an accompanying reference convention. 1 2
Next check the conditions. What was the sample temperature? What reference density was used? Is the reported volume that of the solid material itself, or does it include gaps between particles? These questions determine what is being compared. 1 3
A hydrometer can provide a relative-density measurement by how it floats in a liquid. Interpreting its scale still requires knowing what quantity and reference convention the instrument reports. 3
Where they overlap
Specific gravity is calculated from density, so the two are directly related descriptions. With a specified reference, one can be converted into the other. They are not rival explanations of the same sample. 2
Their numerical resemblance in common laboratory units is convenient but conditional. Water is approximately 1 g/mL, which is also 1 g/cm³. That approximation explains why density in these units and water-referenced specific gravity often look nearly identical. 3
Edge cases
Water is not exactly 1 g/mL in every situation. Its density changes with temperature and composition. Dissolved salt can make water denser than pure water, and ordinary ice is less dense than liquid water. 3
A pile has spaces. The mass divided by the volume occupied by a loose pile includes spaces between its pieces. That bulk value can differ from the density of the solid material composing the pieces. A comparison is meaningful only when the volume conventions match. 1
A value below one needs its reference. It means a sample is less dense than the selected reference under the specified conditions. It does not independently identify the substance or establish every aspect of how a real object will behave in a fluid. 2
Why the distinction exists
Density gives a direct mass-to-volume measurement. Relative density offers a comparison in which the units cancel, making a reference such as water a convenient benchmark. The apparent simplicity of the ratio does not remove the need to define the benchmark and conditions. 1 2
Common misconceptions
- “Specific gravity measures gravitational pull.” Here it names a density ratio. 2
- “Density and specific gravity always have the same number.” That resemblance depends on units and the reference density. 1 2
- “Water’s density is always exactly one.” Temperature and composition matter. 3
- “Heavier means denser.” Total mass must be considered together with volume. 1
Examples
In an illustrative comparison, equal 100 cm³ volumes of two samples have masses of 80 g and 120 g. Their densities are 0.8 and 1.2 g/cm³. Using water approximated as 1.0 g/cm³, their specific gravities are approximately 0.8 and 1.2. The density retains its units; the ratio does not. 1 2 3
Sources
Sources checked October 3, 2026.
- OpenStax, College Physics 2e — Density. Mass per volume, units, and the effect of spaces in a sample.
- IUPAC Gold Book — Relative Density. The density ratio and its reference, also known as specific gravity.
- U.S. Geological Survey, Water Science School — Water Density. Temperature, dissolved substances, and hydrometer measurements.