Specific Gravity Calculator
Calculate specific gravity from density, or choose a substance to compare it with water. Useful for material ID and fluid property checks.
Specific Gravity Calculator
Example specific gravities:
Result will appear here...
What the specific gravity calculator does
Specific gravity, also called relative density, is a material's density compared with water. This calculator works it out from a density you enter, turning it into a single plain number that tells you at a glance how a material stacks up against water. It also lists the specific gravities of common substances for reference.
Below is what specific gravity means, the equation behind it, how to read the result, and a worked example.
How to use it
- Enter the density of the material, in your choice of units.
- Browse the substance list if you want to see typical specific gravities for comparison.
- Press Calculate for the specific gravity, or Reset to clear it.
What specific gravity is
Specific gravity is the ratio of a material's density to the density of water. Instead of saying iron has a density of 7,870 kilograms per cubic metre, you can say its specific gravity is about 7.87, meaning it is 7.87 times as dense as water. It is the same information as density, just expressed as a comparison rather than an absolute figure.
The advantage of this comparison is that it is a pure number, with no units, the same whatever measurement system you work in. That makes it convenient for quickly judging and comparing materials, and it is widely used to identify substances, check the concentration of solutions, and assess the purity of metals and gemstones. Because water is the reference, the number also tells you immediately how the material will behave in water.
The equation it uses
Specific gravity, SG, is the density of the material divided by the density of water, taken as 1000 kilograms per cubic metre:
SG = ρmaterial ÷ ρwater
Because both densities are in the same units, they cancel, leaving a pure number. A material exactly as dense as water has a specific gravity of 1; denser materials are above 1, and lighter ones below.
Reading the number
The value of specific gravity is easy to interpret. A number greater than 1 means the material is denser than water and will sink in it; a number less than 1 means it is lighter than water and will float. Exactly 1 means it matches water and will hover, neither rising nor settling. So the figure doubles as an instant float-or-sink test against water.
The range across materials is wide and tells a clear story. Balsa wood sits near 0.2, very light and a strong floater; ethanol and oils fall a little below 1, which is why oil floats on water; water itself is 1. Above that, common metals climb steeply, iron near 7.87, lead about 11.3, gold around 19.3, up to the densest natural elements like osmium near 22.6. The number places any material on that scale at a glance.
Units and precision
You can enter the density in kilograms per cubic metre, grams per cubic centimetre, pounds per cubic foot, and other units, with the calculator converting internally before comparing it to water. The specific gravity itself has no units, since it is a ratio. The reference used is water at 1000 kilograms per cubic metre; very precise work defines the reference at a specific temperature, but for general comparison this standard value is what matters. Results carry several figures.
A worked example
Take aluminium, with a density of about 2,700 kilograms per cubic metre.
Its specific gravity is 2,700 ÷ 1,000 = 2.7. So aluminium is 2.7 times as dense as water, and being above 1, it sinks. For contrast, ice has a specific gravity of about 0.92, just under 1, which is why it floats, with most of its bulk below the surface.
Questions people ask
How do you calculate specific gravity?
Divide the material's density by the density of water, 1000 kg/m³. The result is a pure number with no units, equal to how many times denser than water the material is.
What is the difference between specific gravity and density?
They carry the same information. Density is an absolute mass per volume with units; specific gravity is that density expressed as a ratio to water, so it is a unitless number.
What does a specific gravity below 1 mean?
The material is less dense than water and will float on it. Above 1 means denser than water, so it sinks; exactly 1 means it matches water.
Does specific gravity have units?
No. It is a ratio of two densities, so the units cancel and it is a pure number, the same in any measurement system.
References
A quick note on where this comes from. Specific gravity, or relative density, as the ratio of a material's density to that of water is standard physics, described in the Wikipedia article on relative density and set out in OpenStax's University Physics. The SI units follow the US National Institute of Standards and Technology.
- Wikipedia, Relative density (specific gravity). https://en.wikipedia.org/wiki/Relative_density
- OpenStax, University Physics Volume 1, Section 14.1, Fluids, Density, and Pressure. https://openstax.org/books/university-physics-volume-1/pages/14-1-fluids-density-and-pressure
- National Institute of Standards and Technology (NIST), Special Publication 811, Guide for the Use of the International System of Units (SI). https://www.nist.gov/pml/special-publication-811
Bibek Lal Karna is a PhD student and graduate teaching assistant at the University of Mississippi, with deep interests in theoretical and gravitational physics. He is also the founder of NRCC and is strongly engaged in scientific teaching and communication. At Eon Tools, he reviews physics tools.