What this calculator does
Specific gravity is the ratio of a substance's density to the density of a reference fluid, water for liquids and solids, air for gases. Because it is a ratio of two densities, it has no units: a specific gravity of 1.03 simply means the substance is 1.03 times as dense as the reference, whatever units the two densities were measured in.
The reference matters. A gas quoted against water comes out as a tiny fraction that tells you almost nothing useful, and a liquid or solid quoted against air comes out as an enormous number for the same reason. This calculator lets the reference be water, air, or a custom density, so the ratio being reported actually matches the comparison intended.
The formula
Divide the substance's density by the reference density. Water at 4°C (1000 kg/m³, the temperature at which it is densest) is the standard reference for liquids and solids; air at 15°C and sea level (1.225 kg/m³) is the standard reference for gases. A custom reference is available for anything else, such as comparing one oil against another rather than against water.
| Term | Meaning |
|---|---|
| Specific gravity (SG) | Substance density ÷ reference density, a dimensionless ratio. |
| Reference density | The density the substance is being compared against: water for liquids and solids, air for gases. |
| Density | Mass per unit volume of the substance itself, in kg/m³. |
The inputs explained
| Field | What to enter |
|---|---|
| Substance density (kg/m³) | The density of the substance being tested, in kilograms per cubic metre. Convert from g/cm³ by multiplying by 1000 if that is what you have measured. |
| Reference fluid | Water is the usual reference for liquids and solids, air for gases. Choose Custom to compare against any other reference density. |
| Custom reference density (used only if reference fluid is Custom) (kg/m³) | Only used when Custom is selected above: enter the reference density to divide by. |
When to use it
Checking a liquid against water
Antifreeze, battery acid, wort in brewing and seawater are all commonly quoted by specific gravity against water, because a hydrometer measures exactly that ratio directly, without needing to know the fluid's actual density in kg/m³.
Comparing a gas against air
Whether a gas rises, sinks or mixes evenly in a room depends on its density relative to air, not its density in isolation, so specific gravity against air is the number that actually answers that question.
Verifying a material identification
Specific gravity is a quick check on what a mineral, metal or plastic sample might be, since many materials have a well-documented and fairly narrow specific gravity range.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What is the specific gravity of common liquids and solids, against water?
A range of everyday substance densities, each divided by water's reference density.
| Substance density | Specific gravity |
|---|---|
| 790 kg/m³ | 0.7900 |
| 800 kg/m³ | 0.8000 |
| 1,000 kg/m³ | 1.000 |
| 1,025 kg/m³ | 1.025 |
| 2,700 kg/m³ | 2.700 |
| 13,600 kg/m³ | 13.600 |
What is the specific gravity of common gases, against air?
A range of gas densities, each divided by air's reference density instead of water's.
| Gas density | Specific gravity |
|---|---|
| 0.0899 kg/m³ | 0.0734 |
| 0.1786 kg/m³ | 0.1458 |
| 1.2506 kg/m³ | 1.021 |
| 1.429 kg/m³ | 1.167 |
| 1.784 kg/m³ | 1.456 |
| 1.977 kg/m³ | 1.614 |
Questions
Why does specific gravity have no units?
It is one density divided by another, so whatever units were used cancel out as long as both densities are in the same units. That is also why specific gravity is the same number whether densities are measured in kg/m³ or g/cm³.
Is specific gravity the same as relative density?
Yes, the two terms describe the same ratio. "Specific gravity" is the more common term in brewing, geology and older engineering texts; "relative density" is more common in modern scientific usage, but both mean substance density divided by reference density.
Why is water at 4°C used as the reference, not room temperature?
Water reaches its maximum density at almost exactly 4°C, so using that temperature as the reference gives a fixed, reproducible baseline of 1000 kg/m³, rather than a figure that drifts slightly with whatever temperature the water happened to be measured at.
What does a specific gravity below 1 mean?
Against water, it means the substance is less dense than water and will float on it, assuming the two do not mix. Against air, it means the substance is less dense than air and will tend to rise through it, as with helium or hydrogen.
For water's own density at a specific temperature rather than as a reference figure, see the water density calculator. For pressure at depth in a fluid of known density, see the hydrostatic pressure calculator.