Specific Gravity Calculator
Enter the substance density and the reference density (water commonly 1000 kg/m³ or 1 g/mL); by SG = ρ_substance / ρ_reference the tool instantly computes the dimensionless specific gravity.
Input Data
Results
At a glance:Specific gravity (SG), also called relative density, is a dimensionless quantity comparing a substance's density to a reference substance's density: SG = ρ_substance / ρ_reference. For liquids and solids the reference is usually pure water at 4 °C (densest, 1000 kg/m³ or 1.000 g/mL); for gases the reference is usually air. Because SG is the ratio of two densities in the same unit, the units cancel, leaving a pure number with no dimension. Using this tool's default: a liquid density ρ = 1250 kg/m³, water 1000 kg/m³ as reference, SG = 1250/1000 = 1.25, meaning the liquid's density is 1.25× that of water (25% heavier). The most direct meaning of SG is judging floating/sinking: when SG > 1 (heavier than water) the substance sinks in water (e.g. iron SG≈7.9, stone); SG < 1 (lighter than water) floats (wood, oil, ice SG≈0.92); SG = 1 stays suspended. That is why oil floats, ice floats, and metals sink. Specific gravity is measured and used widely: first, a hydrometer measures liquid SG — it floats in the liquid, deeper immersion means lower SG. Used for battery electrolyte concentration (sulphuric acid SG reflects charge state), brewing and beverage sugar/alcohol, antifreeze, milk and juice concentration. Second, mineral and gem identification — each mineral has a characteristic SG, used to judge authenticity (diamond SG≈3.5). Third, engineering — whether a material floats and buoyancy calculations. Fourth, medicine — urine SG assesses kidney concentration. Fifth, chemistry — converting concentration via density and SG. Notes: first, substance and reference density must use the same unit (both kg/m³ or both g/mL). Second, density changes with temperature, so precise SG states the measurement and reference temperatures (e.g. '20 °C/4 °C'). Third, SG and density may be numerically close (water ≈ 1 in g/mL) but SG is dimensionless while density has units — different concepts. Fourth, this calculator takes non-negative substance density and reference density > 0. Fifth, for gas SG use air density as the denominator.
Formula
Specific gravity: SG = ρ_substance / ρ_reference (dimensionless).
Reference for liquids/solids: 4 °C water (1000 kg/m³ = 1 g/mL).
SG > 1 sinks, SG < 1 floats, SG = 1 suspended.
Substance and reference density must share units (cancel out).
$$SG = \dfrac{\rho_{\text{substance}}}{\rho_{\text{reference}}}$$How to Use
- Enter the substance density (e.g. kg/m³).
- Enter the reference density (liquids commonly water 1000 kg/m³ or 1 g/mL).
- The right panel instantly shows the dimensionless SG.
Specific gravity of common substances (water as reference)
| Substance | Density (kg/m³) | SG |
|---|---|---|
| Ice | 920 | 0.92 |
| Water | 1000 | 1.00 |
| Seawater | 1025 | 1.03 |
| Iron | 7870 | 7.87 |
SG > 1 sinks, SG < 1 floats; density varies with temperature, so precise SG states the temperature.
Case Studies
SG and floating of a liquid
A liquid density 1250 kg/m³, water 1000 kg/m³.
SG = 1250 / 1000 = 1.25 > 1.
Heavier than water, so the liquid sinks.
Oil floats on water
Edible oil density ≈ 920 kg/m³, water 1000 kg/m³.
SG = 920 / 1000 = 0.92 < 1.
SG below 1, so oil floats on water.
FAQ
What is the difference between SG and density?
Density is mass per unit volume, with units (kg/m³); SG is the ratio of a substance's density to a reference density, a dimensionless number. Since water is ≈ 1 g/mL, the g/mL density and SG are often numerically close, but the concepts differ.
Why is 4 °C water the common reference?
Because water is densest at about 4 °C (1000 kg/m³ = 1.000 g/mL), the most convenient fixed reference. Gas SG commonly uses air as the reference.
What does SG > 1 mean?
It means the substance is heavier than water and sinks in water; SG < 1 means lighter and floats; SG = 1 means suspended. This is the most direct basis for judging floating/sinking.
How does a hydrometer work?
A hydrometer (hydrometer) floats in the liquid by buoyancy; deeper immersion means lower SG, shallower means higher. The scale reads SG directly, used for battery electrolyte, alcohol, antifreeze, etc.
Does temperature affect SG?
Yes. Density changes with temperature, so precise SG states the measurement and reference temperatures (e.g. 20 °C/4 °C). In general engineering and teaching, small room-temperature variation is often negligible.
Related Tools
References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.