Specific Gravity Calculator – Density Ratio & Buoyancy
Calculate specific gravity and buoyancy by comparing substance density to water. Enter mass and volume or input density directly.
Provide substance mass and volume, or enter the density directly. Optionally adjust temperature for accurate water density.
Specific Gravity Calculator – Density Ratio & Buoyancy
Calculate specific gravity and buoyancy by comparing substance density to water. Enter mass and volume or input density directly.
About the Specific Gravity Calculator
Specific gravity (SG), also called relative density, is a dimensionless ratio that compares the density of a substance to the density of a reference substance — usually water at a defined temperature. The formula is straightforward: SG = ρ_substance / ρ_water. Because the reference (water) has a density of approximately 1.000 g/cm³ at 4 °C, specific gravity is numerically equal to the density of the substance expressed in g/cm³ when both are measured at the same temperature.
Water was chosen as the reference because it is abundant, easily purified, and has precisely defined physical properties. The International Association for the Properties of Water and Steam (IAPWS) has established polynomial equations for water density across a wide temperature range. At 4 °C water reaches its maximum density of 1.000 g/cm³; at 20 °C it is 0.9982 g/cm³; at 100 °C it drops to 0.9584 g/cm³. This calculator uses the standard IAPWS polynomial to apply the correct water density for any temperature you specify.
The practical significance of specific gravity is immediate: any substance with SG < 1 will float in water; any substance with SG > 1 will sink. Ice (SG ≈ 0.917) floats, which is why icebergs remain partially above the ocean surface. Wood species vary from about 0.4 (balsa) to above 1.0 (ebony). Metals like aluminum (SG ≈ 2.70), iron (7.87), lead (11.34), and gold (19.32) all sink rapidly.
In engineering, specific gravity appears in hydraulic design, slurry transport, and process chemistry. Petroleum engineers use it to characterise crude oil, measuring it as API gravity. Brewers and winemakers use it to track fermentation progress with a hydrometer. Gemologists use it to identify unknown stones by comparing measured SG against reference tables. Civil engineers test aggregate SG when designing concrete mixes.
This calculator supports two measurement workflows. If you have a sample whose mass and volume you can measure, enter both values and the calculator derives the density and then the specific gravity. If the density is already known — from a datasheet, published table, or previous measurement — you can enter it directly to skip the mass-volume step. Temperature input corrects the water reference density, which matters most for liquid-liquid comparisons where both substances change density with temperature.
Buoyancy is the practical consequence of specific gravity. Archimedes' principle states that the upward buoyant force on a submerged object equals the weight of the fluid it displaces. If SG < 1, the buoyant force exceeds the object's weight and it rises; if SG > 1, the object's weight exceeds buoyancy and it sinks. This calculator notes the buoyancy outcome directly below the specific gravity result, making it easy to assess floatability at a glance.
Specific gravity examples
Common substances showing mass, volume, density, and specific gravity at 20 °C.
| Substance / Conditions | Specific Gravity | Notes |
|---|---|---|
| Aluminum — 27.0 g, 10.0 cm³, 20 °C | SG ≈ 2.70 | Density = 2.70 g/cm³. SG > 1, so aluminum sinks. Widely used in aerospace where strength-to-weight ratio matters. |
| Ethanol — 15.6 g, 20.0 cm³, 25 °C | SG ≈ 0.789 | Density ≈ 0.780 g/cm³ at 25 °C. SG < 1, ethanol floats on water. Important in beverage and pharmaceutical industries. |
| Quartz — 26.5 g, 10.0 cm³, 20 °C | SG ≈ 2.65 | Density = 2.65 g/cm³. Standard reference material used to verify specific gravity measurement equipment. |
| Copper — direct density 8.96 g/cm³, 20 °C | SG ≈ 8.97 | Direct density input. Copper sinks immediately. Used in electrical wiring due to high electrical conductivity. |
How to use the specific gravity calculator
- Enter the substance mass in grams and volume in cm³, OR enter the substance density in g/cm³ directly — you do not need both methods.
- Set the temperature in °C at which the measurement was made. The calculator adjusts the water reference density accordingly.
- Click Calculate to see the specific gravity, the substance density, and whether the material floats or sinks in water.
- Use the example buttons to pre-fill values for aluminum, ethanol, or copper.
- Click Reset to clear all fields and start a new measurement.
Specific gravity FAQ
What is the difference between specific gravity and density?
Density is an absolute measurement (mass per unit volume) with units such as g/cm³ or kg/m³. Specific gravity is a dimensionless ratio: it compares the substance's density to the density of water at the same or a reference temperature. Numerically, SG equals the density in g/cm³ when water is the reference at 4 °C, but the two quantities are conceptually different.
Why does temperature matter for specific gravity?
Both the substance and water change density with temperature. Water density peaks at 4 °C (1.000 g/cm³) and decreases above or below that point. If you measure a liquid at 25 °C and compare it to water at 4 °C, you get a slightly different specific gravity than if you compare both at 25 °C. Specifying the temperature ensures consistent and reproducible results.
How do I measure the volume of an irregularly shaped solid?
Use the water displacement method: fill a graduated cylinder with a known volume of water, submerge the solid, and record the new volume. The difference is the volume of the solid. Alternatively, suspend the solid on a string and weigh it both in air and fully submerged in water; the weight difference equals the buoyant force, from which volume = (weight in air − weight in water) / (ρ_water × g).
What does an SG greater than 1 mean in practice?
An SG greater than 1 means the substance is denser than water and will sink when placed in water under normal gravity. The higher the SG, the denser the material. For example, lead (SG ≈ 11.3) sinks far more quickly than a ceramic tile (SG ≈ 2.3). In slurry transport engineering, SG directly determines the pumping energy required to move suspended solids.
Can I use specific gravity for gases?
Yes, but the reference changes: gas specific gravity is typically measured relative to air (ρ_air ≈ 0.00120 g/cm³ at 15 °C, 1 atm) rather than water. Natural gas has SG ≈ 0.55–0.80 relative to air, meaning it is lighter and rises if released indoors. This calculator is optimised for liquids and solids using water as the reference.
How is specific gravity related to API gravity used in the petroleum industry?
API gravity is an alternative scale defined as API° = (141.5 / SG) − 131.5, where SG is measured relative to water at 60 °F (15.6 °C). A light crude oil with API° = 40 has SG ≈ 0.825, while a heavy crude at API° = 20 has SG ≈ 0.934. Higher API gravity means lighter, more valuable oil that is easier to refine.