How It Works
Specific gravity (SG) is the dimensionless ratio of a substance's density (or specific weight) to that of a reference — usually water at 4 °C (1,000 kg/m³). An SG less than 1 means the substance floats in water; greater than 1 means it sinks. You can calculate SG from either density ratios or specific weight ratios.
Example Problem
Olive oil has a density of 920 kg/m³. What is its specific gravity relative to water?
- Identify the knowns. Substance density ρ = 920 kg/m³ (olive oil at room temperature) and reference density ρ_ref = 1,000 kg/m³ (water at 4 °C, the standard reference).
- Identify what we're solving for. We want the specific gravity SG, a dimensionless ratio with no units.
- Write the density-ratio formula: SG = ρ / ρ_ref. Both densities must be in the same units so the result is unitless.
- Substitute the known values: SG = 920 kg/m³ / 1,000 kg/m³.
- Simplify the arithmetic. The kg/m³ units cancel exactly, leaving a pure number: SG = 0.92.
- State the final result: **Specific gravity SG = 0.92** (dimensionless). Because SG < 1, olive oil floats on water.
Since SG < 1, olive oil floats on water.
When to Use Each Variable
- Solve for Specific Gravity (Density) — when you know the substance and reference densities, e.g., checking if a material will float or sink in water.
- Solve for Substance Density — when you know the specific gravity and reference density, e.g., converting an SG measurement from a hydrometer to a density value.
- Solve for Reference Density — when you know the SG and substance density, e.g., determining what reference fluid was used in a reported SG measurement.
- Solve for Specific Gravity (Specific Weight) — when working with specific weight ratios instead of density ratios, e.g., using weight-based measurements in field conditions.
- Solve for Substance Specific Weight — when you know SG and reference specific weight, e.g., calculating the weight per unit volume of a petroleum product.
Key Concepts
Specific gravity is the dimensionless ratio of a substance's density to a reference density, usually water at 4 degrees C (1,000 kg/m3). Because it is dimensionless, SG has the same numeric value regardless of the unit system. An SG less than 1 means the substance floats in the reference fluid; greater than 1 means it sinks. SG can also be expressed using specific weight ratios, which is equivalent when both measurements are at the same gravitational acceleration.
Applications
- Petroleum industry: API gravity is derived from specific gravity to classify crude oil grades
- Brewing and winemaking: using hydrometers to measure sugar content via SG before and after fermentation
- Geology and mineralogy: identifying minerals by comparing their SG to known reference values
- Quality control: verifying purity of chemicals, acids, and solutions by checking SG against specifications
Common Mistakes
- Confusing specific gravity with density — SG is dimensionless, while density has units (kg/m3 or g/cm3)
- Ignoring temperature — both the substance and reference densities change with temperature, so SG varies unless temperature is specified
- Assuming water density is always 1,000 kg/m3 — at temperatures other than 4 degrees C, water density deviates slightly
- Using SG to predict buoyancy in fluids other than water — a substance with SG = 0.9 floats in water but may sink in a lighter fluid
Frequently Asked Questions
What is specific gravity used for?
Specific gravity is widely used in geology (mineral identification), brewing (measuring sugar content), petroleum (API gravity), and quality control to verify material purity.
Is specific gravity the same as density?
No. Density has units (kg/m³), while specific gravity is a dimensionless ratio. Numerically, specific gravity relative to water at 4 °C equals the density in g/cm³.
Can specific gravity change with temperature?
Yes. Both the substance and reference densities change with temperature, so specific gravity measurements should state the temperature conditions (e.g., SG at 20 °C/4 °C).
How do you measure specific gravity?
The simplest tool is a hydrometer — a weighted glass float that sinks deeper in less dense liquids and reads SG directly off a calibrated scale. Pycnometers and digital densitometers are more accurate for precision work. For solids, weigh the sample in air and in water; SG equals weight-in-air divided by (weight-in-air − weight-in-water).
What is API gravity and how does it relate to specific gravity?
API gravity is a petroleum-industry measure defined as °API = (141.5 / SG) − 131.5, with SG measured at 60 °F. Crude oil with SG = 1 has 10° API; lighter (more valuable) crudes have higher API numbers. West Texas Intermediate sits near 40° API (SG ≈ 0.825); heavy Venezuelan crude is around 15° API (SG ≈ 0.97).
What reference fluid is used for gases?
For gases, the reference is usually dry air at the same temperature and pressure. Specific gravity of a gas equals the ratio of its molar mass to air's (28.97 g/mol). Methane (16 g/mol) has SG_gas ≈ 0.55, so natural gas leaks rise; propane (44 g/mol) has SG_gas ≈ 1.52 and pools near the floor — a real safety consideration in confined spaces.
Can specific gravity be less than 1?
Yes — anything less dense than the reference fluid has SG < 1 and floats. Wood, oil, alcohol, and gasoline all have SG below 1 relative to water. Ice has SG ≈ 0.917, which is why icebergs float with about 8% of their volume above the surface.
Reference:
Lindeburg, Michael R. 1992. Engineer In Training Reference Manual. Professional Publication, Inc. 8th Edition.
Worked Examples
Industrial Refining
What is the specific gravity of mercury relative to water?
Mercury (Hg) is used in barometers and thermometers precisely because of its very high density. Compute its specific gravity using ρ_Hg = 13,534 kg/m³ and water as the 1000 kg/m³ reference at 4 °C.
- Knowns: ρ_s = 13,534 kg/m³ (mercury), ρ_r = 1000 kg/m³ (water)
- SG = ρ_s / ρ_r
- SG = 13,534 / 1000
SG ≈ 13.534
Mercury is 13.5× denser than water, which is why a 760 mm mercury column balances one atmosphere while the equivalent water column would be 10.3 m tall. Specific gravity is dimensionless — it's just the density ratio.
Oil & Gas Pipeline
What is the specific gravity of #2 diesel fuel for pipeline custody transfer?
Pipeline operators use specific gravity to identify the product currently flowing and to compute mass flow from volumetric meters. A diesel sample weighs γ_s = 8.04 kN/m³ at 15 °C; water at the same temperature has γ_r = 9.80 kN/m³. Compute the specific gravity.
- Knowns: γ_s = 8.04 kN/m³ (diesel), γ_r = 9.80 kN/m³ (water at 15 °C)
- SG = γ_s / γ_r
- SG = 8.04 / 9.80
SG ≈ 0.820
Typical #2 diesel ranges 0.82–0.86. Lower-density products (gasoline, jet A) sit near 0.72–0.80; heavier products (heating oil, residual fuel) climb above 0.90. Refineries blend to hit a contracted SG window.
Automotive Cooling
What density does a 50/50 ethylene glycol coolant (SG = 1.07) have?
A 50/50 ethylene glycol / water coolant blend reports specific gravity SG = 1.07 on a refractometer. Compute the substance density using water at 1000 kg/m³ as the reference — useful for converting between volumetric and mass flow in a cooling loop.
- Knowns: SG = 1.07, ρ_r = 1000 kg/m³ (water reference)
- ρ_s = SG × ρ_r
- ρ_s = 1.07 × 1000
ρ_s ≈ 1070 kg/m³
Pure ethylene glycol is 1113 kg/m³; pure water is 1000 kg/m³. A 50/50 blend lands near 1070 kg/m³, which is why coolant feels heavier than water by hand. Specific-gravity refractometers are calibrated against this density range for service shops.
Specific Gravity Formula
Specific gravity is the dimensionless ratio of a substance's density (or specific weight) to that of a reference fluid — almost always water at 4 °C for liquids and solids, or dry air for gases:
SG = ρsubstance / ρreferenceDensity-ratio form
SG = γsubstance / γreferenceSpecific-weight ratio form (equivalent under same g)
Where:
- SG — specific gravity, dimensionless (a pure number)
- ρsubstance — density of the substance under test (kg/m³)
- ρreference — density of the reference fluid (kg/m³); 1000 kg/m³ for water at 4 °C
- γsubstance — specific weight of the substance (N/m³)
- γreference — specific weight of the reference (N/m³); 9810 N/m³ for water
Both forms give the same numeric answer because γ = ρ × g, and the g factor cancels in the ratio. Because SG is dimensionless it has the same value in any unit system. Numerically, SG relative to water at 4 °C equals the density expressed in g/cm³ — so water itself is 1.00, ice is 0.917, mercury is 13.6, and ethanol is 0.789. SG < 1 means the substance floats in the reference; SG > 1 means it sinks.
Specific Gravity of Common Materials
Specific gravity (SG) of everyday materials, computed as the ratio of each material's density to water at 4 °C (ρ_water ≈ 1000 kg/m³). Click a material to load its density into the calculator. For the full sortable list of all materials, see the material density reference.
| Material | Specific Gravity (SG) | Density (g/cm³) | Type | Source |
|---|---|---|---|---|
| Aluminum | 2.7 | 2.7 | Metal | RSC Periodic Table |
| Iron | 7.87 | 7.87 | Metal | RSC Periodic Table |
| Copper | 8.96 | 8.96 | Metal | RSC Periodic Table |
| Lead | 11.3 | 11.3 | Metal | RSC Periodic Table |
| Gold | 19.3 | 19.3 | Metal | RSC Periodic Table |
| Carbon steel | 7.85 | 7.85 | Metal | CRC Handbook |
| Stainless steel | 8 | 8 | Metal | CRC Handbook |
| Oak (air-dried) | 0.75 | 0.75 | Solid | CRC Handbook |
| Ice | 0.917 | 0.917 | Solid | CRC Handbook |
| Concrete | 2.4 | 2.4 | Solid | CRC Handbook |
| Glass (window) | 2.5 | 2.5 | Solid | CRC Handbook |
| Gasoline | 0.745 | 0.745 | Liquid | CRC Handbook |
| Ethanol | 0.789 | 0.789 | Liquid | PubChem (NIH) |
| Water (pure) | 1 | 1 | Liquid | USGS Water Science School |
| Dry air | 0.001204 | 0.001204 | Gas | CRC Handbook |
Sources: RSC Periodic Table (elements), USGS (water), PubChem / NIH (pure compounds), and the CRC Handbook of Chemistry and Physics, 97th ed. (alloys, building materials, gases). Specific gravity is computed against water at 4 °C; alloy and building-material values are typical and vary with composition, temperature, and moisture content.
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