Specific Gravity Calculator
Solve specific gravity problems step-by-step with our free calculator. See formulas, worked examples, and clear explanations.
Reviewed by Manoj Kumar, Mathematics Educator
Formula
SG = Density of Substance / Density of Reference
Where SG is the specific gravity (dimensionless ratio), Density of Substance is in kg/m3, and Density of Reference is typically water at 4 degrees C (997 kg/m3). Alternatively, using the Archimedes method: SG = Weight in Air / (Weight in Air - Weight in Water).
Worked Examples
Example 1: Mineral Identification by Archimedes Method
Problem:A mineral specimen weighs 85 grams in air and 53 grams when submerged in water. Determine its specific gravity and identify the likely mineral.
Solution:Apparent weight loss = 85g - 53g = 32g\nSG = Weight in Air / Apparent Loss = 85 / 32 = 2.656\nDensity = 2.656 x 997 kg/m3 = 2,648 kg/m3\n\nSG of 2.656 closely matches quartz (2.65)\nThis is consistent with common quartz specimens.
Result:Specific Gravity: 2.656 | Likely Mineral: Quartz | Density: 2,648 kg/m3
Example 2: API Gravity of Crude Oil
Problem:A crude oil sample has a density of 850 kg/m3. Calculate its specific gravity and API gravity classification.
Solution:SG = 850 / 997 = 0.8526\nAPI Gravity = (141.5 / 0.8526) - 131.5 = 165.97 - 131.5 = 34.47\n\nAPI > 31.1 classifies this as light crude oil\nThis is similar to Brent crude (API ~38)\nLight crude commands premium pricing.
Result:SG: 0.853 | API Gravity: 34.47 | Classification: Light Crude
Frequently Asked Questions
What is specific gravity and how is it defined?
Specific gravity (SG) is the ratio of the density of a substance to the density of a reference substance, typically water at 4 degrees Celsius (997 kg/m3). Unlike density, specific gravity is a dimensionless number with no units, making it universally comparable regardless of the measurement system used. A specific gravity of 2.7 means the substance is 2.7 times denser than water. If SG is less than 1, the substance floats on the reference fluid; if greater than 1, it sinks. This simple concept has been used for thousands of years, dating back to Archimedes, and remains fundamental in fields ranging from geology and chemistry to brewing and petroleum engineering.
How do you measure specific gravity using the Archimedes method?
The Archimedes method determines specific gravity by weighing an object in air and then while submerged in water. The difference between these two measurements equals the weight of water displaced, which corresponds to the object volume. The formula is SG = Weight in Air / (Weight in Air - Weight in Water). For example, if a rock weighs 150 grams in air and 95 grams in water, SG = 150 / (150 - 95) = 150 / 55 = 2.727. This method works for any solid object that is denser than the reference liquid. For porous materials, the object must be coated in wax or paraffin first to prevent water absorption, which would alter the submerged weight reading.
What instruments are used to measure specific gravity?
Several instruments measure specific gravity depending on the application and required precision. A hydrometer is a glass float calibrated to read SG directly when placed in a liquid, commonly used in brewing, winemaking, and battery testing. A pycnometer is a precision flask used to measure liquid density by comparing the mass of a known volume of liquid versus water. Digital density meters use oscillating tube technology for highly accurate measurements in laboratories. Westphal balances use a plunger and counterweights for liquid SG measurement. For gemstones and minerals, heavy liquids of known SG are used to determine whether a specimen floats or sinks, providing a quick identification method.
How is specific gravity used in the petroleum industry?
The petroleum industry uses API gravity, which is derived from specific gravity, as its standard measure of crude oil density. The formula is API Gravity = (141.5 / SG) - 131.5. Light crude oil (API greater than 31.1) is more valuable because it yields more gasoline and diesel. Heavy crude (API less than 22.3) requires more refining. For example, West Texas Intermediate crude has an API of about 39.6 (SG 0.827), while Canadian oil sands produce crude with API around 8 (SG 1.014). Specific gravity is also used to measure fuel blending ratios, monitor refinery processes, detect product contamination, and calculate pipeline flow characteristics for transportation logistics.
References
Reviewed by Manoj Kumar, Mathematics Educator · Editorial policy