Specific Gravity Calculator
Solve specific gravity problems step-by-step with our free calculator. See formulas, worked examples, and clear explanations.
Reviewed for accuracy by Manoj Kumar, Mathematics Educator
Specific Gravity Calculator
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Formula: SG = Density of Substance / Density of Reference
Worked example — Specific Gravity: 2.656 | Likely Mineral: Quartz | Density: 2,648 kg/m3
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 SG = Weight in Air / Apparent Loss = 85 / 32 = 2.656 Density = 2.656 x 997 kg/m3 = 2,648 kg/m3 SG of 2.656 closely matches quartz (2.65) This 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 API Gravity = (141.5 / 0.8526) - 131.5 = 165.97 - 131.5 = 34.47 API > 31.1 classifies this as light crude oil This is similar to Brent crude (API ~38) Light 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.
Why does specific gravity matter in geology and mineralogy?
Specific gravity is one of the most important physical properties for mineral identification because each mineral has a characteristic SG range. Quartz has SG of 2.65, feldspar about 2.56, and galena (lead ore) around 7.5. Geologists can quickly narrow down mineral identification by estimating SG through a simple heft test or precise Archimedes measurement. In gemology, SG helps distinguish real gemstones from imitations, since a synthetic stone may look identical but have a different SG. For example, diamond has SG of 3.52, while cubic zirconia is 5.80. Mining engineers use SG to calculate ore reserves, estimate tonnage from volume surveys, and design mineral processing equipment for separation based on density differences.
How does temperature affect specific gravity measurements?
Temperature significantly affects specific gravity because density changes with temperature due to thermal expansion. Water reaches maximum density at approximately 4 degrees Celsius (997.05 kg/m3) and becomes less dense at both higher and lower temperatures. At 20 degrees Celsius, water density is 998.2 kg/m3, and at 60 degrees it drops to 983.2 kg/m3. Most specific gravity measurements are standardized at either 4 or 20 degrees Celsius and noted as SG(20/4), meaning the substance was measured at 20 degrees with water at 4 degrees as reference. For precise work, temperature corrections must be applied using published tables. This is especially important in the petroleum industry where product volumes and densities are corrected to a standard temperature of 15 degrees Celsius.
What is the relationship between specific gravity and density?
Specific gravity and density are closely related but distinct concepts. Density is an absolute measure expressed in units like kg/m3 or g/cm3, while specific gravity is a dimensionless ratio comparing a substance density to a reference. In the CGS system (grams per cubic centimeter), the numerical value of density in g/cm3 equals the specific gravity when water at 4 degrees Celsius is the reference, because water density is approximately 1.000 g/cm3. In SI units (kg/m3), you must divide by 997 to convert density to specific gravity. The advantage of specific gravity over density is its independence from units, making it easy to communicate internationally. To convert SG to density: Density = SG x Reference Density.
How is specific gravity used in brewing and winemaking?
Brewers and winemakers use specific gravity to track fermentation progress and estimate alcohol content. Before fermentation, the original gravity (OG) of the wort or must is measured, typically ranging from 1.040 to 1.090 for beer. Sugar dissolved in the liquid makes it denser than water. As yeast converts sugar to alcohol and carbon dioxide, the gravity drops because alcohol is lighter than water (SG 0.789). The final gravity (FG) after fermentation is complete typically ranges from 1.005 to 1.015. Alcohol by volume is estimated using ABV = (OG - FG) x 131.25. For example, OG of 1.050 and FG of 1.010 yields approximately 5.25% ABV. Hydrometers calibrated in specific gravity units are essential tools in every brewery and winery.
What is the Baume scale and how does it relate to specific gravity?
The Baume scale is an older hydrometer scale that provides a convenient way to express specific gravity for industrial liquids. There are two versions: one for liquids heavier than water and one for liquids lighter than water. For heavy liquids: Baume = 145 - (145 / SG). For light liquids: Baume = (140 / SG) - 130. The scale was developed by French pharmacist Antoine Baume in 1768 and remains in use in certain industries. In the sugar industry, a related scale called Brix measures sugar concentration. In the acid industry, Baume degrees indicate sulfuric acid concentration. While modern instruments typically display specific gravity or density directly, the Baume scale persists in legacy industrial processes and older reference materials.
Can specific gravity be used to detect fraud or adulteration?
Yes, specific gravity is a primary tool for detecting adulteration in liquids and authenticating materials. In the food industry, honey adulteration with sugar syrup is detected because pure honey has SG of 1.39-1.45 while corn syrup is around 1.38. Milk dilution with water lowers the SG below the normal range of 1.028-1.035. In the fuel industry, gasoline diluted with cheaper solvents shows abnormal SG readings. Jewelers use SG to verify gemstones, since a cubic zirconia passed off as diamond has dramatically different SG. Gold purity can be estimated by SG because 24K gold has SG of 19.3 while lower karat alloys are lighter. These tests are quick, non-destructive, and require minimal equipment, making SG analysis widely accessible.
References
Reviewed for accuracy by Manoj Kumar, Mathematics Educator · Editorial policy
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