Specific Gravity to Density Converter
Convert specific gravity to density in various units for liquids and solids. Enter values for instant results with step-by-step formulas.
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Formula
Specific gravity is the ratio of a substance's density to the density of water at a specified reference temperature. Multiplying specific gravity by the water density at that temperature gives the actual density. At 4 degrees C, water density is 999.972 kg/m3.
Last reviewed: December 2025
Worked Examples
Example 1: Seawater Density Calculation
Example 2: Petroleum API to Density Conversion
Background & Theory
The Specific Gravity to Density Converter applies the following established principles and formulas. Unit conversion is the process of expressing a quantity in a different unit of measurement while preserving its physical meaning. At the foundation of modern measurement lies the International System of Units (SI), which defines seven base units: the meter for length, kilogram for mass, second for time, ampere for electric current, kelvin for thermodynamic temperature, mole for amount of substance, and candela for luminous intensity. All other units, called derived units, are defined as algebraic combinations of these seven. Dimensional analysis is the principal method for performing unit conversions. By treating units as algebraic quantities that can be multiplied, divided, and cancelled, a conversion factor chain allows a value expressed in one unit to be rewritten in another without altering its physical magnitude. For example, to convert 60 miles per hour to meters per second, one multiplies by a chain of conversion factors each equal to one: (1609.34 m / 1 mile) × (1 hour / 3600 s). Metric prefixes enable compact expression of quantities across extreme ranges of magnitude. Standard prefixes span from nano (10^-9) through micro (10^-6) and milli (10^-3) up through kilo (10^3), mega (10^6), and giga (10^9), and beyond in both directions. These prefixes are strictly multiplicative and apply consistently to any SI base or derived unit. Temperature conversions require affine transformations rather than simple scaling. To convert Celsius to Fahrenheit the formula is °F = (°C × 9/5) + 32, while the conversion to the absolute Kelvin scale is K = °C + 273.15. These formulas reflect the different zero points and degree-size conventions of each scale. Significant figures govern how precision is preserved through calculations. A result should not express more precision than the least precise input value permits. In digital storage, IEEE and IEC standards distinguish between decimal prefixes (kilobyte = 1000 bytes) and binary prefixes (kibibyte = 1024 bytes), a distinction that has practical consequences for how storage capacity is reported by manufacturers versus operating systems. Unit coherence — ensuring that all quantities in an equation share a consistent unit system — is essential for obtaining correct results.
History
The history behind the Specific Gravity to Density Converter traces back through the following developments. Human beings have been measuring and comparing quantities since before recorded history. The earliest known measurement units were body-based: the cubit (the distance from elbow to fingertip), the foot, the hand, and the digit. The furlong originated as the length of a furrow a team of oxen could plow without resting. These anthropomorphic standards were practical for local use but differed between regions and kingdoms, creating persistent difficulties in trade and construction. The ancient Egyptians standardized the royal cubit at approximately 52.4 centimeters and distributed calibrated granite rods to ensure consistency across building projects, including the pyramids. Roman engineers used the mile (mille passuum, one thousand double paces) and spread these standards throughout their empire via road networks. Despite these efforts, measurement diversity persisted across medieval Europe, hampering commerce. The French Revolution created political will for radical standardization. In 1795 France officially adopted the metric system, defining the meter as one ten-millionth of the distance from the equator to the North Pole along the Paris meridian. This gave the world its first fully decimal, rationally constructed measurement system. The Metre Convention of 1875 established the International Bureau of Weights and Measures (BIPM) in Sevres, France, creating a permanent international body to maintain physical artifact standards and coordinate global metrology. For over a century, the kilogram was defined by a platinum-iridium cylinder locked in a vault near Paris. In 1999, a stark demonstration of what unit inconsistency costs occurred when NASA's Mars Climate Orbiter was lost because one engineering team used pound-force seconds while another used newton seconds. The spacecraft entered the Martian atmosphere at the wrong angle and was destroyed, at a cost of 327 million dollars. In 2019 the SI underwent its most significant revision, redefining all seven base units in terms of fixed numerical values of fundamental physical constants such as the speed of light, Planck's constant, and the elementary charge. This eliminated any reliance on physical artifacts and made the measurement system permanently stable and universally reproducible.
Frequently Asked Questions
Formula
Density (kg/m3) = Specific Gravity x Water Density at Reference Temperature
Specific gravity is the ratio of a substance's density to the density of water at a specified reference temperature. Multiplying specific gravity by the water density at that temperature gives the actual density. At 4 degrees C, water density is 999.972 kg/m3.
Worked Examples
Example 1: Seawater Density Calculation
Problem: Seawater has a specific gravity of 1.025 at 4 degrees C. Calculate its density in various units.
Solution: Density (kg/m3) = 1.025 x 999.972 = 1024.971 kg/m3\nDensity (g/cm3) = 1024.971 / 1000 = 1.025 g/cm3\nDensity (lb/ft3) = 1024.971 / 16.0185 = 64.00 lb/ft3\nDensity (lb/gal) = 1024.971 / 119.826 = 8.554 lb/gal\nAPI gravity = (141.5 / 1.025) - 131.5 = 6.59 API
Result: SG 1.025 = 1025.0 kg/m3 = 64.0 lb/ft3 = 8.55 lb/gal
Example 2: Petroleum API to Density Conversion
Problem: Light crude oil has an API gravity of 35. What is its specific gravity and density?
Solution: SG = 141.5 / (API + 131.5) = 141.5 / (35 + 131.5) = 141.5 / 166.5 = 0.8498\nDensity (kg/m3) = 0.8498 x 999.972 = 849.8 kg/m3\nDensity (lb/gal) = 849.8 / 119.826 = 7.09 lb/gal\nThis oil floats on water (SG < 1.0)
Result: API 35 = SG 0.8498 = 849.8 kg/m3 = 7.09 lb/gal (floats on water)
Frequently Asked Questions
What is specific gravity and how is it different from density?
Specific gravity is a dimensionless ratio comparing a substance's density to the density of a reference substance, which is usually water at 4 degrees Celsius (999.972 kg/m3) for liquids and solids, or air for gases. Unlike density, specific gravity has no units because it is a ratio of two densities. Density is an absolute measurement expressed in units like kg/m3 or g/cm3. Because water's density at 4 degrees C is approximately 1.000 g/cm3, the numerical value of specific gravity and density in g/cm3 are nearly identical for most practical purposes. However, they are conceptually different: density tells you the mass per unit volume, while specific gravity tells you how heavy something is relative to water.
How do you convert specific gravity to density in kg/m3?
To convert specific gravity to density in kg/m3, multiply the specific gravity by the density of water at the reference temperature. At 4 degrees Celsius, water density is 999.972 kg/m3, so density in kg/m3 is approximately equal to specific gravity times 1000. For example, a substance with specific gravity 1.5 has a density of approximately 1500 kg/m3 (1.5 times 999.972). For greater precision, you must specify the reference temperature because water density changes with temperature. At 20 degrees C, water density is 998.206 kg/m3, so the same specific gravity of 1.5 would correspond to 1497.3 kg/m3. The difference is small but matters in precision applications like pharmaceutical manufacturing and analytical chemistry.
Why is reference temperature important for specific gravity measurements?
Reference temperature is crucial because the density of the reference substance (water) changes with temperature. Water is densest at approximately 4 degrees Celsius (999.972 kg/m3). At 20 degrees C, water density drops to 998.206 kg/m3, and at 25 degrees C it is 997.048 kg/m3. A specific gravity measurement written as SG 20/4 means the substance was measured at 20 degrees C and compared to water at 4 degrees C. The notation SG 20/20 means both measurements were at 20 degrees C. For precise work in chemistry and quality control, you must know which temperature convention is used, or the resulting density calculation will be slightly wrong. Most standard industrial tables use 60 degrees Fahrenheit (15.56 degrees C) as the reference temperature for petroleum products.
What instruments are used to measure specific gravity?
Several instruments measure specific gravity, each suited to different applications and precision requirements. A hydrometer is a weighted glass float that sinks to a depth proportional to the liquid density, with specific gravity read from the calibrated scale at the liquid surface. A pycnometer is a precisely calibrated glass flask that measures the mass of a known volume of liquid. Digital density meters use oscillating U-tube technology to measure density to five or six decimal places, providing the most accurate results. A Westphal balance uses Archimedes principle with a plunger submerged in the liquid. For solids, the water displacement method compares the weight of the object in air versus submerged in water. Refractometers estimate specific gravity indirectly through the refractive index of a solution.
How is specific gravity used in the brewing and winemaking industry?
Brewers and winemakers use specific gravity to track fermentation progress and calculate alcohol content. The original gravity (OG) is measured before fermentation begins, typically between 1.030 and 1.090 for beer and 1.070 to 1.120 for wine. As yeast converts sugar to alcohol and carbon dioxide, the specific gravity decreases because alcohol is lighter than water (SG 0.789). The final gravity (FG) is measured when fermentation is complete, usually between 1.005 and 1.020. Alcohol by volume (ABV) is estimated as approximately (OG minus FG) times 131.25. A beer with OG 1.060 and FG 1.012 would have roughly 6.3 percent ABV. Brewers also use the Brix and Plato scales, which express sugar concentration as a percentage and are directly related to specific gravity.
What is the API gravity scale used in the petroleum industry?
API gravity is a scale developed by the American Petroleum Institute that inversely relates to specific gravity. The formula is API gravity equals (141.5 divided by specific gravity at 60 degrees F) minus 131.5. On this scale, water has an API gravity of 10. Lighter petroleum products have higher API values: light crude oil is 31 to 45 API, medium crude is 22 to 31 API, and heavy crude is below 22 API. Gasoline is approximately 55 to 65 API, while diesel fuel is around 32 to 38 API. The API scale was developed because petroleum pricing and refining characteristics correlate better with this inverted scale. Light crude oils with higher API gravity generally command higher prices because they yield more valuable refined products like gasoline and jet fuel.
References
Reviewed by Manoj Kumar, Mathematics Educator · Editorial policy