Article — Specific Gravity Calculator
Specific Gravity Calculator: A Dimensionless Density Ratio
Specific gravity is the dimensionless ratio of a substance's density to a reference, conventionally water at 4 °C (1000 kg/m³). Water has SG = 1.0 exactly. Substances with SG < 1 float on water, and substances with SG > 1 sink.
The phrase "specific gravity" sounds redundant — specific gravity is just density divided by another density. But that one cancellation is what makes specific gravity so useful: the result has no units, no temperature dependence to first order, and no need to commit a value of g to memory. It is the same number whether you live in Miami or on the International Space Station.
What specific gravity measures
Specific gravity (SG) compares a substance's density to water at 4 °C, the temperature at which water reaches its maximum density of 1000 kg/m³ (1.000 g/cm³). The ratio is dimensionless because both densities have the same units. A substance with density 1.5 g/cm³ has SG = 1.5; a substance with density 8.96 g/cm³ (copper) has SG = 8.96.
The water reference is convenient because most engineers, brewers, and chemists work with aqueous solutions. For gases, the reference is usually air at standard conditions (1.225 kg/m³ ISA), which gives the specific gravity of natural gas (about 0.55), hydrogen (0.07), and propane (1.55).
The specific gravity formula
The defining equation is SG = ρ_substance / ρ_reference. For water-referenced SG of a liquid or solid: SG = ρ / 1000 kg/m³. For density in g/cm³, the value is numerically identical to SG: water density 1.000 g/cm³ = SG 1.000, mercury 13.59 g/cm³ = SG 13.59.
Gasoline 0.74Ethanol 0.79Ice 0.92Sea water 1.025Glycerin 1.26Mercury 13.59Gold 19.30Specific gravity vs density
Density has units (kg/m³, g/cm³, lb/ft³). Specific gravity has none. Density depends on what unit system you choose; specific gravity does not. Density values vary subtly with temperature; specific gravity values vary too, but the cancellation removes most of the dependence on the reference standard.
For everyday material identification, specific gravity is the friendlier number. Saying "the unknown liquid has a specific gravity of 0.79" is more readable than "the unknown liquid has a density of 789 kg/m³". Both convey the same physics; SG is just easier to remember.
The story of Archimedes detecting fraud in the golden crown of King Hiero II hinges on specific gravity. Pure gold has SG 19.3; silver has SG 10.5. By measuring the crown's volume through water displacement and comparing to an equal-mass gold ingot, Archimedes could detect adulteration without melting the crown. Whether the bath-tub story is literal history or apocryphal, the underlying physics is correct.
Specific gravity and the API scale for oil
The petroleum industry uses API gravity rather than SG directly because the numbers are easier to handle for crude oil grades. API° = 141.5 / SG − 131.5. Water sits at exactly 10 °API. Light crudes have API above 31; medium crudes 22 to 31; heavy crudes 10 to 22; bitumen and tar sands below 10.
The conversion is non-linear: a small change in SG corresponds to a large change in API. SG 0.85 = 35 °API (light); SG 0.95 = 17.5 °API (heavy). Refineries price crude partly on API because lighter crudes yield more gasoline and diesel per barrel.
Baumé scale and brewing
The Baumé scale, named after French chemist Antoine Baumé (1768), survives in brewing, sugar refining, and parts of chemistry. Two formulas exist: 140/SG − 130 for liquids lighter than water (alcohol, oil) and 145 − 145/SG for heavier liquids (sugar syrup, acid).
For homebrewers, the related Plato scale measures dissolved sugar in beer wort: 12 °P ≈ SG 1.048. Wine and mead makers tend to use Brix (essentially equivalent to Plato for our purposes). All three scales are derived from SG and ultimately measure the same thing — the density of the sugary solution before fermentation begins.
Specific gravity and buoyancy
The simplest use of specific gravity is the float-or-sink check. Anything with SG below 1 floats on water. Wood (SG 0.4 to 0.9), ice (SG 0.92), oil (SG 0.85 to 0.95), ethanol (SG 0.79), and gasoline (SG 0.74) all float. Heavy metals (gold 19.3, lead 11.3, mercury 13.59) sink.
Specific gravity in medicine and gemology
In medicine, urine specific gravity is a quick test of hydration and kidney function. Healthy urine has SG between 1.005 and 1.030. Below 1.005 suggests dilute urine (excessive fluid intake, diabetes insipidus, or kidney dysfunction). Above 1.030 suggests dehydration. Most modern clinical labs report it on every routine urinalysis.
In gemology, specific gravity differentiates gems with similar appearances. Diamond has SG 3.52; cubic zirconia has SG 5.85; moissanite has SG 3.21. A hydrostatic balance measures the gem in air and submerged in water, and the ratio identifies the stone non-destructively.
In battery testing, the specific gravity of lead-acid battery electrolyte indicates state of charge. A fully charged 12 V lead-acid battery has electrolyte SG of about 1.275; fully discharged drops to 1.120. Refractometers and hydrometers read this directly.
Common specific gravity mistakes
SG values vary slightly with temperature because both the substance and the reference shift with heat. For precision work, always state the measurement temperature and the reference temperature. Petroleum uses 60 °F (15.6 °C); brewing uses 20 °C or 15.6 °C; gemology uses 20 °C. Mixing temperatures introduces errors of a few percent.
Other recurring slips: confusing SG with density (SG is unitless, density has units), assuming all gases use air as the reference (industrial specs sometimes use hydrogen or methane), and applying Baumé formulas to the wrong density range. For most everyday use, water-referenced SG at 20 °C is the safe default and is what the calculator above uses.
One subtler error involves mixing temperature corrections. Petroleum specs use 60 °F as the reference, and the API formula bakes that temperature into the constants. Brewing uses either 20 °C or 60 °F depending on country. Translating between standards requires a small density correction, typically 0.0007 SG per °F for ordinary liquids. Software like API MPMS Chapter 11 implements the full correction; for casual use the temperature mismatch is usually under 1 percent and can be ignored.
A final point: specific gravity is almost always quoted as a number with three decimal places. SG 1.025 (sea water) and SG 1.030 (whole milk) look similar but represent meaningfully different densities, and your hydrometer reading should resolve at the third decimal. For high-precision metrology, the underlying density must be measured to at least four significant figures, so SG goes to four decimals at minimum. Trust the precision your instrument provides, not more, and do not invent precision the measurement does not actually support.