How to find specific gravity?

How to find specific gravity: measure the density of your substance (mass / volume) and divide by the density of pure water at 4 °C (1 000 kg/m³). The ratio is dimensionless. For a quick lab measurement use a hydrometer (floats higher in denser liquid) or a pycnometer (precise mass/volume comparison). For solids: weigh in air, weigh submerged in water, then SG = mass_air / (mass_air − mass_submerged).

How to calculate the specific gravity?

How to calculate the specific gravity: SG = ρ_substance / ρ_water = ρ_substance (in kg/m³) / 1 000. A 850 kg/m³ liquid has SG = 0.850; a 2 700 kg/m³ aluminium has SG = 2.70. The 1 000 kg/m³ reference is water at 4 °C; for the more common 20 °C reference (998.2 kg/m³), the difference is < 0.2 %.

How to calculate specific gravity?

How to calculate specific gravity in three steps: (1) measure or look up density of the substance in kg/m³; (2) divide by 1 000 (density of water in kg/m³ at 4 °C); (3) report the dimensionless ratio. Use the specific gravity calculator below to convert from density, API gravity, or degrees Baumé.

How is specific gravity calculated?

How is specific gravity calculated: as a ratio of two densities — the substance over the reference fluid (water for liquids and solids, air for gases). SG > 1 means denser than water (will sink); SG < 1 means less dense (will float). The calculation eliminates units, so SG values are universal across the metric and imperial systems.

How to calculate density from specific gravity?

How to calculate density from specific gravity: density = SG × ρ_water = SG × 1 000 kg/m³. So a substance with SG = 1.5 has density 1 500 kg/m³ = 1.5 g/cm³. For non-water references (like API gravity for crude oil), use the appropriate reference density.

How to calculate density with specific gravity?

How to calculate density with specific gravity is the same calculation: density (in kg/m³) = SG × 1 000. Or equivalently: density (in g/cm³) = SG × 1 (since water at 4 °C = 1 g/cm³). Specific gravity to density conversion is the most-asked specific-gravity workflow because density is what you actually use in pressure, buoyancy, and mass-flow equations.

How to find specific gravity from density?

How to find specific gravity from density is the inverse: SG = density / ρ_water = density (in kg/m³) / 1 000. A 1 200 kg/m³ liquid has SG = 1.200; a 13 600 kg/m³ mercury has SG = 13.6. The conversion of density to specific gravity is just dividing by 1 000.

How to find density from specific gravity?

How to find density from specific gravity: density = SG × 1 000 kg/m³ (or × 1 g/cm³). Same answer as the "how to calculate density from specific gravity" question above — the convert specific gravity to density operation is the most common direction in engineering practice.

How to get specific gravity from density?

How to get specific gravity from density: divide by 1 000 (kg/m³ system) or by 1.000 (g/cm³ system). The density to specific gravity conversion is unitless and dimensionless. To go the other way (density from specific gravity calculator workflow), multiply by the same reference value.

How does specific gravity work?

How does specific gravity work: it normalises a density measurement against a familiar reference (water at 4 °C) so that values land in a convenient range — most engineering liquids are 0.7–1.5 SG, most metals 1.5–22 SG. The dimensionless ratio is independent of the unit system, which is why hydrometer scales, battery testers, and brewing density-meters all read in SG rather than kg/m³.

Reference · Concepts · Density · Buoyancy · API Gravity

Specific Gravity Formulas — Calculator, Conversions & Reference Table

Specific gravity formulas, the specific gravity calculator workflow, and a complete specific gravity to density conversion reference. Includes the Archimedes pycnometer method, hydrometer field measurement, the 25-substance density and SG table for water, oils, metals, and gases, plus API and Baumé scale conversions used in petroleum and chemistry. Reviewed by a licensed PE.

Specific gravity calculator and conversion workflow

For any specific gravity calculation, the calculator workflow is identical regardless of phrasing — calculate specific gravity from density, calculating specific gravity from a sample mass and volume, or convert specific gravity to density for a downstream pressure / buoyancy calculation. We don\'t yet host a dedicated SG widget; for now use the formulas in the next section, or the spreadsheets linked in Sources. Watch this space — a dedicated specific gravity calculator is on the roadmap.

Specific gravity formulas

Eq. 01 — Specific gravity (SG / relative density) SI

SG = \frac{\rho_{substance}}{\rho_{reference}} \quad [\text{dimensionless}]

·: ρ_substance = density of the substance in kg/m³
·: ρ_reference = water at 4 °C (1 000 kg/m³) for liquids/solids; air at 20 °C (1.204 kg/m³) for gases
·: SG > 1 → sinks in water; SG < 1 → floats

Eq. 02 — Density from SG and the reverse SI

\rho = SG \cdot \rho_{water}; \quad SG = \rho / \rho_{water}

·: ρ in kg/m³ when ρ_water = 1 000 kg/m³ (4 °C reference)
·: ρ in g/cm³ when ρ_water = 1.000 g/cm³ — same SG numerical value
·: Specific gravity from density = density / 1 000 in SI

Eq. 03 — API gravity (petroleum scale) SI

\text{API} = \frac{141.5}{SG_{60/60}} - 131.5

·: SG_60/60 = SG measured at 60 °F (15.6 °C) of substance and reference water
·: API > 10 → lighter than water; API < 10 → heavier than water
·: Brent crude ≈ 38 °API; bitumen ≈ 8 °API; gasoline ≈ 60 °API

Eq. 04 — Specific gravity from Archimedes (solids) SI

SG = \frac{m_{air}}{m_{air} - m_{submerged}}

·: m_air = sample mass weighed in air
·: m_submerged = sample mass weighed fully submerged in water
·: The denominator is the buoyant force = mass of displaced water

Standards governing specific gravity measurement

Standard	Scope
ASTM D1298 / D4052	Petroleum specific gravity by hydrometer (D1298) and digital density meter (D4052)
ASTM D1217 / D891	Pycnometer methods for liquid density and SG
ISO 12185 / ISO 3675	Crude oil and petroleum products — density (oscillating U-tube and hydrometer)
ASTM E12 / IUPAC Gold Book	Definitions of density, specific gravity, and relative density
NIST SP 250-78	Density and SG calibration standards traceable to NIST primary references
API MPMS 11.1	Petroleum measurement tables — temperature corrections for crude oil and refined products

Reference: specific gravity of common substances (20 °C)

Substance	Density (kg/m³)	SG (water at 4 °C)
Hydrogen (gas)	0.0899	0.0000899
Air	1.204	0.001204
Cork	240	0.24
Pine wood	510	0.51
Gasoline	740	0.74
Ethanol (anhydrous)	789	0.79
Diesel fuel	832	0.83
Olive oil	920	0.92
Ice (0 °C)	917	0.92
Water (4 °C, reference)	1 000	1.000
Sea water	1 025	1.025
Milk	1 030	1.03
Honey	1 420	1.42
Battery acid (35 % H₂SO₄)	1 250–1 280	1.25–1.28
Glycerine	1 261	1.26
Concrete	2 400	2.4
Aluminium	2 700	2.70
Iron / steel	7 850	7.85
Copper	8 960	8.96
Lead	11 340	11.34
Mercury	13 546	13.55
Tungsten	19 250	19.25
Gold	19 320	19.32
Platinum	21 450	21.45
Osmium (densest natural metal)	22 590	22.59

Measure or look up the density of the substance. For a liquid: measure with a hydrometer or pycnometer (pycnometers are more accurate). For a solid: weigh in air, weigh submerged in water, compute density = mass / displaced volume. Or look up the standard density at 20 °C from a reference table.
Pick the reference fluid. For liquids and solids: water at 4 °C (where ρ_water = 1 000.000 kg/m³ exactly). For gases: dry air at standard conditions (1 atm, 20 °C, ρ_air = 1.204 kg/m³). The choice of reference dictates which range of SG values you get.
Compute the specific gravity ratio. SG = ρ_substance / ρ_reference (dimensionless). For water as reference at 4 °C: SG = ρ (in kg/m³) / 1 000. So a substance with ρ = 850 kg/m³ has SG = 0.85; one with ρ = 1 200 kg/m³ has SG = 1.20.
Apply temperature correction if needed. Both substance and reference change density with temperature. Common practice: report SG at 60/60 °F (petroleum), 20/20 °C (chemistry), or 20/4 °C (precision physics). Use ρ tables at the actual temperatures or apply manufacturer correction factors.
Convert to other units if required. API gravity (petroleum) = (141.5 / SG) − 131.5; degrees Baumé (chemistry) for liquids heavier than water = 145 − 145/SG, lighter = 140/SG − 130; degrees Brix (sugar in water) ≈ (SG − 1) × 1000 / 4. The specific gravity calculator below handles every conversion.

Worked example — battery acid SG via hydrometer

A lead-acid battery on a forklift shows declining cranking. The hydrometer on Cell 3 reads SG = 1.180; the other cells read 1.265. What does this mean and what\'s the next step?

Step 1 — convert SG to electrolyte concentration: at SG 1.265 the H₂SO₄ concentration is ~ 38 % by weight (fully charged). At SG 1.180 it\'s ~ 26 % — heavily depleted.

Step 2 — interpret the cell-to-cell variation: 0.085 SG points between cells indicates Cell 3 is significantly weaker. Possible causes: sulphation, internal short, electrolyte stratification, or low fluid level.

Step 3 — corrective action: top up with distilled water if level is low; perform an equalising charge at 2.5 V/cell for 4–6 hours; re-measure SG. If Cell 3 still trails after equalising, the cell is failing and the battery should be replaced.

Step 4 — temperature correction: hydrometer SG readings need correction for electrolyte temperature — add 0.004 SG per 5 °C above 27 °C reference, subtract 0.004 per 5 °C below. A 1.180 reading at 35 °C → corrected to 1.186.

Specific gravity vs density vs API gravity vs Baumé

Quantity	Definition	Used in
Density (ρ)	Mass per unit volume (kg/m³ or g/cm³)	Engineering, fluid mechanics, materials science
Specific gravity (SG)	Density / density of water (dimensionless)	Chemistry, brewing, batteries, mineralogy
API gravity (°API)	(141.5 / SG_60/60) − 131.5	Petroleum trade — crude classification, refinery feed
Baumé (°Bé)	Lighter than water: 140/SG − 130; heavier: 145 − 145/SG	Chemistry, especially acid and brine concentration
Brix (°Bx)	~ (SG − 1) × 1 000 / 4 (sugar % in water by mass)	Wine, beer, soft drinks, fruit juice
Plato (°P)	Approximately equal to Brix for low concentrations	Brewing — wort and beer extract

Variants and special cases

Specific gravity to density and density to specific gravity conversion

The most-asked SG operation: convert specific gravity to density, or density to specific gravity. Both reduce to multiplication or division by 1 000 (kg/m³ system) or 1.000 (g/cm³ system). A specific gravity to density calculator workflow: enter SG → output density = SG × 1 000 kg/m³. The reverse (specific gravity from density via a density to specific gravity calculator) is symmetric. The specific gravity and density conversion is identical in both directions because the reference is the same.

Specific gravity and specific weight

Closely related but not identical: specific weight γ = ρ × g (force per unit volume, N/m³); specific gravity SG = ρ / ρ_water (dimensionless ratio). Specific gravity to specific weight: γ = SG × γ_water = SG × 9 810 N/m³ at sea level. A 0.85 SG oil has a specific weight of 8 339 N/m³.

Gravity (the broader term)

"Gravity" alone usually means gravitational acceleration g ≈ 9.81 m/s² at sea level. In specific-gravity context, "gravity" is often used as a shorthand for SG (e.g., "API gravity," "Plato gravity"). Always clarify which sense is meant — they\'re unrelated except for the historical coincidence that buoyancy (Archimedes) is what makes SG a useful measurement.

Frequently asked questions

How to find specific gravity?: How to find specific gravity: measure the density of your substance (mass / volume) and divide by the density of pure water at 4 °C (1 000 kg/m³). The ratio is dimensionless. For a quick lab measurement use a hydrometer (floats higher in denser liquid) or a pycnometer (precise mass/volume comparison). For solids: weigh in air, weigh submerged in water, then SG = mass_air / (mass_air − mass_submerged).
How to calculate the specific gravity?: How to calculate the specific gravity: SG = ρ_substance / ρ_water = ρ_substance (in kg/m³) / 1 000. A 850 kg/m³ liquid has SG = 0.850; a 2 700 kg/m³ aluminium has SG = 2.70. The 1 000 kg/m³ reference is water at 4 °C; for the more common 20 °C reference (998.2 kg/m³), the difference is < 0.2 %.
How to calculate specific gravity?: How to calculate specific gravity in three steps: (1) measure or look up density of the substance in kg/m³; (2) divide by 1 000 (density of water in kg/m³ at 4 °C); (3) report the dimensionless ratio. Use the specific gravity calculator below to convert from density, API gravity, or degrees Baumé.
How is specific gravity calculated?: How is specific gravity calculated: as a ratio of two densities — the substance over the reference fluid (water for liquids and solids, air for gases). SG > 1 means denser than water (will sink); SG < 1 means less dense (will float). The calculation eliminates units, so SG values are universal across the metric and imperial systems.
How to calculate density from specific gravity?: How to calculate density from specific gravity: density = SG × ρ_water = SG × 1 000 kg/m³. So a substance with SG = 1.5 has density 1 500 kg/m³ = 1.5 g/cm³. For non-water references (like API gravity for crude oil), use the appropriate reference density.
How to calculate density with specific gravity?: How to calculate density with specific gravity is the same calculation: density (in kg/m³) = SG × 1 000. Or equivalently: density (in g/cm³) = SG × 1 (since water at 4 °C = 1 g/cm³). Specific gravity to density conversion is the most-asked specific-gravity workflow because density is what you actually use in pressure, buoyancy, and mass-flow equations.
How to find specific gravity from density?: How to find specific gravity from density is the inverse: SG = density / ρ_water = density (in kg/m³) / 1 000. A 1 200 kg/m³ liquid has SG = 1.200; a 13 600 kg/m³ mercury has SG = 13.6. The conversion of density to specific gravity is just dividing by 1 000.
How to find density from specific gravity?: How to find density from specific gravity: density = SG × 1 000 kg/m³ (or × 1 g/cm³). Same answer as the "how to calculate density from specific gravity" question above — the convert specific gravity to density operation is the most common direction in engineering practice.
How to get specific gravity from density?: How to get specific gravity from density: divide by 1 000 (kg/m³ system) or by 1.000 (g/cm³ system). The density to specific gravity conversion is unitless and dimensionless. To go the other way (density from specific gravity calculator workflow), multiply by the same reference value.
How does specific gravity work?: How does specific gravity work: it normalises a density measurement against a familiar reference (water at 4 °C) so that values land in a convenient range — most engineering liquids are 0.7–1.5 SG, most metals 1.5–22 SG. The dimensionless ratio is independent of the unit system, which is why hydrometer scales, battery testers, and brewing density-meters all read in SG rather than kg/m³.

The original principle of buoyancy

A solid heavier than a fluid will, if placed in it, descend to the bottom of the fluid, and the solid will, when weighed in the fluid, be lighter than its true weight by the weight of the fluid displaced. This is the foundational statement of what we now call the Archimedes principle — and the basis for every specific gravity measurement of a solid by submerged-weight comparison.

Archimedes — On Floating Bodies, Book I, Proposition 7 → ca. 250 BCE; translation by T. L. Heath, 1897

Related calculators and references

Sources and further reading

Archimedes. On Floating Bodies, ca. 250 BCE. The original statement of buoyancy and the principle behind every specific-gravity solid measurement.
ASTM International. ASTM D1298 — Standard Test Method for Density, Relative Density, or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method.
ASTM International. ASTM D4052 — Standard Test Method for Density, Relative Density, and API Gravity of Liquids by Digital Density Meter.
API. Manual of Petroleum Measurement Standards (MPMS) 11.1 — Temperature and Pressure Volume Correction Factors for Generalised Crude Oils, Refined Products, and Lubricating Oils.
IUPAC. Gold Book — Definitions of density, specific gravity, and relative density.
NIST. SP 250-78 — Density Calibration Standards. Reference values traceable to international standards.