Specific Gravity Calculator

Background.

About nine-tenths of an iceberg sits below the waterline, and specific gravity explains the figure. Glacier ice has a density of about 917 kg/m³ and seawater about 1,025 kg/m³. Divide one by the other and you get 0.89, which is also the fraction of a floating berg that must be submerged for it to displace its own weight.

Specific gravity is that comparison made general: a density divided by a reference density, usually water's, leaving a number without units. Below 1 a solid floats in the reference liquid and above 1 it sinks, and the value itself works as a fingerprint. Ethanol sits near 0.79, quartz sand near 2.65, steel near 7.85, mercury near 13.5. A hydrometer reads the ratio directly, which is why brewers, winemakers, fuel traders and gemmologists speak in specific gravity rather than in kg/m³.

The subtle part is the reference. Water has no single density: it peaks at 999.97 kg/m³ near 4 °C and falls to 998.21 kg/m³ at 20 °C. That 0.18 percent difference is irrelevant when you are identifying a mineral and decisive when a refinery or an excise officer is reading the third decimal place. Standards therefore name their conditions as a pair, such as 20/4 °C (sample at 20 °C, water at 4 °C) or the petroleum industry's 60/60 °F, and two specific gravities can only be compared when their pairs match. That is why both densities are entered here rather than one being assumed.

What is specific gravity calculator?

Specific gravity, also called relative density, is the ratio of a substance's density to the density of a reference substance under stated conditions: SG = ρ_sample ÷ ρ_reference. For liquids and solids the reference is water; for gases it is usually dry air at the same temperature and pressure. Both densities carry the same units, so the units cancel and the ratio is dimensionless. Measured against water near 4 °C, specific gravity is numerically almost identical to density in g/cm³, which is convenient and is also why the two are so often confused.

How to use this calculator.

1. 01Enter the sample's density in kg/m³. A value in g/cm³ or g/mL is multiplied by 1,000, so 0.85 g/mL becomes 850.
2. 02Enter the reference density for the convention you are following: 999.97 kg/m³ for water at 4 °C, 998.21 at 20 °C, about 999.0 at 15.6 °C (60 °F), or about 1.204 for dry air at 20 °C and 101.325 kPa when the sample is a gas.
3. 03Check that the sample density belongs to the temperature your convention names. A hydrometer read at a different temperature from its calibration needs the instrument's correction table first.
4. 04Read the ratio and record its temperature pair beside it, for example 0.8508 at 60/60 °F, so anyone comparing it with a published figure knows what was measured.

The formula.

SG = ρ_sample ÷ ρ_reference, and nothing else enters. Because the division cancels units, any matching pair gives the same result: 850 kg/m³ over 1,000 kg/m³ and 0.850 g/cm³ over 1.000 g/cm³ are both 0.85.

Several working scales are transforms of this one ratio. API gravity, used to grade crude oil and petroleum products, is 141.5 ÷ SG − 131.5 with both fluids at 60 °F; it runs backwards, so lighter oils score higher, and water scores exactly 10. For a floating object, its specific gravity divided by that of the liquid gives the fraction of its volume below the surface. When the density is not known, Archimedes' method gives the ratio directly: weigh the object in air and again hanging in water, and SG = W_air ÷ (W_air − W_water), because the weight lost equals the weight of the water displaced.

The calculator performs only the division. It does not convert to API, Baumé or Plato values, and it applies no temperature correction, which is why the reference density is something you choose rather than a fixed constant. Both densities must be positive.

A worked example.

Frequently asked questions.

References& sources.

1. [1]NIST Special Publication 811, Guide for the Use of the International System of Units, 2008 edition.
2. [2]ASTM D1298-12b (2017), Standard Test Method for Density, Relative Density, or API Gravity (PAYWALL/BIBLIOGRAPHIC).
3. [3]BIPM, The International System of Units (SI Brochure), 9th ed., version 3.01, coherent derived units and quantity equations.