Specific Gravity Calculator

Background.

Specific gravity strips the units off density: divide a sample's density by a reference density — nearly always water — and the dimensionless number that remains tells you instantly how the material stands relative to that reference. SG < 1 floats on it, SG > 1 sinks in it, and the digits carry real trade information: 0.79 reads as an alcohol or light solvent, 1.03 as seawater, 2.65 as quartz sand, 13.5 as mercury.

Because it is a pure ratio, specific gravity is what hydrometers actually measure, and whole industries are built on that convenience. Brewers track fermentation as SG falls from a sugary 1.050 toward 1.010 while yeast converts dense sugar to lighter alcohol; clinical labs flag urine outside 1.005–1.030; battery service judges charge from electrolyte near 1.28; and the petroleum trade prices crude with API gravity, a rescaled inverse of SG at 60 °F.

The quiet variable underneath is temperature. Water's density is not one number: 999.97 kg/m³ at 4 °C but 998.21 kg/m³ at 20 °C, and the sample's density drifts with temperature too. That is why standards name their conditions — petroleum's 60/60 °F convention measures both sample and reference at 15.6 °C — and why this calculator makes you enter the reference density explicitly instead of assuming one silently. Two SG values only compare when their temperature conventions match.

Enter both densities in the same unit and the ratio comes out convention-free; the model-scope note beside the result records that everything else — which water, which temperature, which standard — travels with your inputs, not the arithmetic.

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: SG = ρ_sample / ρ_reference. With water as reference — the default for solids and liquids — it is a dimensionless restatement of density that reads directly as buoyancy: less than 1 floats, greater than 1 sinks. Gas work uses air as the reference instead, and precise usage always names the temperature of both substances, since both densities drift with temperature.

How to use this calculator.

1. 01Measure or look up the sample's density and convert it to kg/m³ (g/cm³ × 1,000; g/mL is the same as g/cm³).
2. 02Enter the reference density at your convention's temperature: 999.97 kg/m³ for water at 4 °C, 998.21 at 20 °C, 999.02 at 15.6 °C (60 °F), or air ≈ 1.204 for gas-phase SG.
3. 03Read the ratio: it has no units, and any value within a few percent of 1 deserves a second look at temperature effects before you conclude float or sink.
4. 04For petroleum work, convert to API gravity with 141.5/SG − 131.5 — the industry quotes crudes that way.
5. 05Record the temperature pair (e.g. 20/4 °C, 60/60 °F) beside the result; an SG without its convention is not comparable to a tabulated one.

The formula.

The formula is a single division, SG = ρ_sample / ρ_reference, and its value lies in what division removes: units and instrument calibration factors cancel, leaving a number every lab reproduces regardless of measurement system — the reason century-old hydrometer scales still work. The subtleties are all in the reference. Water's density peaks at 4 °C, which made 4 °C the scientific reference of choice (SG there is numerically almost identical to density in g/cm³), while trade standards prefer matched-temperature conventions like petroleum's 60/60 °F, encoded in ASTM D1298. Derived scales are simple transforms of the ratio: API gravity = 141.5/SG − 131.5, Baumé and Plato likewise. The engine performs the division in Decimal arithmetic and rounds once to twelve significant digits — overkill for a hydrometer reading, but it guarantees the rounding never contributes to a float/sink call near SG = 1.

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.