How this instrument works
Specific gravity states density as a ratio rather than as magnitude: divide your sample by some reference substance and all units cancel, leaving one bare number. That cancellation is its whole purpose. Metallurgists working in kg/m³ and foundries working in lb/ft³ both write 7.87 for iron, and neither has to name the unit for other shops to know exactly what was meant.
Water anchors this scale for solids and liquids, which is why readings look so familiar: because the gram was originally pinned to one cubic centimetre of water, any substance's ratio and its density in g/cm³ agree to within rounding error. Gases get compared against air instead, near 1.204 kg/m³ at 20 °C, so methane quoted as 0.55 has nothing to do with water at all. Measuring by flotation is ancient — Synesius of Cyrene wrote to Hypatia around 400 CE asking for graduated floating tubes, the instrument now called a hydrometer, whose stem is still marked directly in this ratio.
What such arithmetic quietly assumes is that you know which reference you used, and at what temperature. Both densities shift with heat, by different amounts, so the value taken at 15 °C is not one taken at 60 °F — petroleum reports print 60/60 °F on their faces for precisely that reason. Choosing water at its 4 °C peak of 999.972 kg/m³ instead of the round 1000 moves every answer by 28 parts per million: invisible on the shop floor, unacceptable in the calibration lab. ISO and IUPAC now prefer 'relative density' as its name, partly because gravity plays no role whatsoever in what gets measured.
- Type your material's figure into Substance density — that menu accepts kg/m³, g/cm³ or lb/ft³, so handbook values copy straight across.
- Leave Reference density (water) at 1000 kg/m³ for ordinary work, drop it to 999.972 for the 4 °C basis, or set 1.204 kg/m³ to rate gases against air.
- Read Specific gravity. No unit is attached, because none survives division — you get an identical number whichever density units went in.
- Compare against 1: under it your sample floats in fresh water, over it your sample sinks, and 1.025 marks where seawater sits.
Worked example — mercury and the short barometer
Mercury at room temperature measures 13534 kg/m³. Put that into Substance density, leave Reference density (water) on 1000 kg/m³, and Specific gravity returns 13.534 — litre for litre, mercury outweighs water thirteen and half times over.
That single ratio is why Torricelli's 1643 barometer fits on the wall. Atmospheric pressure supports whatever column height satisfies p ⁄ (ρg), so denser liquid means shorter column: 760 mm of mercury against roughly 10.3 metres of water for one identical push. Water barometers want three storeys of stairwell; mercury versions want one metre of glass tubing. 13.534 is exactly what separates them.
Questions
What unit is specific gravity measured in?
None at all. It is one density divided by another, so units cancel and only a bare number remains. Iron reads 7.87 whether you worked in kg/m³ or lb/ft³. This is what separates it from density, which always needs its unit bolted on, and it is why such figures travel safely between metric and imperial workshops with no conversion step in between.
Is specific gravity identical to density in g/cm³?
Numerically almost, but not by definition. Fresh water sits at 0.99997 g/cm³ near 4 °C and drops to about 0.9982 g/cm³ by 20 °C, so dividing by it is not quite dividing by one. Rate mercury against 20 °C water and you get 13.558 rather than 13.534 — 0.2% of shift, which matters in calibration reports and nowhere else. State your reference temperature next to any ratio you publish.
Why is water not always used as reference?
Because gases would produce absurd numbers against it. Methane compared with water reads 0.00067; compared with air at 1.204 kg/m³ it reads 0.55, which is what pipeline engineers actually work with. Set Reference density (water) to 1.204 kg/m³ and this instrument switches to that gas convention. Solids and liquids keep water underneath them.
What does a value below 1 tell me?
That your material floats in fresh water, with its submerged fraction equal to that very ratio. Ice at 0.917 rides with 91.7% of its bulk under fresh water; cork near 0.24 sits mostly proud. Above 1 and it sinks. Exactly 1 and it hovers wherever you leave it — neutral buoyancy, which is what divers chase with weight belts.
Why do brewers and battery shops quote it?
Because the floating hydrometer reads this ratio directly and costs almost nothing. Sugar raises wort density, so brewers see about 1.050 before fermentation and 1.010 after, and that drop converts into alcohol content. Charged lead-acid cells read near 1.265 as sulfuric acid concentrates in their electrolyte, sagging toward 1.120 when flat. In both trades that hydrometer is chemistry analysis disguised as a float.
Is 'specific gravity' even the correct name?
Standards bodies say no. ISO and IUPAC both prefer 'relative density', on reasonable grounds: no gravitational field enters anywhere. You would measure an identical ratio on Mars, because both densities change by nothing when weight does. That older name survives in petroleum, brewing, mining and clinical labs through sheer momentum, and both terms mean precisely one thing.