The specific gravity of water is 1. It is exactly 1 rather than approximately 1, because specific gravity is defined as the ratio of a substance’s density to the density of water, and water compared with itself gives 1. Anything with a specific gravity above 1 is denser than water and sinks in it; anything below 1 floats. Mercury is 13.6, ice is 0.917.
What specific gravity actually means
Specific gravity, also called relative density, answers one question: how heavy is this material compared with an equal volume of water?
SG = ρsubstance / ρwater
Both densities are in the same units, so the units cancel and specific gravity is a pure number. Being dimensionless means it reads the same in every system: water is 1000 kg/m³ in SI, 1 g/cm³ in CGS and 62.4 lb/ft³ in FPS units, but its specific gravity is 1 in all three.
Which water, at what temperature?
Water expands when heated, so its density depends on temperature and the definition needs a reference condition. Water is also odd: it reaches its maximum density of 999.97 kg/m³ at 3.98 degrees C, not at freezing point, which is why lakes freeze from the top down.
Engineering practice rounds that peak to a convenient 1000 kg/m³ at 4 degrees C, which is the reference behind most textbook problems. Two other conventions are in daily use: 20/20 degrees C, with sample and reference water both at 20 degrees C, standard in chemical and food laboratories; and 60/60 degrees F (15.56 degrees C), the petroleum convention behind API gravity, where API = 141.5/SG – 131.5.
The gap between these references is only about 0.2 percent, which never matters in a first-year problem and matters a great deal in fuel trading, where a tanker load is bought on density.
Worked example
Problem. A sample of engine oil has a density of 890 kg/m³. Find its specific gravity, its specific weight and the mass of 20 litres of it.
- SG = 890 / 1000 = 0.89, no units. Below 1, so the oil floats on water.
- Specific weight γ = ρg = 890 × 9.81 = 8730 N/m³.
- Mass = ρV = 890 × 0.020 = 17.8 kg. The same volume of water would be 20 kg.
Density, specific weight, specific volume and specific gravity
Four terms, four different quantities, and mixing them up is the most common mistake here. For water at 4 degrees C:
| Quantity | Symbol and formula | SI unit | Value for water |
|---|---|---|---|
| Density | ρ = m/V | kg/m³ | 1000 (999.97 at its peak) |
| Specific weight | γ = ρg | N/m³ | 9810, that is 9.81 kN/m³ |
| Specific volume | v = 1/ρ | m³/kg | 0.001 |
| Specific gravity | SG = ρ/ρwater | None, dimensionless | 1 |
Specific weight depends on g, so water weighs slightly less on a mountain than at sea level while its density and specific gravity do not change at all. Specific weight is the one to use for hydrostatic pressure, p = γh.
Specific gravity of common substances
| Substance | Specific gravity | Substance | Specific gravity |
|---|---|---|---|
| Petrol (gasoline) | 0.72 to 0.78 | Seawater | 1.020 to 1.030 |
| Diesel | 0.82 to 0.87 | Glycerine | 1.26 |
| Engine oil (SAE 30) | 0.88 to 0.90 | Concrete | 2.4 plain, 2.5 reinforced |
| Ice | 0.917 | Aluminium | 2.70 |
| Water (reference) | 1.000 | Steel | 7.85 |
| Milk (whole cow) | 1.028 to 1.034 | Mercury | 13.6 |
Mercury at 13.6 is worth memorising, because manometer problems use it constantly: 1 mm of mercury equals 13.6 mm of water column.
How specific gravity is measured
Hydrometer. A sealed glass float with a weighted bulb and a graduated stem. Drop it in and it sinks until the weight of liquid it displaces equals its own weight, which is Archimedes’ principle. A denser liquid needs less volume displaced, so the float rides higher, and the scale read at the surface is the specific gravity directly. Variants have their own names: a lactometer for milk, a saccharometer for sugar solutions, a battery hydrometer for acid.
Pycnometer or specific gravity bottle. A small glass bottle of exactly known volume with a capillary-bored stopper, so the filled volume repeats. Weigh it empty, full of distilled water, then full of the sample, and SG = (sample – empty) / (water – empty). Slower than a hydrometer but accurate to four decimal places, and the standard laboratory method.
Digital density meter. A U-shaped glass tube is made to oscillate, and its natural frequency shifts with the mass of liquid inside it. The instrument converts that shift to density and SG from one or two millilitres of sample. This is what refineries and dairies use now.
Why specific gravity decides floating and sinking
A body floating at rest displaces its own weight of fluid. For a body of uniform density that gives a short result:
Fraction of volume submerged = SG of the body / SG of the fluid
Example. A cube of ice 0.5 m on each side floats in fresh water.
- Volume = 0.5³ = 0.125 m³. Density of ice = 0.917 × 1000 = 917 kg/m³.
- Mass = 917 × 0.125 = 114.6 kg, weight = 1124 N. To balance it the cube must displace 114.6 kg of water, which is 0.1146 m³.
- Submerged fraction = 0.1146 / 0.125 = 0.917, that is 91.7 percent below the surface and only 8.3 percent showing.
In seawater at SG 1.025 the fraction becomes 0.917/1.025 = 0.895, so 89.5 percent sits below the waterline. That extra buoyancy from dissolved salt is why swimming in the sea feels easier than in a pool.
Where specific gravity is used in practice
- Lead-acid battery state of charge. Sulphuric acid electrolyte is consumed as the battery discharges, so its SG falls: about 1.265 to 1.280 fully charged, around 1.120 flat. A hydrometer on a car or inverter battery is still the quickest honest health check there is.
- Milk testing. Whole cow milk sits at 1.028 to 1.034. Added water pulls the reading toward 1.000, so a lactometer reading below about 1.026 at a dairy collection centre flags dilution.
- Urine testing. Normal urine runs about 1.005 to 1.030. Low suggests over-hydration or a kidney concentrating problem, high suggests dehydration.
- Fuel quality. Petrol and diesel are bought, taxed and checked on density corrected to a standard temperature, and an off-spec SG at a pump usually means adulteration with a cheaper solvent.
- Drilling mud. The mud column holds back formation pressure, so its SG, often 1.1 to 2.2 after adding barite, is controlled continuously. Too light and the well kicks, too heavy and the formation fractures.
References
- NIST – Density and Specific Gravity, National Institute of Standards and Technology.
- Hydrometer method, Agronomy Journal.
FAQs
What is the specific gravity of water?
Exactly 1. Specific gravity is the ratio of a substance’s density to the density of water, and water divided by water is 1. With water at 1000 kg/m³, SG = 1000/1000 = 1.
Why is specific gravity dimensionless?
Because it divides one density by another. Both use the same unit, so the units cancel and only a number is left, the same number whether the densities were in kg/m³, g/cm³ or lb/ft³.
What is the difference between specific gravity and specific weight of water?
Specific gravity is a dimensionless ratio and equals 1 for water. Specific weight is weight per unit volume, γ = ρg, and for water it is 1000 × 9.81 = 9810 N/m³ or 9.81 kN/m³. Specific weight depends on local gravity; specific gravity does not.
Why is the density of water highest at 4 degrees C and not at 0 degrees C?
Below about 4 degrees C, water molecules begin arranging into the open hydrogen-bonded structure that becomes ice, and that structure takes up more space. Density therefore rises on cooling to 3.98 degrees C, peaks at 999.97 kg/m³, then falls. Ice ends up at SG 0.917 and floats.
How do you find whether an object will float in water?
Compare specific gravities. If the object’s SG is less than 1 it floats, and the fraction of its volume below the surface equals its SG divided by the SG of the liquid. Ice at 0.917 floats with 91.7 percent submerged, while steel at 7.85 sinks.