Specific Gravity Calculator
Enter a density (or mass and volume) and instantly get specific gravity relative to water, seawater, air or a custom reference, plus the density converted into common units.
Convert a Density into Specific Gravity
Specific Gravity Formula
Where:
- \(SG\) — specific gravity (dimensionless)
- \(\rho\) — density of the substance (kg/m³)
- \(\rho_r\) — density of the reference substance (kg/m³)
Specific gravity is the ratio of a substance's density to the density of a reference material. Because it is a ratio of two densities, the result is a pure number with no units. For liquids and solids the reference is normally pure water; for gases it is air.
If you only know the mass \(m\) and volume \(V\) of a sample, its density is found first:
\[ \rho \;=\; \frac{m}{V} \]
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Specific Gravity – Calculation Example
Example: a piece of aluminium has a density of \(\rho = 2.7\ \text{g/cm}^3 = 2700\ \text{kg/m}^3\). Find its specific gravity relative to pure water at 4 °C (\(\rho_r = 1000\ \text{kg/m}^3\)).
\[ SG \;=\; \frac{\rho}{\rho_r} \;=\; \frac{2700\ \text{kg/m}^3}{1000\ \text{kg/m}^3} \;=\; 2.7 \]
Result: the specific gravity of aluminium is 2.7. Since the density of water is \(1\ \text{g/cm}^3\), the numerical value is the same in g/cm³ — a useful shortcut for solids and liquids referenced to water.
How to use the calculator
- Decide how you will describe your substance. If you know its density, leave the checkbox “Enter mass and volume instead of density” unticked and type the value into Density of the substance (ρ), then pick the unit (kg/m³, g/cm³, g/mL, lb/ft³ or lb/US gal).
- If you do not know the density, tick the checkbox and enter the Mass (m) and Volume (V) of your sample with their units — the calculator computes \(\rho = m/V\) for you.
- Choose the Reference substance: water at 4 °C, water at 20 °C, average seawater, air at 20 °C, or a custom reference.
- If you select Custom reference density, enter its value and unit in the Reference density (ρᵣ) field that appears.
- Read the results: the Specific gravity (SG) block shows the ratio, both densities and an interpretation (sinks, floats, rises); the Density of the substance block converts the density into kg/m³, g/cm³, lb/ft³, lb/gal and the mass of one litre.
What is specific gravity?
Specific gravity (SG), also called relative density, compares how dense a substance is to a reference material. It answers a simple question: “How many times denser (or lighter) than the reference is this substance?” Because the units cancel in the division, specific gravity is a dimensionless number — an aluminium part with SG = 2.7 is exactly 2.7 times denser than water, whatever unit system you use.
For solids and liquids the reference is almost always pure water. With water at 4 °C (density exactly \(1000\ \text{kg/m}^3\)), specific gravity is numerically equal to the density in g/cm³, which makes conversions effortless: a liquid with SG = 0.8 has a density of \(800\ \text{kg/m}^3\). For gases, the reference is usually air at 20 °C and 1 atm (about \(1.2\ \text{kg/m}^3\)), and the resulting value is used in safety assessments — for example to predict whether a leaking gas will rise or collect near the floor.
The value also tells you about buoyancy. A substance with SG > 1 sinks in water; SG < 1 floats; SG = 1 is neutrally buoyant. Ice (SG ≈ 0.92) floats with about 8 % of its volume above the waterline, while steel (SG ≈ 7.8) sinks — which is also why a steel ship floats: its average density, hull included, is far below 1 once the air inside is counted.
Reference table
| Substance | Density (kg/m³) | SG vs water at 4 °C |
|---|---|---|
| Gold | 19 300 | 19.3 |
| Lead | 11 340 | 11.34 |
| Steel | 7 850 | 7.85 |
| Aluminium | 2 700 | 2.7 |
| Seawater (average) | 1 025 | 1.025 |
| Pure water at 4 °C | 1 000 | 1.000 |
| Ice | 917 | 0.917 |
| Ethanol | 789 | 0.789 |
| Olive oil | ≈ 910 | ≈ 0.91 |
| Air at 20 °C, 1 atm | 1.204 | 0.0012 |
Assumptions, limitations and tips
- Temperature matters. Density changes with temperature, so a precise specific gravity is only meaningful together with a temperature. Water drops from \(1000\ \text{kg/m}^3\) at 4 °C to about \(998.2\ \text{kg/m}^3\) at 20 °C — a 0.2 % difference. The calculator offers both water references for this reason.
- The substance is assumed uniform. The calculation treats the material as homogeneous. Mixtures, suspensions and porous solids have an effective density that may differ from the density of any single component.
- Mass and volume must describe the same sample. When using the mass/volume mode, make sure both values refer to the same object, measured at the same temperature.
- Hydrometers and lab practice. Commercial instruments (hydrometers, pycnometers, digital density meters) report specific gravity routinely; this calculator is a quick way to check or convert such readings between unit systems.
- Specific gravity is one of the basic material properties used across physics — from buoyancy and fluid statics to problems involving energy and motion. For related topics, see our Conservation of Energy calculator, the Centripetal Force calculator and the Elastic Collision (1D) calculator.
Frequently Asked Questions
What is the difference between specific gravity and density?
Density is a physical property with units — it tells you how much mass fits in a given volume, e.g. \(2700\ \text{kg/m}^3\). Specific gravity is a pure ratio with no units: the density of the substance divided by the density of a reference (usually water). Two materials can have the same SG relative to water but very different densities in lb/ft³. In practice, SG is convenient because it is independent of the unit system and can be measured directly with a hydrometer.
What is a normal or typical specific gravity value?
For common liquids it ranges roughly from 0.7 (gasoline) to about 1.26 (glycerine), with most oils between 0.8 and 0.95 and water itself at 1.0. Metals are much heavier: aluminium is 2.7, steel about 7.8, lead 11.3 and gold 19.3. Gases referenced to air cluster near 1 — methane is about 0.55 (rises) while carbon dioxide is about 1.5 (sinks). There is no single "good" value; it depends entirely on the material and its purpose.
How do I convert specific gravity back to density?
Multiply the SG by the reference density. Referenced to water at 4 °C, a liquid with SG = 0.85 has a density of \(0.85 \times 1000 = 850\ \text{kg/m}^3\), which is also 0.85 g/cm³ or about 53.1 lb/ft³ (divide kg/m³ by 16.018). If the reference is air, multiply by roughly 1.2 kg/m³ instead. Always check which reference was used before converting — seawater and 20 °C water references shift the result by a few tenths of a percent.
How is specific gravity measured experimentally?
The classic method is a hydrometer, which floats deeper in light liquids and higher in dense ones; it is scaled directly in SG and used in brewing, winemaking and battery testing. For an irregular solid, weigh it, then find its volume by water displacement in a graduated cylinder, and divide mass by volume. A pycnometer (density bottle) gives precise liquid densities by comparing the filled and empty weights. This calculator is a quick way to turn any of these measurements into SG or between unit systems.
Does temperature change specific gravity?
Yes, in two ways: the substance's density and the reference density both shift with temperature. Most liquids expand when heated, so their SG drops slightly — water itself goes from exactly 1000 kg/m³ at 4 °C to about 998.2 kg/m³ at 20 °C. For everyday work the difference is negligible, but in laboratories and custody transfer a reading is meaningless without its temperature. Hydrometers are calibrated for a specific temperature (often 20 °C), and correction tables exist for converting readings taken at other temperatures.
Can I use this calculator for brewing, API gravity or Baumé scales?
Yes, as a starting point. Beer and wine recipes use SG readings from a hydrometer (typically 1.000 for water, 1.040–1.100 for wort must); the drop in SG during fermentation estimates alcohol content. In the petroleum industry, API gravity is derived from SG at 60 °F via \(API = \frac{141.5}{SG} - 131.5\), where heavier oils give lower (even negative) values. This calculator gives you the plain SG, which you can then feed into those conversions.
What are common mistakes when calculating specific gravity?
The most frequent errors are using different units for the substance and the reference (always convert both to the same unit first, e.g. kg/m³), and forgetting that gases must be referenced to air, not water — an SG of 0.0012 for air versus water confuses buoyancy arguments. Also watch for mass and volume measured at different temperatures or from different samples, and remember that dissolved substances, bubbles or porous materials make the effective density differ from tabulated values for the pure substance.