The specific gravity calculator above turns a density into a ratio: how heavy a substance is compared with a reference, usually water for solids and liquids and air for gases. The result carries no units, which is the whole point. A specific gravity of 2.7 for aluminium is 2.7 in every unit system on earth, and that portability is why the quantity survives in industries that otherwise abandoned it decades ago.
Arb Digital builds free physics calculators that make the hidden assumptions visible. In specific gravity the hidden assumption is always the reference — which water, at which temperature. This page makes you choose, shows what that choice does to the answer, and reports the API gravity and the floating draft that fall out of the same number.
What This Specific Gravity Calculator Does
It divides a density by a reference density. You can supply the density directly in any of five common units, derive it from a mass and a volume you measured, or run the whole thing backwards from a published specific gravity to get an absolute density in the units you need.
The grid gives the density in kilograms per cubic metre and in pounds per cubic foot, so the answer is usable in either system without a second conversion. It gives the API gravity, which is the petroleum industry's inverted scale built directly on specific gravity. And it gives the fraction of the object that would sit below the waterline if it floated freely, which is the most intuitive physical meaning the number has.
The tool also states, in plain terms, whether the substance floats or sinks in the reference. That is not decoration: a specific gravity above 1 against water means it sinks, and below 1 means it floats, and a great many practical questions reduce to exactly that comparison.
How to Use It
- Choose your input. A known density is the fastest route. Mass and volume is the route from a bench measurement. The reverse mode turns a quoted specific gravity back into a density.
- Set the reference before you read the answer. Water at 4 °C is the classical definition and gives exactly 1,000 kg/m³. Water at 60 °F is the petroleum convention. For a gas, switch to air.
- Enter mass and volume in whatever you measured. The unit selectors cover grams and millilitres for laboratory work and pounds and gallons for field work.
- Ignore the API figure for anything that is not a petroleum liquid. It is defined only on the 60/60 water basis and is meaningless for solids and gases.
- Read the submerged fraction as a physical check. Ice at 0.917 sits with 91.7 per cent of its volume under water, which is the familiar iceberg result and a good way to confirm the answer is sane.
The Formula: How Specific Gravity Is Calculated
Specific gravity is SG = ρ ÷ ρreference. That is the entire definition. OpenStax University Physics Volume 1, section 14.1 on fluids, density and pressure, defines it as the ratio of a material's density to that of water at 4.0 °C and one atmosphere, which is 1,000 kg/m³, and notes that this is precisely why aluminium has a specific gravity of 2.7 regardless of the units the densities were expressed in.
Where you start from a mass and a volume, the density comes first: ρ = m ÷ V, with both converted to SI internally before the division. Running backwards, ρ = SG × ρreference.
API gravity is a rescaling of specific gravity used throughout the oil industry, defined as degrees API = 141.5 ÷ SG60/60 − 131.5, where SG60/60 is the specific gravity of the liquid at 60 °F against water at 60 °F. The US Energy Information Administration glossary entry for API gravity gives exactly this expression. The scale is inverted, so a lighter liquid has a higher API number, and water itself sits at exactly 10.
Work the defaults. Ethanol at 789 kg/m³ against water at 4 °C gives SG = 789 ÷ 1,000 = 0.789. In imperial units that is 789 ÷ 16.0185 = 49.3 lb/ft³. The API figure is 141.5 ÷ 0.789 − 131.5 = 179.34 − 131.5 = 47.8. Floating freely, it would displace its own weight of water, so 78.9 per cent of the volume sits below the surface.
The Reference Is Not a Detail
Every specific gravity figure carries an unstated pair of temperatures — the temperature of the sample and the temperature of the reference water. They are not usually the same, and different industries have settled on different conventions. Chemistry commonly uses 20/20 or 25/4. Petroleum uses 60/60 °F. The classical textbook definition uses water at 4 °C, its density maximum, which makes the reference exactly 1,000 kg/m³.
The differences are small but not negligible. Water at 20 °C is 998.21 kg/m³, about 0.18 per cent below its 4 °C value. That shifts a specific gravity in the fourth decimal place, which is irrelevant for a school problem and very relevant in custody transfer of a liquid, where a fraction of a per cent is money. This is why the tool asks rather than assumes.
The sample temperature matters more than the reference temperature, because most liquids expand considerably more than water does. A hydrocarbon can change density by nearly one per cent for every ten degrees Celsius, so a specific gravity quoted without a sample temperature is incomplete information rather than a slightly imprecise one.
Where Specific Gravity Still Earns Its Place
It survives because it makes a comparison immediate. Will this sink? Will this layer float on that? Is this concentrate stronger than the last batch? All of those are answered by one dimensionless number, without anyone converting anything.
Brewing, winemaking and distilling use it constantly, because the sugar dissolved in a wort raises its density in a predictable way, and the fall in specific gravity during fermentation tracks how much sugar has been converted. Battery servicing uses it because the sulfuric acid concentration in a lead-acid cell, and therefore its state of charge, shows up directly as a specific gravity reading on a hydrometer. Mineral identification uses it because it is one of the few properties of a rough stone that can be measured non-destructively in minutes.
For gases the reference switches to air, and the number tells you immediately whether a leak will pool at floor level or rise to the ceiling. Carbon dioxide at a specific gravity of about 1.5 against air collects in low spaces; methane at about 0.55 rises. That single comparison drives where detectors are mounted.
Where This Sits Next to the Density Tools
The density calculator returns an absolute density in whichever units you ask for, and solves for mass or volume as well. This page returns a ratio, which is a different quantity that happens to be built from the same measurement. If you want the number in kilograms per cubic metre, that page is the right one; if you want to know how it compares with water, this one is.
The density converter rescales a density between units without changing what it describes. The water density calculator gives the reference figure at a temperature of your choosing, which is what you need if your convention is not one of the four presets here. The air density calculator does the same job for the gas reference, including the altitude correction. And the buoyancy calculator takes the floating behaviour further, computing the actual buoyant force rather than the fraction submerged.
Arb Digital builds free tools like this one because useful pages earn attention. If you want tools, calculators or content built for your own audience, we can help.
Browse All Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Quoting a specific gravity without its temperatures — 20/20 and 60/60 are different conventions, and a hydrocarbon's density can move a per cent over ten degrees.
- Using water as the reference for a gas — gas specific gravities are referred to air, and using water gives a number a thousand times too small.
- Applying API gravity outside petroleum liquids — the scale is defined on the 60/60 water basis and is meaningless for solids, gases and aqueous solutions.
- Confusing specific gravity with specific weight — specific weight is a weight per unit volume in newtons per cubic metre and depends on gravity. Specific gravity is dimensionless and does not.
- Measuring volume by displacement on a porous sample — anything that absorbs the liquid or traps air gives a volume that is wrong, and the error goes straight into the ratio.
Related Free Tools From Arb Digital
Get an absolute density with the density calculator and move it between units with the density converter. Pin down your reference with the water density calculator or the air density calculator. For what happens when the object is actually in the fluid, use the buoyancy calculator, and for solutions the concentration converter and solution concentration calculator cover the composition side. The mass to volume converter handles the everyday conversion this calculation rests on. Everything is on the free online tools hub.
Frequently Asked Questions
Density is an absolute quantity with units, such as kilograms per cubic metre. Specific gravity is that density divided by the density of a reference substance, so it has no units at all. The same material has one density expressed differently in each unit system, but one specific gravity everywhere.
It depends on your field. The classical physics definition uses water at 4 degrees Celsius, where its density is exactly 1,000 kilograms per cubic metre. Petroleum work uses 60 degrees Fahrenheit for both sample and reference. Laboratory chemistry often uses 20 degrees Celsius. Always state which you used.
It means the liquid has the same specific gravity as water on the 60/60 basis. Anything above 10 API is lighter than water and will float on it; anything below sinks. Light crudes typically run in the 35 to 45 range and heavy crudes below 22.
It was designed so that lighter, more valuable crude oils get larger numbers, which suits an industry that prices on lightness. The formula 141.5 divided by specific gravity minus 131.5 produces that inversion and was chosen so that hydrometers of the day read conveniently.
Easily, for dense materials. Lead is about 11.3, gold about 19.3, and osmium about 22.6. Against air as the reference the numbers are far larger still, which is one reason gases are always quoted against air rather than water.
Because a floating object displaces its own weight of fluid. The displaced volume divided by the total volume works out to be exactly the ratio of the two densities, which is the specific gravity. Ice at 0.917 therefore floats with 91.7 per cent of its volume under water.
Yes, because the density of the sample changes and usually the reference does too. Most liquids expand more with temperature than water does, so their specific gravity falls as they warm. That is why a specific gravity is only fully specified once both temperatures are stated.
This tool is provided for educational and general reference use. It uses the reference density you select and does not correct for sample temperature, dissolved gas or measurement conditions, so for custody transfer or laboratory work follow the measurement standard that applies to your material.
