Specific gravity is the ratio of a material's density to the density of water
Specific gravity compares how heavy a substance is to how heavy the same volume of water would be. If a material has a specific gravity of 2, it is twice as dense as water. If it has a specific gravity of 0.5, it floats because it is half as dense as water. The formula is straightforward: divide the density of your material by the density of water at 4°C (which is 1 gram per cubic centimetre or 62.4 pounds per cubic foot).
You do not need expensive equipment. A scale, a measuring cup or ruler, and basic arithmetic are enough for most materials. The calculation works the same way whether you are testing a rock, a piece of metal, a liquid, or a gas — only the method of measuring volume changes.
Key Takeaways
- Specific gravity is calculated by dividing the density of a material by the density of water (1 g/cm³ or 62.4 lb/ft³).
- You need two measurements: the mass of your material and its volume, then divide mass by volume to find density.
- Water always has a specific gravity of 1.0 by definition, so any material denser than water sinks and any less dense floats.
- The same formula works for solids, liquids, and gases, but measuring volume differs for each state of matter.
Gather the mass and volume of your material
Weigh your material on a scale and record the mass in grams (or convert to grams if your scale reads in ounces). For solids, measure volume by water displacement: fill a graduated cylinder or measuring cup with a known amount of water, place the object in it, and subtract the original water level from the new level. The difference is your volume in millilitres (which equals cubic centimetres).
For liquids, pour a measured amount into a graduated cylinder — 50 millilitres or 100 millilitres works well. Weigh an empty container, pour the liquid into it, weigh it again, and subtract the container weight to get the liquid's mass. For gases, the process is more complex and usually requires a gas pycnometer or a sealed container of known volume, so most people leave gas calculations to a laboratory.
Write down both numbers clearly. Errors in measurement compound in the final answer, so measure twice if you are uncertain.
Calculate the density of your material
Density is mass divided by volume. If your material weighs 150 grams and occupies 50 cubic centimetres, the density is 150 ÷ 50 = 3 grams per cubic centimetre. Keep the units consistent — if you measure mass in grams and volume in cubic centimetres, your density will be in grams per cubic centimetre. If you measure mass in pounds and volume in cubic feet, your density will be in pounds per cubic foot.
Write the density down before moving to the next step. This number is what you will divide by the density of water.
Divide by the density of water to find specific gravity
Water has a density of 1 gram per cubic centimetre at 4°C (the temperature at which water is densest). If you measured your material in grams and cubic centimetres, divide your density by 1. If you measured in pounds and cubic feet, divide your density by 62.4 pounds per cubic foot.
Using the example above: 3 g/cm³ ÷ 1 g/cm³ = 3. This material has a specific gravity of 3, meaning it is three times as dense as water and will sink. A piece of wood with a density of 0.6 g/cm³ would have a specific gravity of 0.6 and would float.
Specific gravity has no units — it is a pure ratio. This makes it straightforward to compare materials measured in different systems or at different times.
Common specific gravity values for reference
| Material | Specific Gravity | Sinks or Floats |
|---|---|---|
| Water | 1.0 | Reference point |
| Ice | 0.92 | Floats |
| Wood (oak) | 0.75 | Floats |
| Aluminum | 2.70 | Sinks |
| Iron | 7.87 | Sinks |
| Lead | 11.34 | Sinks |
| Gasoline | 0.72 | Floats |
| Olive oil | 0.92 | Floats |
These values assume room temperature and standard conditions. Specific gravity changes slightly with temperature because density changes — hot materials expand and become less dense. For most practical purposes, the difference is small enough to ignore unless you are working in a laboratory or an industrial setting where precision matters.
Why specific gravity matters in real work
Engineers use specific gravity to predict whether a material will float or sink, which matters for shipping, construction, and mining. A mining company needs to know the specific gravity of ore to design separation equipment. A boat designer needs to know the specific gravity of hull materials to calculate how much weight the boat can carry. A chemist uses it to identify unknown liquids — if a liquid has a specific gravity of 0.79, it is likely ethanol.
In manufacturing, specific gravity helps quality control: if a batch of plastic pellets has a different specific gravity than expected, something went wrong in the production process. In geology, specific gravity helps identify minerals — gold has a specific gravity of 19.3, which is why it feels surprisingly heavy for its size.
Common mistakes to avoid
The most common error is confusing density with specific gravity. Density is an absolute measurement (mass per unit volume). Specific gravity is a comparison to water. They are related but not the same — density has units, specific gravity does not.
Another mistake is using the wrong density for water. Water's density changes with temperature: at 20°C it is 0.998 g/cm³, at 25°C it is 0.997 g/cm³. For everyday calculations, using 1 g/cm³ is close enough. For precise laboratory work, look up the exact density of water at the temperature you measured.
A third mistake is measuring volume incorrectly for irregular solids. If your object is too large for a graduated cylinder, use a larger container and measure the water displacement more carefully. If the object is porous (like a sponge or pumice), it will absorb water and give you a false reading — seal it first or use a different method.
Frequently Asked Questions
Can specific gravity be greater than 1?
Yes. Any material denser than water has a specific gravity greater than 1. Metals, stones, and most minerals sink because their specific gravity is well above 1. Lead has a specific gravity of 11.34, meaning it is more than eleven times as dense as water.
What if my material is less dense than water?
Materials with a specific gravity less than 1 float. Wood, ice, and most plastics float because they are less dense than water. This is why a block of ice floats in a glass of water — ice has a specific gravity of about 0.92.
Do I need to measure at a specific temperature?
Room temperature (around 20°C) is fine for most purposes. Specific gravity changes slightly with temperature because density changes, but the difference is usually small. If you are doing laboratory work or need high precision, record the temperature and look up the exact density of water at that temperature.
How do I measure the volume of an irregular object?
Use water displacement. Fill a graduated cylinder with water to a known level, gently place the object in it, and read the new water level. The difference between the two levels is the volume of the object. Make sure the object is fully submerged and does not float.
What is the difference between specific gravity and relative density?
They are the same thing. Specific gravity and relative density are two names for the same measurement — the ratio of a material's density to the density of water. You may see either term used in textbooks or technical documents.