How dense a substance is compared to water is called its specific gravity (also known as relative density). It’s expressed as a simple number: water itself has a specific gravity of 1.000, so anything above 1 is denser than water and anything below 1 is less dense. A substance with a specific gravity of 2.5, for example, is two and a half times as dense as water.
How Specific Gravity Works
Specific gravity is calculated by dividing the density of a substance by the density of water. Water’s density is 1.000 g/mL at 4°C, which is the temperature where water is at its densest. Because you’re dividing one density by another, the units cancel out. A mineral with a density of 3 g/cm³ divided by water’s density of 1 g/cm³ gives a specific gravity of simply 3, with no units attached. This makes specific gravity a dimensionless number, which is one reason it’s so convenient. You can compare substances without worrying about whether someone measured in grams per milliliter or pounds per cubic foot.
Because water’s density rounds to 1 in metric units, specific gravity and density often look identical for practical purposes. A liquid with a density of 0.79 g/mL has a specific gravity of 0.79. The key difference is that density carries units and specific gravity does not.
Sinking, Floating, and What the Number Tells You
The most intuitive way to think about specific gravity is through floating and sinking. A substance with a specific gravity less than 1 floats in water. A substance with a specific gravity greater than 1 sinks. This is why oil slicks sit on top of oceans and why gold drops straight to the bottom of a river.
Here are some common substances and their specific gravity values to give you a sense of the range:
- Gasoline: 0.67 (floats easily on water)
- Ethyl alcohol: 0.791 (floats on water)
- Water: 1.000 (the baseline)
- Gold: 19.32 (nearly twenty times denser than water)
Ice has a specific gravity just under 1, which is why it floats. This is actually unusual. Most solids are denser than their liquid form, but water expands slightly when it freezes, making ice about 9% less dense than liquid water. That quirk is why lakes freeze from the top down instead of the bottom up.
How Specific Gravity Is Measured
The simplest and oldest tool for measuring specific gravity in liquids is a hydrometer. It’s a sealed glass tube, weighted at the bottom with lead shot or mercury, that you lower into a column of the liquid you want to test. The hydrometer floats upright, and you read the specific gravity value off a scale printed on the glass stem at the point where the liquid’s surface touches it.
Hydrometers work on a principle first described by Archimedes: a floating object is pushed up by a force equal to the weight of the liquid it displaces. In a denser liquid, the hydrometer doesn’t need to sink as far to displace enough liquid to support its weight, so it rides higher. In a less dense liquid, it sinks lower. The scale on the stem translates that float height into a specific gravity reading. Hydrometers are calibrated using water at 60°F (about 15.5°C) as the baseline of 1.000.
You’ll find hydrometers used by homebrewers (to track how much sugar yeast has converted to alcohol), by mechanics (to check the condition of battery acid or antifreeze), and in fuel testing.
Specific Gravity in Medicine
One place specific gravity shows up in everyday health is urine testing. A urine specific gravity test measures how concentrated your urine is compared to water, which reflects how well your kidneys are balancing fluid. Normal urine specific gravity falls between 1.005 and 1.030. Pure water would be 1.000, so urine is always slightly denser because it contains dissolved waste products like salts, urea, and other compounds.
A reading toward the high end of that range, or above it, typically means concentrated urine. Dehydration is the most common cause, but heart failure, high blood sodium, and certain hormonal conditions can also push the number up. A reading toward the low end can indicate that you’ve been drinking a lot of fluid, but persistently low values sometimes point to kidney damage, kidney infection, or diabetes insipidus (a condition where the body can’t properly concentrate urine). The test is quick, inexpensive, and often part of a routine urinalysis.
Why Temperature Matters
Liquids expand when they warm up and contract when they cool down, which changes their density. Since specific gravity is a comparison to water at a set temperature, the temperature of both the substance and the reference water need to be accounted for. Most laboratory measurements use water at 4°C as the reference point, because that’s where water reaches its maximum density of 1.000 g/mL. Industrial applications sometimes use 60°F (15.5°C) instead. If you see a specific gravity value written as “1.026 at 20/4°C,” it means the substance was measured at 20°C and compared to water at 4°C.
For casual measurements, like checking the sugar content of homebrew or testing aquarium salinity, small temperature differences won’t throw your reading off by much. For precise scientific or industrial work, temperature correction tables are standard practice.