Specific Gravity of Water and Common Liquids
Water is the yardstick every other liquid gets measured against. Here's what specific gravity actually means, how it differs from density, and where dozens of everyday liquids fall on the scale.
Specific gravity is the ratio of a substance's density to the density of water, which makes water the built-in benchmark of 1.000. A liquid with a specific gravity above 1.0 is denser than water and sinks in it; below 1.0, it's lighter and floats. It's one of the simplest numbers in fluid science, but it quietly drives everything from why oil slicks sit on top of the ocean to how brewers track fermentation and how mechanics check a car battery.
Skip the math Specific Gravity Calculator Open →What specific gravity means
Specific gravity (sometimes called relative density) answers one question: how does this substance's density compare to water's? You get it by dividing the density of the substance by the density of water, measured under the same conditions. Because density divided by density cancels out the units — kilograms per cubic meter over kilograms per cubic meter, or pounds per cubic foot over pounds per cubic foot — specific gravity has no units at all. It's a pure number, which is exactly what makes it so convenient: a specific gravity of 1.03 means "3% denser than water," whether you're working in metric or imperial.
That unitless quality also makes specific gravity easy to compare across wildly different substances and industries. A winemaker, a geologist, and a petroleum engineer can all quote a specific gravity and mean the same thing, even though they'd normally express density in completely different units.
Why water is the reference point
Water was chosen as the reference because it's abundant, easy to purify, and — critically — its density has been measured with extreme precision at every practical temperature. By convention, the reference is pure water at 4 °C (39.2 °F), the temperature at which water reaches its maximum density of almost exactly 1,000 kg/m³ (62.4 lb/ft³). At that single point, water's specific gravity is defined as 1.000.
Watch the reference temperature. Some industries — petroleum and brewing in particular — quote specific gravity against water at 60 °F (15.6 °C) or 20 °C (68 °F) instead of the strict 4 °C standard, because it's closer to normal working conditions. The difference is small (water is only marginally less dense at 60 °F than at 39 °F), but for precise lab or engineering work, always check which reference temperature a chart or instrument assumes.
Specific gravity of common liquids
Once water is fixed at 1.000, everything else falls into place around it. Here are typical values for liquids people ask about most, measured near room temperature:
| Liquid | Specific gravity | Floats or sinks in water |
|---|---|---|
| Gasoline | ~0.72–0.75 | Floats |
| Ethanol (ethyl alcohol) | ~0.79 | Floats |
| Ice (frozen water) | ~0.92 | Floats |
| Olive oil | ~0.91–0.92 | Floats |
| Diesel fuel | ~0.85 | Floats |
| Water (reference) | 1.000 | — |
| Whole milk | ~1.03 | Sinks |
| Seawater | ~1.02–1.03 | Sinks |
| Glycerin (glycerol) | ~1.26 | Sinks |
| Honey | ~1.36–1.45 | Sinks |
| Concentrated sulfuric acid | ~1.84 | Sinks |
| Mercury | ~13.6 | Sinks |
These are typical reference values; exact figures shift with temperature, concentration, and purity — honey's specific gravity, for instance, depends heavily on its water content.
The pattern in that table is the whole point of specific gravity: line up several liquids and you can immediately see which will layer on top of which. Pour gasoline, water, and honey into the same container undisturbed and, given time, they'll settle into three distinct bands in exactly the order their specific gravity predicts.
Specific gravity vs. density
It's easy to blur specific gravity and density together because they describe the same underlying property — how tightly packed a substance's mass is. The difference is in the packaging:
- Density is mass per unit volume, expressed in units like kg/m³, g/mL, or lb/ft³. It's an absolute measurement.
- Specific gravity is density divided by water's density, so the units cancel and you're left with a dimensionless ratio.
Because it has no units, specific gravity is what most portable field instruments — hydrometers, refractometers calibrated for liquids, and many digital density meters — are built to read directly. Once you know a liquid's specific gravity, converting to actual density is one multiplication away.
How to calculate it
The formula is the same however you get your numbers — from lab measurements, a reference table, or a hydrometer reading:
Worked example: a lab sample of a liquid weighs in at 1,260 kg/m³. Divide by water's reference density of 1,000 kg/m³:
The formula also runs in reverse, which is often the more useful direction: if you already know a substance's specific gravity, multiply it by water's density to get an actual density in whatever units you need.
Why it matters
Specific gravity shows up anywhere someone needs a fast, portable way to characterize a liquid without a full lab setup:
Buoyancy
SG < 1.0
Anything with an overall specific gravity below water's floats; above it, sinks. This is why ice floats, why life jackets work, and why a ship's hull design has to account for the water it displaces.
Brewing & winemaking
Hydrometer
Brewers track a beer or wine's specific gravity before and after fermentation — as sugar converts to alcohol, the liquid gets less dense, and the drop tells them how fermentation is progressing.
Automotive
Battery acid
A battery hydrometer checks the specific gravity of a lead-acid battery's electrolyte, which falls as the battery discharges — a quick way to judge state of charge.
Fuel & fluid QC
Spot-check
Refineries and fuel depots use specific gravity as a fast field check that a fuel or oil shipment matches its specification, without needing a full lab analysis.
The bottom line
Specific gravity is simply density expressed relative to water, which is fixed at exactly 1.000. Anything less than 1.0 floats on water, anything more sinks, and the size of the gap tells you how much lighter or heavier the liquid really is. It's a small piece of math, but because it strips away units entirely, it's become one of the most practical numbers in fluid science — easy to measure in the field and easy to compare across industries.
Frequently asked questions
What is specific gravity in simple terms?
Specific gravity is how many times heavier or lighter a substance is than an equal volume of water. It's a plain ratio with no units, so a specific gravity of 2 means the substance is twice as dense as water, and 0.5 means it's half as dense.
What is the specific gravity of water?
Exactly 1.000, by definition. Water at its temperature of maximum density (about 4 °C / 39 °F) is the reference point that every other substance's specific gravity is measured against.
How is specific gravity different from density?
Density is mass per unit volume and carries units such as kg/m³ or lb/ft³. Specific gravity is density divided by the density of water, so the units cancel out and you're left with a plain number. Density tells you how heavy something is; specific gravity tells you how it compares to water.
What is the specific gravity of common liquids like milk, oil, and mercury?
As a rough guide: gasoline is about 0.74, olive oil about 0.91, whole milk about 1.03, seawater about 1.025, honey about 1.4, and mercury about 13.6. Anything below 1.0 floats on water; anything above 1.0 sinks.
Why does specific gravity matter for buoyancy?
An object floats in water if its overall specific gravity is less than 1.0 and sinks if it's greater than 1.0, which is why ice (about 0.92) floats and a steel ball bearing (around 7.8) sinks. The same logic is why oil sits on top of water and why a hydrometer riding higher or lower in a liquid reveals its density.
What reference temperature is used for specific gravity?
The strict scientific reference is water at 4 °C (39.2 °F), where it reaches maximum density. Many industries instead reference water at 60 °F (15.6 °C) or 20 °C (68 °F) because it's a more practical working temperature, so it's worth checking which reference a chart uses before comparing figures precisely.
How do you measure specific gravity at home?
A hydrometer is the simplest tool: it's a weighted, calibrated float that you drop into a liquid, and the depth it settles to reads directly as specific gravity. This is exactly how brewers check a beer's fermentation progress and how a battery hydrometer checks lead-acid battery electrolyte.
Sources & references
The figures in this guide follow standard reference values for the density and specific gravity of water and common liquids:
- U.S. Geological Survey, Water Science School — Water Density. Pure water reaches its maximum density (0.99984 g/mL) at about 4 °C.
- The Engineering ToolBox — Specific Gravity of Liquids. Reference specific gravity values for common liquids.
- The Engineering ToolBox — Densities of Common Liquids. Density data used to derive comparative specific gravity figures.
Water Systems Specialist · Austin, Texas
A Texas-licensed Professional Engineer with 13+ years in residential water systems, John leads the science, flow and pressure guides on Water Calculator. Read full bio
Licensed Professional Engineer (PE) · Specializes in water density, flow and pressure