In short: specific gravity (SG) is how many times denser a substance is than water — density ÷ 999.972 kg/m³ (water at 4 °C, its densest point). Above 1 means denser than water; below 1 means less dense. Seawater runs about 1.025, ethanol about 0.79, and mercury about 13.5.
This figure is a planning aid based on published density data. For lab, safety or regulatory work, confirm the density and reference standard with a qualified professional.
How to calculate specific gravity
Specific gravity compares a substance's density to the density of water at a chosen reference temperature. Because both sides of the ratio are a density, the units cancel — SG is always a bare number, not a measurement in kilograms or pounds.
Worked example. Say you weigh 227 g of an unknown liquid and find it fills 250 mL. Density = 227 g ÷ 250 mL = 0.908 g/cm³ = 908 kg/m³. Divide by the 4 °C reference: 908 ÷ 999.972 = 0.908. That places the liquid in the range of a common vegetable oil, well below water and well above a typical spirit.
Why not just compare kilograms and liters? Because SG is a ratio, it stays the same no matter what volume you actually measured — 227 g in 250 mL gives the same 0.908 as 2.27 kg in 2.5 L. That portability is why SG, rather than raw density, is the number quoted on datasheets and hydrometers.
Why the water reference you pick matters
Two things change the number you get: the substance's own density, and which water reference you divide by. Water is unusual in that it reaches its maximum density at about 4 °C, not at freezing, which is why 999.972 kg/m³ is the traditional strict reference. Many laboratories instead use the 20/20 convention — sample and water both held at 20 °C (998.20 kg/m³) — simply because a 20 °C room is easier to hold steady than a 4 °C one.
The two references differ by only about 0.18%, but that is enough to matter for precise chemistry or brewing work. If you need to see how density itself shifts across the full temperature range, the density-by-temperature calculator lays out the curve; this tool assumes you already have a density figure for your substance and focuses on the comparison to water.
Specific gravity of common substances
Typical published densities compared to the 4 °C water reference (999.972 kg/m³). °API is a petroleum-industry scale — shown only where it is conventionally used, for fuels and oils.
| Substance | Density (kg/m³) | Specific gravity | °API |
|---|---|---|---|
| Gold | 19,300 | 19.30 | — |
| Mercury | 13,534 | 13.53 | — |
| Aluminum | 2,700 | 2.700 | — |
| Table salt (solid) | 2,170 | 2.170 | — |
| Milk, whole | 1,030 | 1.030 | — |
| Seawater (3.5%) | 1,025 | 1.025 | — |
| Water (reference) | 999.972 | 1.000 | 10.0 |
| Ice (0 °C) | 916.7 | 0.9167 | — |
| Motor oil (SAE 30) | 875 | 0.875 | 30.2 |
| Ethanol | 789 | 0.789 | 47.8 |
| Gasoline | 745 | 0.745 | 58.4 |
Worked examples
Is seawater heavier than tap water?
SG 1.025
Seawater at about 1,025 kg/m³ comes out to SG 1.025 against the 4 °C reference — about 2.5% denser than fresh water. That extra density is why boats sit slightly higher in the ocean than in a freshwater lake.
Identifying a mystery kitchen liquid
SG 0.908
227 g in 250 mL works out to 908 kg/m³, or SG 0.908. That sits in the range published for common vegetable oils (roughly 0.91–0.93), not water or a spirit.
Checking a bottle of ethanol
SG 0.789
Pure ethanol at 789 kg/m³ gives SG 0.789 and an API gravity of about 47.8° — consistent with near-pure ethanol rather than a diluted spirit, which typically sits closer to 0.95.
What else affects the result
Temperature of the sample itself. Most liquids expand and lose density as they warm, so a density measured at 30 °C will read slightly lower than the same liquid at 20 °C. If your datasheet states a temperature, measure — or at least note — your sample at the same temperature.
Dissolved gas or air bubbles. A liquid sample with trapped air reads artificially light. Let carbonated or freshly shaken liquids settle before weighing a known volume.
Porosity, for solids. A porous solid — wood, an aggregate, a ceramic — has a lower bulk density than its particle density because of the air inside it. Which one you want depends on the question you're answering: use buoyancy or displacement calculations for how an object behaves in water, rather than SG alone.
Dissolved solids in a liquid. Salinity, sugar or mineral content raises a liquid's density in a fairly predictable way. The water density calculator handles that adjustment directly if you are starting from a known salinity rather than a measured density.
Frequently asked questions
What is specific gravity?
Specific gravity (SG) is the ratio of a substance's density to the density of water, so it is a plain number with no unit. A value above 1 means the substance is denser than water — it will generally sink in it — and a value below 1 means it is less dense and will tend to float.
Should I use water at 4°C or 20°C as the reference?
Water at 4°C (999.972 kg/m³) is its point of maximum density and is the traditional, strict reference. Many labs instead use the 20/20 convention — both the sample and the water measured at 20°C (998.20 kg/m³) — because it is easier to hold a room at 20°C than at 4°C. The two references differ by about 0.18%, so state which one you used for precise work.
How do you calculate specific gravity from density?
Divide the substance's density by the reference water density, in the same units: SG = density of substance ÷ density of water. For example, a liquid measured at 1,025 kg/m³ divided by 999.972 kg/m³ (the 4°C reference) gives a specific gravity of 1.025.
What is the specific gravity of water?
Water compared to itself at the same temperature has a specific gravity of exactly 1.000 by definition. But if you compare water at a different temperature to the 4°C reference this calculator defaults to, the number moves — water at 20°C reads about 0.9982 against the 4°C standard, reflecting the roughly 0.18% it has already expanded.
What is API gravity and how does it relate to specific gravity?
API gravity is a petroleum-industry scale defined as 141.5 divided by specific gravity, minus 131.5. Water has an API gravity of exactly 10. Most crude oils and refined fuels fall between about 10 and 70° API; anything below 10° API is denser than water. It is conventionally used for petroleum liquids rather than other substances.
Sources & references
- Kell, G. S. (1975). Density, thermal expansivity, and compressibility of liquid water from 0° to 150 °C. Journal of Chemical & Engineering Data, 20(1), 97–105.
- National Institute of Standards and Technology (NIST). Thermophysical Properties of Fluid Systems.
- American Petroleum Institute. API Gravity, ASTM D1298 / API MPMS Chapter 9.
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)