Specific Gravity Calculator: convert between density and specific gravity using water as the reference fluid

Specific Gravity Calculator
Select Input Unit:
Density of reference fluid: Grams/Liter: Lb/Gallon: Lb/in³: S.G.:
Reference Fluid (Water) Temperature Unit:
°F: °C: K: °R:
Water Density (kg/m³):  
Lb / Gallon:  
Lb / in³:  
Lb / ft³:  
Kg / m³:  
Specific Gravity (S.G.):  

*60°F is the most common reference temperature for specific gravity*

Specific Gravity is a Contentious Unit

Water Densities per this page.

Also See:  

Temperature Compensated Universal Head Pressure Calculator
Reference Guide: Specific Gravity
01

What Is Specific Gravity?

Specific gravity is a fluid's density divided by the density of water. It is a ratio, so it has no units: SG = 0.80 means the fluid is 80% as dense as water.

Specific gravity of a liquid\[ \text{SG} = \frac{\rho_{\text{liquid}}}{\rho_{\text{water}}} \]

The catch is the word water. Water's density changes with temperature, from 999.97 kg/m³ at 39.2 °F to 998.2 kg/m³ at 68 °F, so an SG is only exact when you know which water it was compared to. This calculator lets you choose.

Read why specific gravity is a contentious unit before trusting a fourth decimal place.
02

Density to Specific Gravity, Unit by Unit

To get SG from a density in US units, convert the density to kg/m³ and divide by the water density. Every unit cancels, leaving a pure number:

lb/gal → SG, unit by unit\[ \text{SG} = \rho\ \frac{\cancel{\text{lb}}}{\cancel{\text{gal}}} \times \frac{1\ \cancel{\text{gal}}}{231\ \cancel{\text{in}^3}} \times \frac{27{,}679.90471\ \cancel{\text{kg}/\text{m}^3}}{1\ \cancel{\text{lb}}/\cancel{\text{in}^3}} \times \frac{1}{\rho_{\text{water}}\ \cancel{\text{kg}/\text{m}^3}} \]
lb/in³ → SG, unit by unit\[ \text{SG} = \rho\ \frac{\cancel{\text{lb}}}{\cancel{\text{in}^3}} \times \frac{27{,}679.90471\ \cancel{\text{kg}/\text{m}^3}}{1\ \cancel{\text{lb}}/\cancel{\text{in}^3}} \times \frac{1}{\rho_{\text{water}}\ \cancel{\text{kg}/\text{m}^3}} \]
Shortcuts\[ \text{SG} = \frac{119.826 \times \rho_{\text{lb/gal}}}{\rho_{\text{water}}} \qquad \text{SG} = \frac{27{,}679.90471 \times \rho_{\text{lb/in}^3}}{\rho_{\text{water}}} \]
Example: jet fuel at 6.670 lb/gal, referenced to water at 60 °F (999.0 kg/m³): SG = 119.826 × 6.670 / 999.0 = 0.8000. The constants are exact: 1 lb = 0.45359237 kg, 1 in = 0.0254 m and 1 US gallon = 231 in³ by definition.
03

Density & Specific Gravity Conversions

Specific gravity (SG) is the fluid's density divided by the density of water. It has no units, which makes it easy to quote, but it is only exact once you know which water it is compared to.

Specific gravity\[ \text{SG} = \frac{\rho_{\text{fluid}}}{\rho_{\text{water}}} \qquad \rho_{\text{fluid}} = \text{SG} \times \rho_{\text{water}} \]

Water is 999.0 kg/m³ at 60 °F and 1,000.0 kg/m³ at 4 °C. Fuel specifications usually quote SG at 60 °F. The water-density-compensated calculators compare against water at the fluid's own temperature instead. See why specific gravity is a contentious unit.

kg/m³ → lb/gal and lb/in³\[ \begin{gathered} \rho_{\text{lb/gal}} = \rho_{\text{kg/m}^3} \times \frac{231}{27{,}679.9} = \frac{\rho_{\text{kg/m}^3}}{119.83} \\ \rho_{\text{lb/in}^3} = \frac{\rho_{\text{kg/m}^3}}{27{,}679.9} \end{gathered} \]
UnitEquals
1 lb/in³27,679.9 kg/m³
1 lb/gal (US)119.83 kg/m³
1 lb/ft³16.018 kg/m³
1 US gallon231 in³ = 3.7854 L

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