Specific Gravity to Density Calculator (and Density to SG)
Convert specific gravity to density in g/cm³, kg/m³, lb/ft³ and lb/gal, or density back to specific gravity, against water at 4, 15, 20 or 25 °C, 60 °F, any temperature from 0 to 40 °C, air or a custom reference. Also gives API gravity, Baumé degrees, specific volume and the weight of a volume.
How it works
density = SG × ρ(reference) · SG = density ÷ ρ(reference) · ρ(water, t) = 999.97495 × [1 − (t − 3.983035)²(t + 301.797) ÷ (522528.9 (t + 69.34881))] kg/m³ · API = 141.5 ÷ SG(60 °F) − 131.5 · °Bé = 145 − 145 ÷ SG (heavy) or 140 ÷ SG − 130 (light)
Specific gravity (relative density) compares a substance’s density with the density of a reference, almost always water for liquids and solids and air for gases. Because it is a ratio it has no units, so turning it into a density only means multiplying by the reference density, and dividing a density by the reference gives the specific gravity back. The temperature of the reference water matters from the fourth decimal: water is densest near 4 °C and about 0.2% lighter at 20 °C. This calculator uses the Tanaka et al. (2001) equation for air-free water, so you can also pick any water temperature from 0 to 40 °C; the 4 °C option follows the common convention of exactly 1.000 g/cm³. Results are given in g/cm³ (the same number as g/mL and kg/L), kg/m³, lb/ft³ and lb per US and UK gallon, with specific volume, a sink-or-float check, the API gravity and Baumé degrees used for petroleum, acids and syrups, and optionally the mass of a volume you enter. The second table converts the same density to a specific gravity against every other reference, which helps when two data sheets quote different bases.
Worked example
Seawater with a specific gravity of 1.025 against water at 20 °C (0.998207 g/cm³): density = 1.025 × 0.998207 = 1.02316 g/cm³, or 1,023.16 kg/m³. In US units that is 63.8739 lb/ft³ and 8.5387 lb per US gallon. Against water at 60 °F (0.999017 g/cm³) it is a specific gravity of 1.02417, which is 3.42 °Bé on the heavy-liquid scale and 6.66 °API, and it sinks in fresh water.
Sources: Tanaka et al. (2001) water density equation — chempy implementation · Specific gravity, API and Baumé conversion — pipeflowcalculations.com · API gravity — Wikipedia · Density of air — Wikipedia
Frequently asked questions
Is specific gravity the same as density?
No. Density has units (g/cm³, kg/m³, lb/ft³) while specific gravity is a unitless ratio. They are the same number only when the reference is water at 4 °C taken as 1.000 g/cm³ and the density is in g/cm³.
Which reference temperature should I use?
The one your instrument or standard states. Laboratory and pharmacopeia values are often 20/20 °C or 25/25 °C, petroleum and many US industries use 60/60 °F, and older tables use 4 °C. The choice changes the result by up to about 0.3% between 4 and 25 °C.
How do I convert specific gravity to lb/gal?
Multiply the specific gravity by the weight of a gallon of the reference water: about 8.34 lb per US gallon at 60 °F, or about 10.02 lb per UK gallon. The calculator gives both from the exact density.
Can specific gravity be less than 1?
Yes. Anything less dense than the reference has an SG below 1, such as oils, alcohols and gasoline, and floats on water. Values above 1 sink.
What is API gravity?
A petroleum scale defined from the specific gravity at 60 °F: API = 141.5 ÷ SG − 131.5. Water is 10 °API; light crude oils are above 31 °API and heavy oils below 22 °API.
How do I find the specific gravity of a gas?
Divide the gas density by the density of air at the same temperature and pressure. Choose the air reference here to convert a gas SG to a density at 15 °C and 1 atm.
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This calculator is for educational purposes. Double-check important results before acting on them.