Specific Gravity Calculator

Specific gravity is simply the ratio of a substance's density to a reference density:

$$SG = \frac{\rho_{\text{substance}}}{\rho_{\text{reference}}}$$

For liquids and solids, the reference is almost always water. For gases, the reference is usually air. The temperature must be specified because both densities change with temperature. Common conventions include SG at 20°C/20°C (substance at 20°C, reference water at 20°C) or 60°F/60°F in the petroleum industry.

API Gravity is an inverse density scale used by the petroleum industry:

$$°\text{API} = \frac{141.5}{SG_{60/60}} - 131.5$$

Light crude oils have high API gravity (>31.1°), medium crudes range 22.3–31.1°, and heavy crudes are below 22.3°. Water has an API gravity of 10°.

Baumé Gravity is a historical scale still used in some industries:

• For liquids heavier than water: $$°\text{Bé} = 145 - \frac{145}{SG}$$
• For liquids lighter than water: $$°\text{Bé} = \frac{140}{SG} - 130$$

Buoyancy behavior:

• SG < 1: The substance is less dense than the reference — it floats. Oil (SG ≈ 0.8–0.95) floats on water.
• SG = 1: Neutral buoyancy — the substance neither rises nor sinks.
• SG > 1: The substance is denser than the reference — it sinks. Mercury (SG ≈ 13.5) sinks in water.

Specific gravity is measured with hydrometers (graduated floating instruments), pycnometers, or digital density meters. In brewing, a hydrometer measures the sugar content of wort (original gravity vs. final gravity) to determine alcohol content.

The calculated specific gravity immediately tells you the relative density of your substance compared to water. SG < 1 means the substance floats; SG > 1 means it sinks. The API gravity is particularly useful for classifying petroleum products, while Baumé gravity is used in some chemical and food industries. For practical applications in buoyancy design, the float/sink indicator shows the expected behavior.