Specific Gravity Calculator

About the Specific Gravity Calculator

The **Specific Gravity Calculator** converts between specific gravity (SG), density, and API gravity for liquids and solids. Specific gravity is the dimensionless ratio of a substance's density to the density of water at 4 °C (999.97 kg/m³). An SG below 1 means the substance floats; above 1 it sinks. The calculator includes a library of 21 common substances, buoyancy analysis with float/sink visualization, and mass computation for any volume.

Specific gravity is widely used in petroleum engineering (API gravity for crude oil classification), brewing (measuring sugar content via wort gravity), gemology (identify minerals by density), and materials science. API gravity is the petroleum industry standard: API = (141.5/SG) – 131.5. Light crude oil has API > 31.1° (SG < 0.87), medium crude 22.3–31.1°, and heavy crude below 22.3°.

Three input modes let you work in whatever unit is most convenient: enter SG directly from a hydrometer reading, input measured density in kg/m³, or enter API gravity from a petroleum lab report. Each mode immediately converts to all other scales plus Baumé degrees, and computes the mass and buoyancy properties for a user-specified volume.

When This Page Helps

Specific gravity is the fundamental property that determines whether substances float or sink, how liquids layer in tanks, and how to size pumps and pipes. In petroleum engineering, API gravity classifies crude oils and determines pricing — lighter oils command higher prices because they yield more gasoline and diesel. Brewers measure wort gravity to track fermentation progress.

The buoyancy analysis included here goes beyond simple density conversion. It computes the exact fraction submerged (Archimedes' principle), the buoyant force, and the net force when fully submerged. This is directly applicable to ship design, submarine ballast calculations, floating platform engineering, and hydrometer calibration.

How to Use the Inputs

1. Choose an input mode: Specific Gravity, Density, or API Gravity.
2. Select a substance from the built-in library or enter a custom value.
3. Enter the volume of interest and select the unit (liters, gallons, or cubic meters).
4. Review the SG, density, API gravity, Baumé, and mass results.
5. Check the buoyancy indicator to see if the substance floats or sinks in water.
6. Use the reference table to compare densities across 21 common materials.

Example Calculation

Tips & Best Practices

• SG is dimensionless — it has no units. It equals the numerical value of density in g/cm³.
• API gravity is inversely related to density: lighter liquids have higher API values.
• Fresh water has SG ≈ 1.000, seawater ≈ 1.025, Dead Sea water ≈ 1.24.
• Ice floats because its SG (0.917) is less than 1, with about 8.3% above the waterline.
• A hydrometer directly measures SG by floating at different heights in different liquids.
• Temperature affects density significantly: always note the reference temperature for SG values.

Understanding Specific Gravity Scales

The petroleum industry standardized on API gravity because it provides better resolution for classifying oils in the SG 0.80–0.95 range. Light crude (API > 31.1°) is most valuable — it is easy to refine into gasoline. Medium crude (22.3–31.1° API) produces a mix of products. Heavy crude (API < 22.3°) requires expensive cracking processes. Extra-heavy crude and bitumen (API < 10°) are denser than water and must be heated for pipeline transport.

The Baumé scale predates API and is still used in some chemical industries. "Heavy liquid" Baumé reads increase with increasing density above water, while "light liquid" Baumé reads increase with decreasing density below water. The American Baumé scale was modified to create API gravity by setting water at exactly 10° API.

Buoyancy and Hydrostatics

Archimedes' principle connects specific gravity directly to floating behavior. A substance with SG < 1 displaces its own weight of water before being fully submerged, so it floats. The equilibrium draft (depth below waterline) is exactly SG times the object height for a uniform prism. Ship designers use this principle to calculate displacement, draft, and freeboard.

Layered liquids in tanks separate by density: the densest liquid sinks to the bottom. Oil-water separators exploit the density difference (oil SG ≈ 0.85–0.95 vs. water SG = 1.00) to separate crude from produced water. Adding salt increases water density, allowing denser objects to float — this is why swimming is easier in the Dead Sea.

Industrial Applications of Specific Gravity

In mining and gemology, specific gravity helps identify minerals. Quartz (SG 2.65), pyrite (5.0), galena (7.5), and gold (19.3) have distinctive densities. Heavy liquid separation uses fluids of known SG to sort mineral grains by density, a key technique in ore processing and geological analysis.

Frequently Asked Questions

• What is the difference between specific gravity and density?

  Density has units (kg/m³ or g/cm³). Specific gravity is dimensionless — it is the ratio of the substance density to the reference density (water at 4 °C). Numerically, SG equals the density in g/cm³ because ρ_water ≈ 1.000 g/cm³.

• Why use API gravity instead of SG?

  API gravity was developed by the petroleum industry to amplify differences between oils. Light and heavy crude oils have very similar SG values (0.80-0.95), but their API gravities span 17° to 45°, making distinctions clearer. Pricing, taxation, and pipeline specifications all reference API gravity.

• How does Archimedes principle relate to specific gravity?

  Archimedes' principle states that an object floats when the weight of displaced water equals the object weight. For a floating object, the fraction submerged equals exactly its specific gravity. Ice (SG = 0.917) floats with 91.7% submerged and 8.3% above water.

• Can specific gravity change with temperature?

  Yes. Most substances expand when heated, decreasing their density and SG. Water is unusual — it is densest at 4 °C. Petroleum products are typically reported at 60 °F (15.6 °C). A correction of about 0.00035 per °F is common for petroleum SG measurements.

• What specific gravity scales are used besides API?

  Baumé (°Bé) is used in chemistry and sugar industries. Brix (°Bx) measures sugar concentration. Plato (°P) is used in brewing. Twaddell (°Tw) = 200(SG−1) is used for liquids heavier than water. Each scale is optimized for a specific industry's density range.

• How do I measure specific gravity in a lab?

  Common methods: hydrometer (glass float, reads SG directly), pycnometer (precise glass flask, weigh empty and full), density meter (vibrating U-tube, digital readout), and Westphal balance. Hydrometers are simplest but least precise. Digital density meters achieve 5-decimal accuracy.