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Specific Gravity Calculator

Specific gravity measures how dense a substance is relative to water at 4°C (39.2°F), the temperature at which water reaches maximum density. Engineers, brewers, geologists, and manufacturers use specific gravity to predict buoyancy, identify materials, and estimate composition. A value below 1.0 means the substance floats; above 1.0 it sinks. This calculator divides your material's density by water's reference density—1000 kg/m³—giving you a dimensionless ratio instantly.

Last updated:

Creators Steven Wooding, BSc Physics
6,981 people find this calculator helpful

The Specific Gravity Formula

Specific gravity (also called relative density) expresses how much denser or lighter a substance is compared to pure water at its densest state. The calculation is straightforward: divide the material's density by water's reference density.

SG = ρ_material ÷ ρ_water

where ρ_water = 1000 kg/m³ (or 62.43 lb/ft³)

  • SG — Specific gravity (dimensionless ratio)
  • ρ_material — Density of the substance being tested
  • ρ_water — Density of freshwater at 4°C; always 1000 kg/m³

Understanding Specific Gravity vs. Density

Density measures mass per unit volume and depends on units (g/cm³, kg/m³, lb/ft³). Specific gravity is a ratio without units, making it universal and easy to compare across measurement systems.

For example, ice has a density of 916.7 kg/m³. Its specific gravity is 916.7 ÷ 1000 = 0.9167. This single number immediately tells you that ice floats because it's less dense than water.

The key advantage: specific gravity gives instant buoyancy insight. Anything with SG > 1 sinks; anything with SG < 1 floats. Density alone doesn't convey this information without knowing water's properties.

How to Use This Calculator

The process requires just one input:

  • Enter the density of your material (in kg/m³, g/cm³, or lb/ft³—the calculator handles conversions).
  • Read the specific gravity result in the second field.
  • Check the buoyancy indicator showing whether the substance floats, sinks, or remains neutrally buoyant in freshwater.

Optional: search our built-in reference table for common materials like mercury (13.6), lead (11.3), aluminum (2.7), or wood (0.4–0.9). Select a substance to autofill its density.

Practical Considerations When Using Specific Gravity

These nuances prevent misinterpretation of results:

  1. Temperature sensitivity — Specific gravity assumes water at 4°C (1000 kg/m³). Warmer water is slightly less dense—at 20°C it's 998 kg/m³. For precision work (brewing, gemstone identification), measure at or adjust for the actual temperature of your reference liquid.
  2. Salinity changes the reference — Seawater (density ≈1025 kg/m³) is denser than freshwater. Materials that sink in freshwater may float in saltwater. Always confirm which reference liquid applies to your use case before drawing conclusions about buoyancy.
  3. Material purity and structure — Air pockets, porosity, and impurities shift apparent density. A solid block of wood sinks differently than a hollow wooden boat, even though both are made of the same material. Measure the actual sample's volume and mass, not theoretical values.
  4. Pressure and phase changes — Deep submersion increases pressure, compressing gases and liquids. Dissolved air in water also affects buoyancy. For underwater or industrial applications, account for these secondary factors beyond the basic SG ratio.

Real-World Applications

Brewing and fermentation: Brewers track specific gravity during fermentation to monitor yeast activity and alcohol content. A drop from 1.050 to 1.010 over days signals sugar-to-alcohol conversion.

Geology and mining: Ore identification relies on specific gravity. Gold (19.3) versus fool's gold/pyrite (5.0) are easily distinguished by this property, making it a quick field test.

Shipping and naval design: Hull engineers calculate ballast and cargo load limits using specific gravity to predict whether vessels list, trim, or achieve stable buoyancy.

Gemology: Rubies (3.97) versus red glass (2.5) are differentiated by specific gravity, a non-destructive test that guides valuation and authenticity assessment.

Frequently Asked Questions

What does a specific gravity value of exactly 1.0 mean?

A specific gravity of 1.0 means the substance has identical density to freshwater at 4°C. It will neither sink nor float—remaining neutrally buoyant, suspended in water at any depth. Few natural materials achieve precisely 1.0; most cluster slightly above or below. This point marks the transition between sinkers and floaters.

Can specific gravity be negative or zero?

No. Specific gravity is always a positive number because density is always positive. A value approaching zero would mean the substance is nearly weightless relative to water (like a gas or foam), while values approaching infinity represent infinitely dense materials (theoretically impossible). All real substances fall between very small positive numbers and very large ones.

Why is 4°C chosen as the reference temperature for water?

At 4°C (39.2°F), water reaches its maximum density—approximately 1000 kg/m³. This is a unique property of water due to hydrogen bonding. Below or above 4°C, water becomes less dense. Choosing this temperature provides a stable, reproducible reference point across all calculations and industries, ensuring consistency in buoyancy predictions and material comparisons.

How do I measure the density of an irregular solid object?

Use the water displacement method: fill a graduated cylinder with water and note the level. Submerge the object completely and record the new level. The difference is the object's volume. Weigh the object separately. Divide mass by volume to find density. This method works for any solid that doesn't dissolve or absorb water, and handles irregular shapes effortlessly.

Is specific gravity the same as API gravity used in oil industry?

No. API gravity is a different scale specific to petroleum, measured in degrees and inversely related to density. While both are density-based metrics, API uses a different formula and reference system. Oil with high API (light crude) has low API gravity values, whereas specific gravity is always a direct ratio with no degree scale involved.

Can I use specific gravity to identify unknown minerals?

Yes, it's a reliable first-pass test. Each mineral has a characteristic specific gravity range—quartz (2.65), feldspar (2.55–2.75), galena (7.4–7.6). By comparing a sample's SG to known values, you narrow down the possibilities. Combine this with hardness, color, and crystal structure tests for confident mineral identification in the field.

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