Specific Gravity Calculator

Calculate the specific gravity (relative density) of a substance from its density, or convert a known specific gravity back to density. Choose water or air as the reference.

Specific Gravity — SG = density / reference density

Density (ρ), kg/m³ Density of the substance in kilograms per cubic metre.

Reference Water is standard for liquids and solids; air is standard for gases.

To Density — density = SG × reference density

Specific gravity (SG) A dimensionless ratio, e.g. 1.2.

Reference Water is standard for liquids and solids; air is standard for gases.

Reference Densities

Specific gravity is always relative to a standard reference substance. This calculator uses the two most common references:

Reference	Density	Condition	Used for
Water	1,000 kg/m³	4°C (maximum density)	Liquids and solids
Air	1.225 kg/m³	Sea level, 20°C	Gases and vapors

Switch back to the Specific Gravity or To Density tab to run a calculation — this tab is informational only.

Result

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Enter values above to compute.

3 min read 3 steps 6 terms 3 examples 6 FAQs SG = ρ / ρ_ref

Specific gravity is one of the simplest ways to describe how dense a substance is without getting tangled in units.

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Walk-through

How to Use This Calculator

3 steps

Pick a tab for what you know

Use the Specific Gravity tab when you know a substance's density and want its specific gravity. Use the To Density tab when you know a specific gravity and want the density back. Check the Reference tab any time you want to see the exact reference densities the calculator uses.

Choose water or air as the reference

Water (1,000 kg/m³ at 4°C, its point of maximum density) is the standard reference for liquids and solids. Air (1.225 kg/m³ at sea level, 20°C) is the standard reference for gases and vapors. Pick whichever matches your substance.

Read the result and interpretation

The result card shows the specific gravity or converted density instantly. When the water reference is selected, the interpretation line also tells you whether the substance floats or sinks in water — SG below 1 floats, SG above 1 sinks.

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Reference

Formula & Methodology

2 formulas

Specific Gravity from Density

SG = ρ / ρ_ref

SG is specific gravity (also called relative density), ρ (rho) is the density of the substance, and ρ_ref is the reference density — 1,000 kg/m³ for water or 1.225 kg/m³ for air. Because both densities use the same units, SG is a pure, dimensionless ratio. Example: a substance with ρ = 1,200 kg/m³ compared to water gives SG = 1,200 / 1,000 = 1.2.

Density from Specific Gravity

ρ = SG × ρ_ref

Rearranging the SG formula recovers density from a known specific gravity: multiply SG by the reference density. Example: SG = 1.2 against water gives ρ = 1.2 × 1,000 = 1,200 kg/m³.

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Glossary

Key Terms Explained

6 terms

Specific Gravity The ratio of a substance's density to the density of a reference substance (usually water), written SG = ρ/ρ_ref. Because it's a ratio of two densities in the same units, specific gravity is dimensionless — it has no units at all.

Relative Density Another name for specific gravity, used interchangeably in most technical contexts. Both terms describe how dense a substance is compared to a standard reference.

Reference Density The density of the standard substance a specific gravity is measured against. Water at 4°C (1,000 kg/m³, its temperature of maximum density) is the near-universal reference for liquids and solids; air at sea level and 20°C (1.225 kg/m³) is the standard reference for gases.

Water The default reference substance for specific gravity, with a density of exactly 1,000 kg/m³ (1 g/cm³) at 4°C. Because water's SG is defined as 1, any substance with SG less than 1 is less dense than water, and any substance with SG greater than 1 is denser.

Buoyancy The upward force a fluid exerts on a submerged or floating object, governed by Archimedes' principle. Specific gravity is a quick proxy for buoyancy in water: an object with SG below 1 displaces enough water to float, while an object with SG above 1 sinks.

Dimensionless Having no physical units. Specific gravity is dimensionless because it's a ratio of two quantities measured in the same units (density divided by density) — the units cancel out, leaving a pure number like 1.2 or 0.8.

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Scenarios

Real-World Examples

3 worked examples

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Solid Sample

Specific gravity from density

Density (ρ) 1,200 kg/m³ Reference Water

SG = ρ/ρ_ref = 1,200 / 1,000 = 1.2. Because SG is greater than 1, this substance is denser than water and will sink if placed in it.

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Labeled Liquid

Converting a known SG back to density

Specific gravity (SG) 1.2 Reference Water

ρ = SG × ρ_ref = 1.2 × 1,000 = 1,200 kg/m³. This is the reverse of the first example — converting a bottle label's stated SG back into an actual density value.

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Float-or-Sink Check

Reading the floats/sinks note

Density (ρ) 900 kg/m³ Reference Water

SG = 900 / 1,000 = 0.9. Since SG is less than 1, the calculator's interpretation line reports that this substance floats in water — this is exactly why ice (SG ≈ 0.92) floats on liquid water.

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Reference

Cite This Calculator

APA & MLA

Use either format to cite this calculator in a paper, report, or resource list.

APA

MLA

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Deep Dive

Understanding Specific Gravity and Relative Density

3 chapters · 3 min read

Specific gravity is one of the simplest ways to describe how dense a substance is without getting tangled in units. By comparing a substance's density to a fixed reference — almost always water — specific gravity turns an awkward number like 1,200 kg/m³ into an intuitive one like 1.2: this substance is 20% denser than water.

Why Divide by a Reference Density?

Density on its own depends entirely on which unit system you're using — kg/m³, g/cm³, lb/ft³ — and the raw numbers can be hard to compare at a glance. Dividing by a fixed reference density cancels the units and produces a single dimensionless number that means the same thing no matter which unit system you started from. A specific gravity of 1.2 always means "20% denser than the reference," whether the underlying density was measured in metric or imperial units.

Choosing the Right Reference: Water vs. Air

Water at 4°C is the near-universal reference for liquids and solids because it's a stable, reproducible standard with a density defined as exactly 1,000 kg/m³ (1 g/cm³). Gases and vapors instead use air at sea level and 20°C (1.225 kg/m³) as their reference, since comparing a gas to water would produce impractically tiny numbers. This calculator lets you switch between the two references so the ratio you get is meaningful for the kind of substance you're measuring.

A Quick Float-or-Sink Test

When the reference is water, specific gravity doubles as a fast float-or-sink check: anything with SG below 1 is less dense than water and floats, and anything above 1 is denser and sinks. This is why ice (SG ≈ 0.92) floats on liquid water while a steel ball bearing (SG ≈ 7.8) sinks immediately. It's an approximation — real buoyancy also depends on shape and surface tension for small or oddly-shaped objects — but for a solid, roughly uniform substance, the SG-vs-1 comparison is a reliable rule of thumb.

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Questions

Frequently Asked Questions

6 questions

What is the formula for specific gravity?+

SG = ρ/ρ_ref, where ρ is the density of the substance and ρ_ref is the reference density — 1,000 kg/m³ for water or 1.225 kg/m³ for air. To go the other direction, density = SG × ρ_ref.

What does it mean if specific gravity is less than 1?+

A specific gravity below 1 means the substance is less dense than the reference. When the reference is water, this means the substance floats in water — for example, ice has an SG of about 0.92.

What reference density does this calculator use?+

Water at 4°C (1,000 kg/m³), its temperature of maximum density, is the default reference for liquids and solids. You can switch to air at sea level and 20°C (1.225 kg/m³) for gases and vapors.

What units does specific gravity use?+

None — specific gravity is dimensionless. It's a ratio of two densities measured in the same units, so the units cancel out, leaving a plain number like 1.2 or 0.9.

How do I convert specific gravity back to density?+

Multiply the specific gravity by the reference density: density = SG × ρ_ref. Use the To Density tab to do this conversion automatically for either the water or air reference.

Can specific gravity be used for gases?+

Yes — for gases, specific gravity is measured against air instead of water, since comparing a gas's density to water's would produce impractically small numbers. Switch the reference selector to Air for gas-phase substances.

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