Specific Gravity Calculator
Calculate specific gravity, relative density, and compare material density to water.
SG = ρ_material ÷ ρ_water (1000 kg/m³ at 4°C)
Material presets:
What Is the Specific Gravity Calculator?
The Specific Gravity Calculator computes SG = ρ_material / ρ_water (1000 kg/m³ at 4°C), shows float/sink verdict, calculates API gravity (petroleum standard) and Baumé scale, and optionally computes the Archimedes buoyancy force if you enter the object volume. A comparison bar shows your material against iron, lead, gold, and other references. Fifteen material presets span wood to gold.
- ›SG is dimensionless, the same value in any unit system
- ›SG < 1: less dense than water, floats. SG > 1: denser, sinks
- ›API gravity (petroleum): light crude > 31.1°API; heavy crude < 22.3°API
- ›Buoyancy force F_b = ρ_water × V × g (Archimedes' principle)
Formula
Specific Gravity Formulas
SG definition
SG = ρ_substance / ρ_water
Reference
ρ_water = 1000 kg/m³ at 4°C
Floats if
SG < 1 (less dense than water)
Buoyancy
F_b = ρ_water × V × g
API gravity
API = 141.5/SG − 131.5
Baumé (heavy)
Bé = 145 − 145/SG
How to Use
- 1Enter the material density and select the unit (kg/m³, g/cm³, or lb/ft³)
- 2Optionally enter the object volume in litres for a buoyancy force calculation
- 3Click a material preset button to auto-fill density for a known material
- 4Click Calculate SG
- 5The float/sink verdict, API gravity, Baumé, and buoyancy force are shown
- 6The comparison bar places your material on a density scale with common references
Example Calculation
Copper (ρ = 8,960 kg/m³):
SG > 1 → sinks (8.960× denser than water)
API gravity = 141.5/8.960 − 131.5 = −115.7° (not applicable for metals)
Baumé = 145 − 145/8.960 = 145 − 16.18 = 128.8°Bé
Ethanol (ρ = 789 kg/m³), 2 litre volume:
21.1% less dense than water
Buoyancy = 1000 × 0.002 m³ × 9.81 = 19.62 N
Weight = 0.789 × 0.002 × 9.81 = 15.48 N → net upward 4.14 N
Archimedes and the crown
Archimedes famously tested the gold content of a crown by measuring displaced water. Pure gold (SG = 19.32) displaces far less water per gram than a gold-silver alloy (SG ≈ 14). The crown's specific gravity betrayed the jeweller's deception.
Understanding Specific Gravity
Specific Gravity of Common Materials
| Material | SG | Density (kg/m³) | Floats? |
|---|---|---|---|
| Cork | 0.24 | 240 | Yes |
| Ice (0°C) | 0.917 | 917 | Yes, 91.7% submerged |
| Ethanol | 0.789 | 789 | Yes |
| Water | 1.000 | 1,000 | Neutral buoyancy |
| Seawater | 1.025 | 1,025 | No |
| Concrete | 2.30 | 2,300 | No |
| Aluminum | 2.70 | 2,700 | No |
| Iron/Steel | 7.87 | 7,870 | No |
| Lead | 11.34 | 11,340 | No |
| Mercury | 13.53 | 13,530 | No |
| Gold | 19.32 | 19,320 | No |
Frequently Asked Questions
What is specific gravity and why is it dimensionless?
Being dimensionless makes SG universally comparable, it doesn't change whether you measure in SI or imperial units, as long as you use the same unit system for both densities.
- ›SG = ρ_material (kg/m³) / 1000 kg/m³ = ρ_material (g/cm³) / 1 g/cm³
- ›Either unit gives the same SG, units cancel
- ›Water reference: 4°C chosen because water is densest at this temperature
- ›For gases: SG is relative to air (1.225 kg/m³) rather than water
What is API gravity and why does it matter?
API gravity was developed by the American Petroleum Institute and is the standard for pricing and classifying crude oil worldwide. The inverted scale was chosen so that lighter (more valuable) oils have higher numbers.
- ›Light crude: API > 31.1° (SG < 0.870), yields more gasoline
- ›Medium crude: 22.3°–31.1° API
- ›Heavy crude: API < 22.3° (SG > 0.920), lower value, harder to refine
- ›Gasoline: API ≈ 60° (SG ≈ 0.740)
How does specific gravity relate to buoyancy?
Buoyancy force depends on the volume displaced, not the weight of the object. A steel ship floats because its hull shape (containing air) displaces more water than a solid steel block of the same mass would.
- ›Object floats when: ρ_object × V_object × g ≤ ρ_water × V_submerged × g
- ›Floating equilibrium: V_submerged / V_object = SG (fraction submerged = SG)
- ›Ice at SG=0.917: 91.7% submerged when floating (8.3% above water, iceberg rule)
- ›Submarines: adjust ballast water to change effective SG from <1 to >1
How is specific gravity used in brewing?
Wort (unfermented beer) has SG > 1 due to dissolved sugars. Fully fermented beer has SG close to 1.008–1.012. Spirits can have SG below 1 (alcohol is less dense than water).
- ›Typical beer OG: 1.040–1.075 (40–75 points above water)
- ›Typical beer FG: 1.008–1.016
- ›ABV ≈ (OG − FG) × 131.25 (e.g., 1.050 → 1.010 = 5.25% ABV)
- ›Refractometers also measure sugar concentration via refractive index
How is specific gravity used in geology?
SG measurement in geology is typically done with a pycnometer (precise volume measurement) or by weighing in air and water (Archimedes method).
- ›Silicate rocks: SG 2.6–3.0 (granite 2.65, basalt 2.9)
- ›Iron ore (magnetite): SG ≈ 5.2, identifying characteristic
- ›Gold: SG = 19.32, much denser than common rocks (quick field check)
- ›Gemstones: diamond 3.52, ruby 3.99, sapphire 3.99, emerald 2.72
What is the Baumé scale?
The Baumé scale was widely used before digital density meters. You may still encounter it in older chemical references and some food/wine production contexts.
- ›Sulfuric acid (SG=1.84): Bé_heavy = 145−145/1.84 = 66.2°Bé
- ›Honey (SG≈1.4): Bé = 145−145/1.4 ≈ 41.4°Bé
- ›Wine must sugar: Baumé reading predicts final alcohol content
- ›1°Bé ≈ 1.8% potential alcohol for grape must (approximate rule)
How do you measure specific gravity experimentally?
The choice of method depends on the material (solid vs. liquid), required precision, and available equipment. Digital density meters are now the industrial standard.
- ›Hydrometer: simple, cheap, ±0.002 accuracy, used in brewing, battery testing
- ›Pycnometer: ±0.0001 accuracy for liquids, analytical chemistry standard
- ›Archimedes (solid): weigh in air (W_a), then submerged (W_w): SG = W_a/(W_a−W_w)
- ›Digital density meter: ±0.00001 accuracy, pharmaceutical and petroleum quality control