Specific gravity (SG) is density relative to water. A simple ratio with surprisingly wide application — beer brewers use it to track fermentation, geologists to identify minerals, gemologists to spot fakes. Here's where SG matters in practice.
The definition
SG = density of object / density of water at 4°C (1000 kg/m³).
SG is dimensionless (just a number). For practical purposes:
- SG < 1: floats in water.
- SG = 1: neutral.
- SG > 1: sinks.
Beer brewing
Brewers measure SG of wort (unfermented beer) and finished beer. SG drops as yeast converts sugars to alcohol.
- Original gravity (OG): SG before fermentation. Typically 1.040–1.060 for ale, higher for strong beers.
- Final gravity (FG): SG after fermentation. Typically 1.005–1.015.
- Drop in SG indicates alcohol production. ABV = (OG − FG) × 131.
A wort with OG 1.050 fermenting to FG 1.012 produces about 5% ABV beer.
Brewers use a hydrometer (floating instrument calibrated to specific gravity) or a refractometer to measure SG. Measurement during fermentation tracks progress.
Wine making
Same idea. Grape must has SG around 1.080–1.100 before fermentation. Drops to 0.990–0.998 in dry wine. The lower the FG, the drier the wine.
Sweet wines have FG 1.010+ (residual sugar). Dry wines complete fermentation to ~0.995.
Battery testing
Lead-acid car batteries have sulfuric acid that grows weaker as the battery discharges:
- Fully charged: SG 1.275–1.290.
- 50% discharged: SG ~1.190.
- Fully discharged: SG ~1.150.
A hydrometer-based battery tester reveals the state of charge. Some batteries have built-in indicators that change color based on electrolyte SG.
Gemstone identification
Different gemstones have characteristic SGs:
| Gemstone | SG |
|---|---|
| Diamond | 3.52 |
| Cubic zirconia | 5.6–5.8 |
| Sapphire / Ruby | 3.9–4.05 |
| Emerald | 2.7 |
| Quartz | 2.65 |
| Amethyst | 2.65 |
| Opal | 2.0–2.2 |
| Pearl | 2.6–2.85 |
| Glass (imitation) | 2.4–2.8 |
| Plastic (imitation) | 1.0–1.5 |
A gemologist places a stone in a heavy liquid and measures whether it sinks or floats. Or weighs it dry and submerged for a precise SG reading.
Cubic zirconia (lab-made diamond simulant) has SG 5.6 vs diamond's 3.52. A simple SG test distinguishes them.
Mineral identification
Geologists use SG to identify minerals in the field:
- Quartz: 2.65
- Feldspar: 2.5–2.7
- Mica: 2.7–3.0
- Amphibole: 3.0–3.4
- Pyrite (fool's gold): 5.0
- Hematite: 5.3
- Galena (lead ore): 7.5
- Gold: 19.3
A specimen "feels heavy for its size" usually has high SG. Pyrite vs gold: pyrite is 5.0, gold is 19.3. Heavy specimens are likely gold; lighter pyrite.
This is the basis of gold panning — gold's density makes it settle to the bottom while lighter sand washes away.
Pearl testing
Real pearls have SG 2.6–2.85. Plastic imitations are 1.0–1.5. Simply weighing a pearl and measuring its volume reveals authenticity.
Real pearls feel heavier than expected for their size. The difference is large enough that gemologists can tell by hand alone.
Drilling fluids
Oil and gas drilling uses "drilling mud" with specific gravity adjusted to balance subsurface pressures. Too light and the well "kicks" (gas pressure escapes). Too heavy and the drill bit slows down.
Mud SG is monitored constantly during drilling. Adjusted with weighting agents (barite, hematite).
Forensics
Crime labs use SG to identify glass fragments. Different glass types have specific SGs:
- Window glass: 2.5
- Auto glass: 2.5
- Bottle glass: 2.4–2.6
- Optical glass: varies widely (1.5–4.0)
SG combined with refractive index narrows down glass source.
Buoyancy in diving
Scuba divers carry weights to make their average SG match seawater. Without weights, gear would float them up; with too many, they'd sink.
Body composition matters: muscle is SG ~1.06; fat is SG ~0.9. So muscular people are slightly denser than fatty ones, and need less weight to descend.
Concrete and construction
Different concretes have different SGs:
- Standard concrete: 2.4
- Lightweight concrete (with foam aggregate): 1.5
- Heavyweight concrete (with iron ore): 3.5
Lightweight is used for thermal insulation; heavyweight for radiation shielding (medical and nuclear).
Fuel quality
Petroleum products have characteristic SGs:
- Gasoline: 0.74
- Diesel: 0.85
- Kerosene: 0.81
- Crude oil: 0.85–0.95 depending on type
SG (along with API gravity, the petroleum equivalent) helps identify and grade fuels.
Body composition testing
Hydrostatic weighing (underwater weighing) measures body density. Weight in air vs underwater gives volume; mass / volume = density. Body fat % is calculated from density.
Modern alternatives (DEXA scan, BodPod, BIA) replaced hydrostatic weighing in clinical settings, but it's still used in research and elite athlete testing.
Density column experiment
A classic chemistry demo: layered liquids of different densities don't mix:
- Honey (1.42): bottom layer.
- Corn syrup (1.36): above honey.
- Dish soap (1.06): above corn syrup.
- Water (1.00): above soap.
- Vegetable oil (0.92): above water.
- Rubbing alcohol (0.79): top layer.
Drop different objects in to see which density level they settle at. A fun demonstration of density.
Temperature affects density
Most fluids expand when heated, decreasing density. SG values are reported at a specific temperature (often 20°C or 4°C for water).
For precise SG work, temperature compensation matters. Brewers correct hydrometer readings for temperature.
Calculate SG
Our density calculator outputs density in kg/m³ and specific gravity (relative to water). Useful for any of these applications — brewing, geology, gemology, or just satisfying curiosity.