Specific Gravity Calculator

Specific gravity (SG) is a dimensionless ratio that compares the density of a substance to the density of a reference substance, normally water at 4°C (39.2°F), which has a density of exactly 1,000 kg/m³.

The Core Formula:

SG = ρ_substance / ρ_reference

For liquids and solids (referenced to water):

SG = ρ_material / 1,000 kg/m³

For gases the reference is air at sea level, 1.225 kg/m³ (the ISA standard at 15°C). Some tables use air at 0°C instead, which is 1.293 kg/m³, so check which one a published gas SG assumes.

Using Archimedes’ Principle (for irregular solids):

SG = Weight in Air / (Weight in Air − Weight in Water)

Worked Example 1: liquid sample

• A 500 mL oil sample weighs 430 g
• Density = 430 g / 500 mL = 0.860 g/cm³
• SG = 0.860 / 1.000 = 0.860 (floats on water, since SG < 1)

Worked Example 2: irregular solid

• A rock weighs 250 g in air and 155 g submerged in water
• Buoyant force = 250 − 155 = 95 g equivalent
• SG = 250 / 95 = 2.63 (similar to granite)

Reference Table:

Applications: Used in brewing (alcohol content), mining (ore separation), geology (mineral identification), and fluid dynamics. A hydrometer directly measures SG of liquids.

Why brewers care more than anyone

Brewing is where specific gravity gets used hardest, because sugar dissolved in water raises SG and alcohol lowers it. A hydrometer reading before fermentation (original gravity, often around 1.050) and another after (final gravity, maybe 1.010) give the alcohol content directly: ABV is roughly (OG − FG) × 131.25, so that pair works out near 5.25%. Nothing else in the process is measured this precisely with a five-dollar glass tube.

The temperature trap

Specific gravity is defined against water at a stated temperature, and water is not the same density hot as cold. A hydrometer calibrated at 20°C reads low in a warm sample, which is why brewing and petroleum both publish correction tables. For everyday work the error is small, but at 30°C against a 20°C calibration you are already out by about 0.002 SG, enough to shift an ABV estimate by a quarter of a percent.

Floating is not about weight

An aircraft carrier floats and a coin sinks. Specific gravity settles it: what matters is the density of the object as a whole, hull and enclosed air together, against the fluid. Steel has SG 7.8, but a steel hull full of air averages well under 1. That is the entire principle behind ship design, and the reason SG rather than mass answers the question.