Specific Gravity / Relative Density

SG = ρ_substance / ρ_reference — dimensionless ratio comparing a substance's density to a reference fluid (water for liquids, air for gases).

Fluid Properties & Statics

Common substances — click to auto-fill

liquids

solids

gass

Substance density ρ (kg/m³)

Reference fluid

Water at 4°C

999.97 kg/m³ — Standard for liquids

Water at 15°C

999.1 kg/m³ — Petroleum standard

Water at 20°C

998.2 kg/m³ — Common lab temp

Air at 20°C, 1 atm

1.204 kg/m³ — Standard for gases

Substance	ρ (kg/m³)	SG (water 4°C)	Floats in water?
Water (20°C)	998	0.998	Yes
Seawater	1025	1.025	No
Glycerin	1261	1.261	No
Milk	1030	1.03	No
Gasoline	720	0.72	Yes
Diesel	850	0.85	Yes
Crude oil	900	0.9	Yes
Ethanol	789	0.789	Yes
Mercury	13600	13.6	No
Honey	1400	1.4	No
Ice (0°C)	917	0.917	Yes
Aluminum	2700	2.7	No
Steel	7850	7.85	No
Lead	11340	11.34	No
Oak wood	720	0.72	Yes
Pine wood	500	0.5	Yes
Concrete	2300	2.3	No

Definition:

SG = ρ_substance / ρ_reference (dimensionless)

API Gravity (petroleum):

°API = 141.5 / SG₁₅ − 131.5 (SG at 15°C / 60°F)

SG < 1 relative to water → substance floats (less dense than water)
SG > 1 relative to water → substance sinks (denser than water)
Gases are compared to air (SG_gas = M_gas / 28.97)
SG is numerically equal to density in g/cm³ when using water at 4°C as reference