Specific Gravity vs. Specific Weight: Key Differences Explained
Specific gravity is the ratio of a substance’s density to the density of water at 4 °C—pure numbers, no units. Specific weight is that same density multiplied by gravity, giving weight per volume (e.g., N/m³).
Engineers often grab either term when sizing pumps or mix designs, then discover units clash on the shop floor. The mix-up happens because both start with density, yet one tells you “how heavy” and the other “how much heavier than water.”
Key Differences
Specific gravity: dimensionless, compares to water, unchanged by gravity. Specific weight: carries force units, changes with altitude or planet, and must be converted to mass before calculations.
Which One Should You Choose?
Use specific gravity for quick fluid identification or hydrometer readings. Choose specific weight when you need forces on submerged structures, buoyancy checks, or pump power ratings.
Examples and Daily Life
Antifreeze labeled “SG 1.10” tells you it’s 10 % denser than water. A fuel oil spec of 8.8 kN/m³ reveals how heavy a tank wall must be. Mix them and your pressure gauge lies.
Can I convert specific gravity to specific weight?
Yes—multiply SG by the density of water (1000 kg/m³) then by 9.81 m/s² to get N/m³.
Does temperature affect both equally?
It alters density equally, so SG stays relative, but specific weight shifts with both density and local gravity.