What is the approximate specific gravity of natural gas relative to air?

• A. 1.00, equal to air
• B. 0.60, lighter than air
• C. 1.60, heavier than air
• D. 0.90, slightly lighter than air

Answer: (B)

Specific gravity is a comparison of how dense a gas is to the density of air at the same conditions. Natural gas is mostly methane, with a molecular weight around 16, while air has an average molecular weight near 29. Since density roughly follows molecular weight, natural gas is about 16/29, which is roughly 0.55—often rounded to about 0.6. So it is lighter than air. That means a natural gas leak will tend to rise and accumulate near the ceiling, affecting where detectors and ventilation are most effective. The other numbers would imply the gas is the same as air or heavier, which doesn’t match natural gas’s actual properties.