Which statement is correct regarding PSI to water and Hg?

• A. 1 psi = 27.7 in H2O
• B. 1 psi = 2.04 in Hg
• C. 1 psi equals 4 inches of water
• D. 1 psi equals 27.7 inches of water and 2.04 inches of Hg

Answer: (D)

Think of pressure as the height of a fluid column that would balance that pressure. Because different fluids have different densities, the same pressure will correspond to very different column heights for each fluid. Mercury is much denser than water, so only a short mercury column is needed, while a much taller water column is required to balance the same pressure.

For 1 psi, the balanced heights are about 27.7 inches of water or about 2.04 inches of mercury. The large water height and the small mercury height reflect mercury’s density being about 13.6 times that of water, so the mercury column is roughly 1/13.6 as tall as the water column for the same pressure.

The statement that 1 psi equals roughly 27.7 inches of water and 2.04 inches of Hg captures both standard conversions, so it’s the correct one. The other options give only one conversion or an incorrect water height, which doesn’t align with these widely used reference values.