Bulk modulus $B = -V(dP/dV)$. Pressure at depth h is $P = dgh$. The fractional change in density is approximately equal to the fractional change in pressure divided by the bulk modulus, $Δd/d \approx P/B = dgh/B$. Therefore, $Δd = d^2gh/B$. [cite: 1, 1383, 1386]