A manometer is like the blood pressure meter that a doctor uses to measure BP. It has a vertical column of a liquid in a thin glass tube of circular cross section.
For measuring blood pressure the liquid used is mercury. It has a density of 13.600 kg/m^3. A manometer can be also used to measure the atmospheric pressure.
The pressure P exerted by the column of liquid of height is measured by the weight W of the liquid and the area of cross section A of the tube:
Pressure = P = W / A = density * volume * g / A = density * Height * g
For measuring atmospheric pressure, we have a closed glass tube placed in a wide beaker or container filled with mercury exposed to the atmosphere. Then the mercury in the tube raises by 760 mm above the level outside the tube, then
P = 13,600 * 0.760 * 9.8 = 1.0129 * 10^5 Pa