Barometric pressure is a measure of the weight of a column of air at any given point. This measurement is typically stated in millibars (mb) -- 1 mb is equal to 100 Newtons of force per square metre -- under the metric system and inches of mercury in old-style Imperial measurements. Standard atmospheric pressure at sea level is a constant 101,325 Newtons or 1,013.25 mb (29.92 inches).
Determine what type of barometer you are using. Mercury or water-based barometers look similar to a thermometer, while aneroid barometers resemble clocks, with a face and hands, called needles.
Note the point at which the mercury has risen on the scale on a mercury barometer or the point at which the needle crosses the scale on an aneroid barometer. This point will correspond to a value in millibars, and this reading tells you how many millibars of pressure the atmosphere above you is exerting downwards.
Determine whether the reading is low or high. If the measurement is below 1,013 mb (29.92 inches) the pressure is considered "low," because it is lower than atmospheric pressure at sea level. This usually indicates stormy weather. If the measurement is above 1,013 mb (29.92 inches), this means the pressure is considered "high," because it is higher than atmospheric pressure at sea level. This usually indicates fair weather.
Most barometers show pressure readings in both millibars and inches.