What is the standard average sea-level atmospheric pressure used in meteorology?

The standard average sea-level atmospheric pressure used in meteorology is 1013.25 hectopascals.

This value is also equal to 101,325 pascals, 1 standard atmosphere, or 29.92 inches of mercury. It represents a defined reference pressure rather than the exact pressure at every sea-level location. Real atmospheric pressure changes with weather systems, temperature, altitude, and the weight of air above a place.

Meteorologists reduce station pressure to sea level so readings from locations at different elevations can be compared. A barometer measures pressure directly, while weather maps commonly draw lines called isobars through places with equal pressure. High-pressure systems generally bring sinking air and more settled conditions; low-pressure systems are commonly associated with rising air and clouds.

The hectopascal is numerically identical to the millibar, so 1013.25 hPa is also 1013.25 millibars. Confusing standard sea-level pressure with a local station reading is a common mistake, especially in mountainous areas.