Barometric Pressure Converter

NOAA gives standard sea-level pressure as 29.92 inches of mercury, or 1,013.25 millibars. That is the reference, not a fixed value: readings rise under high pressure systems, shown as H on weather maps, and fall under low pressure systems, shown as L. Air pressure also follows a small twice-daily cycle, lowest around 4 a.m. and 4 p.m. and highest around 10 a.m. and 10 p.m., according to NOAA.

NOAA puts the millibar values used in meteorology at about 100 to 1,050 mb. That range covers the whole depth of the weather maps, from the upper atmosphere down to the ground, so it is not the range you will see on a home barometer, which stays near the standard 1,013.25 mb and moves with the weather.

Sea Level Pressure

Weather stations sit at different heights, and air pressure drops with height, so stations convert their readings to sea level before reporting them, as NOAA explains. The pressure in a forecast is that sea-level value, not the pressure at your elevation, which is why a home barometer at altitude reads lower unless it is set to match. The Oxygen Saturation Altitude Estimator shows one effect of that lower pressure.

• Inches of mercury (inHg): the US weather unit. The conventional inch of mercury is 25.4 conventional millimeters of mercury, about 3,386.389 Pa in NIST SP 811.
• Millibars (mb) and hectopascals (hPa): the same size, 100 Pa each. NOAA notes that meteorology has used the millibar since 1929.
• Millimeters of mercury (mmHg): common on older barometers and in medicine.
• Kilopascals (kPa): 10 hPa each, used on some instruments.

The National Weather Service conversion formulas use a factor of 33.8639 mb per inHg, which matches the exact definition to four decimal places. Some weather tools label millimeters of mercury as torr; the two differ by less than one part in a million, so it does not matter for weather readings.

Barometer readings in inches of mercury converted to hectopascals (millibars), millimeters of mercury and kilopascals, rounded to two decimal places.

Millibars to Inches of Mercury

Readings in millibars (hPa) converted to inches of mercury, rounded to two decimal places.

Weather apps often give temperature in one scale and pressure in another; the Temperature Converter handles the temperature side.

Single conversions that start from or end in this page's units, one page per pair.

• Kilopascals to mmHg Converter: 1 kilopascal is about 7.500616 millimeters of mercury
• mmHg to Atmospheres Converter: 1 millimeter of mercury is about 0.00131579 standard atmospheres
• Torr to Atmospheres Converter: 1 torr is about 0.00131579 standard atmospheres

Each converter below shows one pressure in many units at once, starting from a different unit.

• Pressure Unit Converter: one pressure in every common unit at once
• PSI Converter: psi to bar, kPa, atm, inHg, inches of water and more
• Kilopascal Converter: kilopascals to psi, bar, atm, hPa and more
• Megapascal Converter: megapascals to psi, ksi, bar, GPa and more
• Pascal Converter: pascals to hPa, kPa, MPa, GPa, psi and more
• Atmosphere Converter: standard atmospheres to psi, kPa, torr, mmHg and more