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Barometric Pressure Converter

Your barometer says inches of mercury, the forecast says millibars, and an app abroad says hectopascals. Enter one reading and see it in every unit, with the factors the National Weather Service uses.

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Pressure converter

Common readings, converted

inHghPa / mbmmHg
28.50965.1723.9
29.00982.1736.6
29.50999.0749.3
29.921,013.2760.0
30.001,015.9762.0
30.201,022.7767.1
30.501,032.8774.7
31.001,049.8787.4

29.92 inHg is the rounded value of standard sea-level pressure. NOAA gives the exact value as 1013.25 in both millibars and hectopascals, which is why 29.92 inHg converts to 1013.2 hPa here.

Convert it yourself

No calculator at hand? These are the factors from the National Weather Service conversion sheet that this tool uses:

Inches of mercury to hectopascals (or millibars):

hPa = inHg × 33.8639

Hectopascals back to inches of mercury:

inHg = hPa ÷ 33.8639

Inches to millimeters of mercury (1 inch is 25.4 mm):

mmHg = inHg × 25.4

Example: 30.00 inHg × 33.8639 = 1,015.9 hPa. And 1 hPa is exactly 1 mb, so that is also 1,015.9 mb.

The units at a glance

inHg and mbInches of mercury and millibars are the two most common units in the United States, according to NOAA.
hPa = mbOne hectopascal is exactly one millibar, so 1013 hPa and 1013 mb are the same reading.
kPaA kilopascal is 10 hectopascals: 1013.25 hPa is 101.325 kPa.
atmOne standard atmosphere is 1013.25 hPa, the standard pressure at sea level.

Why your station shows a different number than the airport

Weather reports give pressure converted to the value it would have at sea level (NOAA JetStream), so places at different heights can be compared. Your station measures the real pressure at your height, which is lower the higher you live: NOAA notes that half of all air molecules are in the lowest 18,000 feet of the atmosphere.

Absolute pressureWhat the sensor really measures at your height. Lower in the mountains than at the coast.
Relative pressureConverted to sea level, like the forecast. This is the number to compare with weather reports.
Set it onceThe C83100 manual says its relative pressure is programmable; the S77925 manual says to allow up to 10 days for automatic calibration or to enter your local pressure.

Our guide to barometric pressure explains more.

Rising or falling: what it can mean

The trend matters more than the number. NOAA's JetStream school quotes some old barometer rules:

⬇️ Falling

"The barometer sinks lowest of all for wind and rain together."

⬆️ Rising

"In wet weather, if the mercury rises high and remains so, expect continued fine weather in a day or two."

NOAA adds that these rules hold true for many locations, "but not all of them". Want to read the signs yourself? See how to tell if a storm is coming.

Quick answers

What is normal barometric pressure?

Standard air pressure at sea level is 29.92 inches of mercury, or 1013.25 millibars (hectopascals), according to NOAA. Real readings move above and below that number as weather systems pass.

Are millibars and hectopascals the same?

Yes. 1 mb is exactly 1 hPa, so you never need to convert between them.

Why is my station lower than the forecast?

Your station probably shows absolute pressure, measured at your own height. Forecasts use sea-level pressure. Switch your station to relative pressure and set it to your local value, as explained in its manual.

My result says "outside the range". What now?

Check the unit you picked: 29.92 typed as hectopascals instead of inches gives a number no home station shows. The La Crosse C83100 and S77925 manuals list 800 to 1100 hPa.

Sources: National Weather Service El Paso: Pressure Conversion (PDF) · NOAA JetStream: Air Pressure · La Crosse C83100 manual (PDF) · La Crosse S77925 manual (PDF)