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What is inHg? Inches of mercury, explained

inHg stands for inches of mercury: the height, in inches, of a column of mercury that the weight of the atmosphere can hold up. Standard sea-level pressure is 29.92 inHg (1013.25 hPa), and the conversion is 1 inHg = 33.864 hPa — or, the other way, 1 hPa = 0.02953 inHg.

What inHg means

It is a pressure unit named after a measurement. Air has weight — the weight of the air above you is what barometric pressure is. Stand a sealed glass tube of mercury upside down in a dish of mercury, and the air pressing on the dish holds the mercury up in the tube until the weight of the column balances it. At sea level the column stands a little under 30 inches tall. Read the height and you have read the pressure. Hg is simply the chemical symbol for mercury.

Nobody needs a tube of mercury to use the unit. A phone sensor, an aneroid dial or a weather model works the pressure out by other means and reports it as the column height that pressure would support. The familiar 29.92 inHg is the standard atmosphere — 1013.25 hPa, or 760 mmHg — expressed that way. It is a defined reference point, not an average anyone measured; pressure at sea level sets it out in every unit.

The same unit turns up on vacuum gauges and on aircraft manifold-pressure gauges, where it measures something other than the weather. This page is about the atmospheric use only.

Where the unit comes from

The mercury barometer was invented by Evangelista Torricelli in Florence in 1643 — sometimes dated 1644, the year he first described the experiment in writing (Britannica; Wikipedia, Barometer). He showed that the atmosphere could support a column of mercury roughly 30 inches high, and in a 1644 letter to Michelangelo Ricci he put the idea in one sentence: “We live submerged at the bottom of an ocean of the element air, which by unquestioned experiments is known to have weight” (Wikipedia, Torricelli’s experiment).

Why mercury, and why about 30? Mercury is roughly 13.6 times denser than water. The same atmosphere that holds up about 30 inches of mercury would hold up a water column of about 34 feet — an impractical instrument. So the 30 is not a chosen round number; it is what the weight of the air comes to once mercury is the working fluid. Read in millimeters instead, the same column gives mmHg, which is where 760 mmHg comes from: 760 mm is 29.92 inches.

Why not psi? Both measure the same quantity — 29.92 inHg is 14.696 psi — but inHg began as a reading off a mercury column used in weather and aviation, while psi is a force-per-area unit from engineering. In psi, a 0.10 inHg weather move is 0.049 psi, so the changes that matter for weather sit in the second and third decimal places.

Which unit you see depends mostly on where you are. In practice, US public forecasts and the altimeter settings US pilots use are given in inHg, and the National Weather Service lists it first in its own pressure unit converter. Most of the rest of the world, and most atmospheric science, uses hectopascals or millibars — which are the same unit under two names: 1 hPa = 1 mb = 100 Pa. That is convention and history, not physics. The quantity is identical; only the yardstick differs.

inHg conversion chart

The first table covers the realistic range of sea-level weather, 28.00 to 31.20 inHg, in steps of 0.10 — wide enough to take in every record quoted further down. Hectopascals and millibars share a column because they are the same number.

inHg to hPa, mb, mmHg and kPa

Computed from 1 inHg = 33.86389 hPa and 1 inHg = 25.4 mmHg; hPa and mmHg to one decimal place, kPa to two. Because of rounding, 29.92 inHg computes to 1013.2 hPa; the standard atmosphere is 29.9213 inHg, which is 1013.25 hPa exactly.

hPa or mb to inHg

For a reading that arrives in hectopascals or millibars — a foreign forecast, a weather model, a device set to metric units — read across from the nearest 10 hPa and add about 0.03 inHg for each extra hectopascal.

inHg at this site’s fixed factor, 1 hPa = 0.02953 inHg, rounded to two decimal places — the same rounding the city pages use.

The exact factors

1 inHg = 3,386.389 Pa, the NIST SP 811 value for mercury at 0 °C under standard gravity, as reproduced by MegaCalc and Intemodino; mmHg, psi and atm per xconvert. The per-hPa column is derived from those figures and rounded; the inHg figure is the factor this site uses everywhere.

Two shortcuts worth remembering

• Levels: multiply inHg by about 33.9 to get hPa. 29.50 inHg × 33.9 gives about 1000 hPa; the exact figure is 999.0. For mmHg, multiply by 25.4 — exactly, because an inch is 25.4 mm.
• Changes: 0.10 inHg is about 3.4 hPa, so 0.01 inHg is about a third of a hectopascal. Working the other way, 10 hPa is about 0.30 inHg.

Why your barometer’s inHg differs from the forecast

The pressure in a forecast is sea-level pressure: a station’s reading adjusted to what it would be at sea level, so places at different heights can be compared. A barometer, phone or watch that shows the raw pressure where it sits — station pressure — reads lower, and at altitude far lower. That is the usual reason two inHg figures for the same place and moment disagree, and neither of them is wrong.

Denver shows the size of the gap. In this site’s ten years of Open-Meteo ERA5 hourly reanalysis (2015–2024), Denver’s mean sea-level pressure is 29.97 inHg (1014.8 hPa). Its mean station pressure, at 1,615 m, is 24.74 inHg (837.7 hPa). A Denver barometer reading near 24.7 inHg is not a record low; it is the elevation. Miami, at 6 m, has almost no gap: a sea-level mean of 30.03 inHg (1017.1 hPa) against a station mean of 30.01 inHg (1016.4 hPa).

Every city page lists its mean station pressure beside the sea-level figures, so you can tell which kind your device is showing. How to correct a device to sea level — and why even a corrected one will not match the official number every day — is the subject of pressure at sea level, which covers it in full.

Reading inHg on this site

This site gives pressure in both units, converted with one fixed factor, 1 hPa = 0.02953 inHg: tables show both, and the live chart on each city page has an inHg/hPa toggle (methodology).

The five pressure zones drawn behind every chart are fixed sea-level bands, the same in every city: very low below 29.38 inHg (995 hPa); low from 29.38 to 29.68 inHg (995–1005 hPa); normal from 29.68 to 30.12 inHg (1005–1020 hPa); high from 30.12 to 30.42 inHg (1020–1030 hPa); very high above 30.42 inHg (1030 hPa). The bands are anchored in hPa, so the inHg boundaries are conversions of them — a reading in the low band is low in either unit. And because they are sea-level values, a device showing station pressure at altitude will read below every band.

This page does not try to answer whether a particular number is high. Normal barometric pressure covers the normal range and whether 30.00 inHg (1015.9 hPa) counts as high, and the 29.95 inHg lookup places individual readings from 29.50 to 30.50 inHg (999.0–1032.8 hPa), 29.90 among them. For where the low and high cut-offs sit, and why other sources draw them differently, see low and high barometric pressure.

inHg as a change

The level is half the story; the site’s city statistics are mostly about change. A swing day is a day containing a 24-hour pressure change of at least 0.18 inHg (6 hPa) in either direction; a big-swing day is the same at 0.30 inHg (10 hPa) or more; a drop day is a 24-hour fall of at least 0.18 inHg (6 hPa). The methodology defines all three. They are conventional bins, not thresholds with any special physical meaning, and they exist so cities can be ranked on one yardstick.

In inHg terms, a swing day is a move of a little under two-tenths of an inch in 24 hours — eighteen hundredths on a barometer that reads to two decimals — and a big-swing day is three-tenths. How often that happens varies enormously. Miami averages 19.8 swing days and 3.2 big-swing days a year; Boston, 257.6 and 158.7; Denver, 281.6 and 169.8. The national medians across the 50 cities are 180.7 and 86.1.

Two caveats. The 0.18 inHg threshold saturates in volatile interior cities — Denver crosses it on about 77% of days — so the 0.30 inHg count is the one that separates them. And reducing a high-altitude reading to sea level inflates its apparent variability, which is part of why Denver and Colorado Springs (279.6 swing days a year) top the list. The city counts are computed on sea-level reanalysis, so a home barometer will not reproduce them exactly; what it will show you is direction and speed, which is the part that carries the weather.

How far real readings go

The chart above runs from 28.00 to 31.20 inHg so that real readings like these fall inside it. Across ten years of hourly values, these four cities stayed between 28.66 and 31.16 inHg — a band 2.50 inches wide.

Single highest and lowest hourly sea-level values, Open-Meteo ERA5 hourly reanalysis, 2015–2024. Reanalysis smooths the sharpest local excursions, so a station barograph may have recorded slightly more extreme values.

The two Colorado highs are sea-level-reduced readings from more than 1,600 m up, and carry the altitude caveat above. Boston’s low is the deepest of the four and sits at 19 m, so it is weather, not elevation. Miami is the steady one: its standard deviation is 0.11 inHg (3.57 hPa), against 0.25 inHg (8.44 hPa) in Boston and a national median of 0.20 inHg (6.87 hPa). All four average inside the normal band.

Every one of the 50 city pages carries the same record table for its own city — start from the full list of cities, or check any US location if yours is not among them.

All 50 city pages · Check any US location · Methodology