Kilopascals to inHg

A Canadian Pressure Report in kPa, an Altimeter That Wants inHg

Environment and Climate Change Canada publishes surface pressure to the public in kilopascals — the hourly observation for a city reads something like 101.3 kPa. The aircraft on the ramp at the same aerodrome has a Kollsman window calibrated in inches of mercury, and the ATIS or METAR altimeter group that feeds it is coded the North American way, as A2992. Same air, same moment, two different numbers, and pilots on this side of the border cross between them constantly.

The factor: 1 kPa = 0.29529983 inHg, so multiply kilopascals by roughly 0.2953. An hourly report of 101.3 kPa works out to 29.91 inHg — a hair under the 29.92 standard setting.

What the Two Numbers Actually Describe

Station pressure

The raw reading of the barometer where it stands, at the elevation of the field. Nothing has been added back for height, so a mountain aerodrome always shows a low figure.

Altimeter setting

Station pressure worked back to sea level through the standard atmosphere, so an altimeter with the value dialled in reads field elevation while still on the ground.

Sea-level pressure

The meteorologist's reduction, which leans on the recent temperature record of the site rather than a fixed lapse rate. It shows up in the METAR remarks and rarely matches the setting exactly.

Height per unit

An inch of mercury is worth close to a thousand feet of altitude near the surface; in the other currency, a single kilopascal buys a little under 300 ft.

Getting From the Weather Page to the Kollsman Window

Most of the time the figure arrives from a public forecast page, a climate archive or a colleague working in metric units, and it has to land in a field that expects inches.

Put the kilopascal figure in the left box

Type 101.3, 98.7, 88.9 — whatever the observation shows. Inches of mercury appear beside it as you type, and a comma is accepted in place of the decimal point if that is how your keyboard is laid out.

Compare it with the setting you were issued

A gap of a tenth of an inch between a city sea-level report and the aerodrome altimeter setting is ordinary. A gap of two or three inches means you are holding station pressure from a high-elevation field, not a setting.

Take the bare number away with you

Each field carries its own copy button and hands over the digits alone — no unit, no spacing — which is what a performance calculator or a planning form wants. Ctrl + C inside the field does the same thing.

Run it the other way for a climate log

The swap button (↔) turns the page into inHg → kPa, the direction you need when a North American setting has to be filed in an archive kept in kilopascals. By hand it is a multiplication by 3.38638816, so 29.92 inHg becomes 101.32 kPa.

A converted number is not an altimeter setting. Converting station pressure into inches gives a perfectly valid inHg value of the wrong quantity. Only the setting published for that aerodrome belongs in the subscale.

Altimeter and Station Pressure Reference Values

These rows pair the kilopascal value a Canadian observation would show with the inches-of-mercury value that reaches the cockpit. The station-pressure entries are standard-atmosphere pressures at the stated field elevation — what a barometer sitting at that height reads on an ISA day.

Reference point	kPa	inHg	Note
Standard atmosphere, sea level	101.325 kPa	29.92 inHg	The value set above the transition level
A round 30.00 in the subscale	101.59 kPa	30.00 inHg	Mildly high pressure
Strong winter high	103.40 kPa	30.53 inHg	Toward the upper end of ordinary settings
Passing low-pressure system	99.00 kPa	29.23 inHg	About 0.69 inHg below standard
Deep storm QNH	97.50 kPa	28.79 inHg	Near the low end of a typical subscale range
Station pressure, 1 000 ft field	97.72 kPa	28.86 inHg	Standard-atmosphere value at that elevation
Station pressure, 2 000 ft field	94.21 kPa	27.82 inHg	Standard-atmosphere value at that elevation
Station pressure, 3 557 ft field	88.96 kPa	26.27 inHg	Elevation of a foothills prairie aerodrome

Notice how far the station-pressure rows sit below the setting rows. At a field two thousand feet up, the barometer reads about 27.8 inHg while the published setting may well be 29.9 — the entire difference is the column of air between the instrument and sea level.

What the Page Adds to a Weather Briefing

Both boxes stay live

Either side accepts typing and the other follows, so you can walk down a column of hourly observations without clearing anything between entries.

Flip to match the source

One press of the swap arrows reverses the pair — the direction to use when a setting given in inches has to be logged in kilopascals.

Hectopascals are one search away

The searchable lists on both sides carry all 26 pressure units, so a Q-code setting in hPa or an old millibar chart value drops straight in.

Digits without decoration

Results run to eight decimals and copy as a plain number, ready for a spreadsheet of observations or a performance worksheet.

Questions About kPa and inHg in Aviation Weather

Why does a Canadian weather page give kilopascals when the ATIS gives inches?

Public meteorology in Canada went metric and settled on the kilopascal, while aviation kept the North American convention of coding the altimeter setting in hundredths of an inch of mercury. Canada and the United States are the two countries that still transmit the setting that way; almost everywhere else it goes out as a Q-code in whole hectopascals.

Is the kilopascal figure on a forecast site the same thing as the altimeter setting?

Close, but not identical. Public pages normally publish a sea-level reduced value that takes the recent temperature record of the site into account, whereas the altimeter setting reduces station pressure through the standard atmosphere. The two agree within a few hundredths of an inch at low-lying stations and drift further apart at elevated or very cold ones.

How much height is one inch of mercury worth?

The classic rule of thumb is a thousand feet per inch, and it is deliberately rounded: in the standard atmosphere the first thousand feet costs about 1.07 inHg, so an inch is nearer 940 ft down low and buys more feet as you climb. Expressed the metric way, the same first thousand feet costs roughly 3.6 kPa.

When does the local setting come out of the subscale and 29.92 go in?

At the transition to flight levels. In Canadian and US airspace that boundary is 18 000 ft, above which every altimeter carries the standard setting so all traffic shares one reference regardless of the weather underneath. Written in the metric column, 29.92 inHg is 101.32 kPa — the standard-atmosphere sea-level value.

Does very cold air throw the altitude off even with the correct setting dialled in?

Yes. The subscale corrects for the pressure of the day but not its temperature, and in air colder than standard the true height above the ground is lower than the instrument indicates. That is why cold-temperature correction tables exist for approach minima in Canadian winters — no amount of unit conversion, in kPa or in inHg, touches that error.