QNH in inches of mercury vs hectopascals — altimeter setting by region

Published 3 June 2026Reference article

FAA airspace uses inches of mercury for altimeter settings. The UK, the EU, and most of the rest of the world use hectopascals. Canada uses inches. The unit mismatch is the single most common source of confused readbacks on transatlantic sim flights — and the easiest to fix once you can see the full picture.

The two units

Standard sea-level pressure is 29.92 inHg or 1013.25 hPa (hectopascals; the same unit was once called the millibar). Both numbers describe the same atmosphere; the unit is a regional convention, not a physical difference. Conversion is approximately 33.86 hPa per inHg.

For pilots, the only practical consequence is that the altimeter setting you receive on the radio looks completely different in each region. “Altimeter 29.92” and “QNH 1013” are the same setting; reading back one when the controller said the other indicates an instrument unit mistake, not just a slip of the tongue.

FAA — inches of mercury

“N534CT, altimeter 29.92.”
“29.92, N534CT.”

Below 18,000 feet MSL, FAA airspace uses the current local altimeter setting, in inHg, broadcast by the controller or read from ATIS. At or above FL180 (which is the FAA transition altitude), all aircraft set the standard 29.92 inHg. QFE (pressure at field elevation) is not used in FAA operations.

Source: FAA AIM Para 7-2 (Barometric Altimeter Errors and Setting Procedures).

UK CAA (CAP 413) — hectopascals, with QFE

“G-PILT, QNH 1018.”
“QNH 1018, G-PILT.”

UK and Irish operations issue QNH in hectopascals. Some UK aerodromes also issue QFE for circuit operations:

“G-PILT, Heathrow QFE 1015.”

QFE altimeter setting makes the altimeter read zero on the runway threshold — useful in the visual circuit, dangerous if confused with QNH. The prefix in the readback is therefore not optional. UK transition altitudes are variable (typically 3,000 to 6,000 feet); above the transition altitude the altimeter is set to the standard 1013.25 hPa.

Source: CAP 413 Edition 24.1, Chapter 7.

EASA SERA — hectopascals, QFE rare

“OH-PILT, QNH 1011.”
“QNH 1011, OH-PILT.”

SERA harmonises EU member states on QNH in hectopascals. QFE survives on request in some operations but is largely historical. Transition altitudes vary by state — typically 3,000 to 6,000 feet, with Spain and a few others higher.

Source: SERA Annex Section 14 (August 2025 revision, © European Union).

Transport Canada — inches of mercury

“C-FXYZ, altimeter 29.94.”
“29.94, C-FXYZ.”

Canada is the odd member of the ICAO family on altimeter units: it uses inches of mercury, aligned with the US, for North-American interoperability. Above 18,000 feet (the Canadian transition altitude, matching the FAA) the standard 29.92 inHg is set.

Source: TC AIM 2026-1, RAC Chapter 7.

Common pitfalls

Mixing units in the readback.

Reading back “29.92” when ATC issued “1013” means the wrong unit went into the altimeter. Both numbers describe the same standard pressure, but only one of them matches the dial in front of the pilot in that region.

Missing the QNH or QFE prefix.

In UK and some EU operations, “1018” alone is ambiguous between QNH and QFE. The prefix carries the meaning; do not drop it from the readback.

Forgetting to switch at the transition altitude.

FAA pilots set 29.92 at FL180; UK pilots set 1013 at transition altitudes that may be as low as 3,000 feet, and the value moves between aerodromes. Brief the destination’s transition altitude before descent.

Carrying FAA habit into Europe.

A KSFO-EGLL leg starts at 29.92, transitions to 1013 at FL180, then descends through 1013 again into a UK QNH that is in hectopascals. The unit changes during the descent.

Online network implications

Network controllers will read the appropriate unit for their airspace. A pilot reading back consistently in the wrong unit on a regional frequency reads as a sim pilot who didn’t brief the destination. Most controllers will correct gently the first time; consistent mismatch suggests a problem with the altimeter setting itself, which is a flight-safety issue the controller will treat as urgent.