Why does ATC give an altimeter setting and how do I use it?

ATC gives an altimeter setting so aircraft operating in the area share the same local pressure reference and indicate comparable altitudes. Set the stated pressure—usually QNH—in the altimeter’s barometric window. With local pressure selected, indicated altitude corresponds approximately to altitude above mean sea level.

In real-world aviation and flight simulation, this shared reference supports vertical separation, terrain clearance and consistent approach minima. The setting is normally based on pressure measured at a nearby reporting station and adjusted to mean sea level; our plain-language explanation of QNH and its pressure reference covers that calculation in more detail.

Why does the ATC altimeter setting matter?

Atmospheric pressure changes with weather, time and location, so an altimeter left on an old setting gradually indicates the wrong altitude. ATC distributes a local value so aircraft and controllers work from the same vertical reference.

Near sea-level pressure, a difference of 0.10 inHg is roughly 100 feet, while about 4 hPa produces a similar error. If the selected pressure is too high, the altimeter overreads and the aircraft is lower than indicated—the hazard behind the saying “high to low, look out below”.

Pressure setting does not eliminate every altimetry error. Very cold air can leave true altitude lower than indicated, so published cold-temperature corrections and operator procedures may still apply.

How do I enter an altimeter setting from ATC?

1. Copy the complete number. In the United States, “altimeter two niner eight five” means 29.85 inHg. In many other regions, “QNH one zero one five” means 1015 hPa. Ask ATC to confirm any clipped or ambiguous digit.
2. Check the units. Glass cockpits may display IN, HPA or MB; hPa and millibars use the same numerical value. Change the unit mode or convert the value rather than entering an inHg number as hPa.
3. Use the setting at the correct phase of flight. Set local pressure when operating below the applicable transition level. If ATC gives QNH while you are still flying on standard pressure, record it and change at the prescribed transition point.
4. Turn the BARO control. Adjust the barometric window or digital pressure readout to the stated value. Set both pilots’ instruments and the standby altimeter where they are independent.
5. Cross-check the result. On the departure airfield, current QNH should make the altimeter indicate approximately the published field elevation. A large discrepancy usually means wrong units, stale weather or the wrong pressure reference.
6. Continue using the cleared altitude. Changing the pressure setting is not a new altitude clearance. Once it is set, fly the assigned indicated altitude or flight level as appropriate.

Required readbacks vary by jurisdiction, but never guess an unclear setting. In a simulator, avoid relying blindly on an automatic altimeter command: it may use the pressure loaded by the weather system rather than the value just spoken by built-in ATC.

When should I use QNH instead of 29.92 or 1013?

Use local QNH for altitude above mean sea level and standard pressure for flight levels above the applicable transition boundary.

Under ICAO-style procedures, change to standard pressure passing the transition altitude on climb and to local QNH passing the transition level on descent. The boundary varies by country and airport; do not assume the United States convention of 18,000 feet applies elsewhere. Our timing guide for changing altimeter settings explains the climb, cruise and descent sequence.

What if ATC’s setting does not match the METAR?

In real flying, use the issued setting or ask ATC to confirm it rather than silently substituting another value. The METAR may be from a different station or observation time; the pressure groups used in METAR reports include formats such as A2992 and Q1013.

In an offline simulator, built-in ATC, displayed airport weather and the active weather engine can fall out of synchronisation. For an accurate approach, use the pressure associated with the weather actually loaded at the airport. If built-in ATC continues using a stale value, treat its vertical instructions cautiously and follow our guidance for handling unsafe or inconsistent simulator ATC instructions.

Common errors to rule out

• Leaving STD selected during descent below the transition level.
• Setting the captain’s altimeter but not the first officer’s or standby instrument.
• Confusing QFE, which may read near zero on the airfield, with QNH.
• Using pressure from a different airport or an older weather observation.
• Mixing inHg and hPa or overlooking a cockpit’s unit selector.