Cold temperature altimeter correction explained

By Zulu Calculator · Updated 12 October 2026 · Aviation

In cold air your altimeter over-reads: it shows the published altitude while the aircraft is actually lower. To keep the obstacle clearance the approach was built with, you add a cold temperature correction to the published altitudes, using the reported airport temperature and the height of each altitude above the airport. At a 1,000 ft airport reporting −25 °C, a 3,000 ft FAF needs about 308 ft added, so you fly 3,400 ft.

This guide explains why the error happens, the ICAO formula and table, the 4% rule of thumb, and the FAA Cold Temperature Airport rules. To work out a whole approach with your own numbers, use the cold temperature correction calculator.

Why do altimeters over-read in cold air?

An altimeter does not measure height. It measures air pressure and turns it into height using the International Standard Atmosphere (ISA): 15 °C at sea level, cooling 1.98 °C for every 1,000 ft. When the real air is colder than that, it is denser, and the pressure levels sit closer together. So the altimeter reaches a given reading at a lower true height.

The error is small near the ground and builds up through every layer of cold air between you and the place the altimeter setting comes from. That is why the correction grows with height above the airport, not with height above sea level. If you want to see how far a day is from standard, the ISA deviation calculator works out the ISA temperature at any altitude and the difference from it.

Who needs to correct, and which altitudes?

The minimum altitudes that keep you clear of terrain need correcting in cold weather:

• initial and intermediate approach altitudes (IAF, IF);
• the final approach fix (FAF) and any step-down fixes;
• the MDA or DA;
• the missed approach altitude.

What you should not do:

• Do not correct altitudes ATC gives you on radar vectors.
• Do not change the altimeter setting to make a correction. Set the reported QNH or altimeter setting as normal and add the correction to the altitudes.
• Do not correct when the airport is at or above ISA. Warm air puts you higher than the altimeter shows.

ICAO leaves the details to each state. Many states and operators set their own rules for when corrections start, which altitudes to correct and how to round. Follow your AIP, the chart notes and your operator's procedures.

How is the cold temperature correction calculated?

The standard method is the approximate formula in ICAO Doc 8168 (PANS-OPS), Volume I, Part III, Section 1, Chapter 4 (ICAO Doc 8168). In words:

t0 = airport temperature + 0.00198 × airport elevation (ft)
correction = H × (15 − t0) ÷ (273 + t0 − 0.5 × 0.00198 × (H + Hss))
H = published altitude − altimeter setting source elevation
Hss = elevation of the altimeter setting source (normally the airport)
altitude to fly = published altitude + correction, rounded up

In metres, use 0.0065 °C per metre in place of 0.00198 °C per foot, and H in metres. The ISA temperature at the airport is 15 − 0.00198 × elevation, so at 1,000 ft it is 13.02 °C.

The 4% rule of thumb

A quick mental check is to add 4% of the height above the airport for every 10 °C below ISA at the airport:

correction ≈ H × 4% × (degrees below ISA ÷ 10)

It lands close to the formula for normal approach heights. Use it to catch a typing mistake, not as your main method.

The ICAO cold temperature error table

ICAO Doc 8168 also gives a table, and the FAA reprints it as Table 7-3-1 in the Aeronautical Information Manual, chapter 7, section 3. You read it by height above the airport (rows) and reported airport temperature (columns). These are the feet to add, for a selection of rows:

The full table, with every row from 200 ft to 5,000 ft, is on the calculator page. The table assumes a sea-level airport, so at higher airports it gives a bigger correction than the formula. Between printed rows and columns you interpolate in a straight line. The table and the AIM 7-3 rules were checked on 12 October 2026.

Worked examples

Example 1: a 1,000 ft airport at −25 °C

The approach has an IF at 4,000 ft, a FAF at 3,000 ft, a step-down at 2,000 ft, an MDA of 1,500 ft and a missed approach altitude of 4,000 ft. ISA at the airport is 13.02 °C, so −25 °C is 38.02 °C below ISA, and t0 = −25 + 1.98 = −23.02 °C.

• FAF: H = 2,000 ft. 2,000 × 38.02 ÷ (273 − 23.02 − 0.5 × 0.00198 × 3,000) = 76,040 ÷ 247.01 = 308 ft. Fly 3,308 ft, rounded up to 3,400 ft.
• Step-down: H = 1,000 ft. Correction 153 ft, so 2,153 ft, rounded up to 2,200 ft.
• MDA: H = 500 ft. Correction 76.5 ft, so 1,576.5 ft, rounded up to the next 10 ft: 1,580 ft.
• IF and missed approach: H = 3,000 ft. Correction 464 ft, so 4,464 ft, rounded up to 4,500 ft.

The 4% rule gives 2,000 × 4% × 3.802 = 304 ft for the FAF, only 4 ft less than the formula. Flown uncorrected, the 3,000 ft FAF would put you at about 2,733 ft: 267 ft lower than the chart expects.

Rounding here is MDA/DA up to the next 10 ft and the other altitudes up to the next 100 ft. Your operator may use different steps.

Example 2: a high airport, formula against table

An airport at 5,000 ft reports −20 °C. ISA there is 15 − 9.9 = 5.1 °C, so the airport is 25.1 °C below ISA. t0 = −20 + 9.9 = −10.1 °C.

• FAF at 7,000 ft: H = 2,000 ft. 2,000 × 25.1 ÷ (273 − 10.1 − 0.5 × 0.00198 × 7,000) = 50,200 ÷ 255.97 = 196 ft. Fly 7,200 ft.
• DA at 5,250 ft: H = 250 ft. Correction 24 ft, so 5,274 ft, rounded up to 5,280 ft.
• Missed approach at 9,000 ft: H = 4,000 ft. Correction 395 ft. Fly 9,400 ft.

The ICAO table, read at 2,000 ft and −20 °C, says 280 ft for the FAF, 84 ft more than the formula. That is because the table is built for a sea-level airport. Both are accepted methods; use the one your operator and state require.

Example 3: an FAA Cold Temperature Airport

A US airport at 1,000 ft is on the FAA Cold Temperature Airport list. Its chart shows a snowflake and −20 °C, and the list marks the final segment only. The airport reports −22 °C, which is at or below −20 °C, so corrections are required in the final segment.

• FAF at 3,000 ft (2,000 ft above the airport): the AIM table between −20 °C (280 ft) and −30 °C (380 ft) gives 300 ft. Fly 3,300 ft.
• MDA at 1,500 ft (500 ft above): between 70 ft and 100 ft gives 76 ft, so 1,576 ft, rounded up to 1,580 ft.
• The IF and missed approach altitudes are not in a marked segment here, so correcting them is optional.
• You don't need to tell ATC about corrections in the final segment.

FAA Cold Temperature Airports (United States)

In the US, the FAA publishes a list of Cold Temperature Airports in the Notices to Air Missions publication (NTAP), and the procedure is in AIM 7-3. In short:

• Approach charts at these airports carry a snowflake symbol and a temperature.
• When the reported airport temperature is at or below that temperature, correct the altitudes in the segments the list marks: intermediate, final, missed approach, or all.
• Use the reported airport temperature and the height above the airport with the ICAO table in AIM Table 7-3-1.
• Tell ATC when you fly a corrected altitude in the intermediate or missed approach segment. You don't need to for the final segment.

Check the current NTAP list before each winter, because airports and segments change.

Other countries

Some states publish their own table and starting temperature. Canada, for example, has its own rules in the Transport Canada AIM, section RAC 9.17. Australia and Europe have their own notes too. Your AIP is the place to look.

Common mistakes

• Using height above sea level. The correction uses the height above the altimeter setting source. Using the full altitude gives a far too large correction.
• Using the HAT or HAA as the altitude. Correct the MDA or DA itself, and work out its height above the airport from it.
• Forgetting a remote altimeter setting. If the QNH comes from another airport, the height is measured from that source. For example, with a source at 200 ft, a 1,500 ft MDA is 1,300 ft above it and needs 199 ft at −25 °C, not 76.5 ft.
• Rounding down. Always round the altitude to fly up.
• Correcting ATC-assigned altitudes or changing the altimeter setting. Neither is part of the procedure.
• Mixing up the altimeter setting error with the temperature error. A wrong QNH is a different problem. The altimeter setting calculator shows how much a wrong setting moves you, and also does a single cold temperature correction.

What to do if you are unsure

Your approved flight manual, operator procedures and official charts always come first. If your operator's method gives a different number from this guide, use the operator's method and ask your training or standards department why. If you are flying a corrected altitude in a segment where ATC must be told, say so on the radio before you get there. If you cannot fly the corrected altitudes, for example because a baro-VNAV approach is below its published lowest temperature, ask for a different approach.

This guide explains the rules in plain English for planning. It is not legal, financial or flight-operations advice; check your contract, the official source linked above, your employer or your operator.