What is ISA deviation?

ISA deviation is the difference between the actual outside air temperature and the International Standard Atmosphere temperature for that pressure altitude. ISA+ means warmer than standard, ISA- means colder.

How is the standard temperature calculated?

SkyStudy uses the ISA lapse rate of about 1.98 °C per 1000 ft from 15 °C at sea level, held at -56.5 °C at and above the tropopause (around 36,090 ft).

How do I work out ISA deviation in my head?

Round the lapse rate to 2 °C per 1000 ft. Standard temperature ≈ 15 − 2 × (altitude in thousands of feet). Subtract that from the actual OAT: a positive answer is ISA+, a negative answer is ISA-.

Why does ISA deviation matter?

ISA deviation feeds true altitude corrections, density altitude, and true airspeed. Warmer-than-standard air is less dense, so it degrades performance and makes the aircraft true-altitude higher than indicated.

What is the ISA temperature at the tropopause?

About −56.5 °C, reached at roughly 36,090 ft. Above the tropopause the standard temperature no longer falls with altitude: it stays at −56.5 °C through the lower stratosphere, which is why deviation questions at very high flight levels use a fixed reference instead of the lapse rate.

Standard atmosphere maths

ISA deviation calculator.

Work out the standard temperature for any pressure altitude and how far the real air sits above or below it, the starting point for density altitude and true airspeed.

ISA temperature & deviation

Enter the pressure altitude and the actual outside air temperature (OAT). SkyStudy returns the standard temperature at that level and how far the real air is from it.

Pressure altitude (ft)

Outside air temp (°C)

Enter a pressure altitude and outside air temperature to see the ISA temperature and the deviation.

In the EASA ATPL exam

ISA deviation is bread-and-butter maths in ATPL Meteorology (subject 050) and slips into Performance and General Navigation questions as a hidden first step. Meteorology asks it directly, either "what is the standard temperature at this level" or "the OAT at FL200 is −40 °C, what is the deviation". Navigation and Performance questions use it silently: any true-altitude correction, density altitude or TAS problem starts by comparing the actual temperature with standard.

Exam-style example

You are cruising at FL350 and the static air temperature is −47 °C. What is the ISA deviation?

Standard temperature at 35,000 ft: 15 − (2 × 35) = −55 °C.
Deviation = actual − standard: −47 − (−55) = +8 °C.

The air is ISA +8, eight degrees warmer than standard. Note: this worked example rounds the lapse rate to 2 °C per 1,000 ft for mental maths. The calculator uses the precise 1.98 °C per 1,000 ft, so it shows ISA+7 for these inputs. In the exam, use the rounded value the question expects.

Common trap: Double negatives and the tropopause. Subtracting −55 means adding 55, and sign slips flip the answer from ISA+8 to ISA−8. And above about 36,090 ft the reference is a constant −56.5 °C, so extending the 2 °C per 1,000 ft lapse to FL400 gives a standard temperature that does not exist.

ATPL Meteorology guide True airspeed calculator Density altitude calculator

How to calculate ISA deviation

The definition, the formula, a worked example and how it shows up in the ATPL exams.

The standard atmosphere in one paragraph

The International Standard Atmosphere (ISA) is the agreed reference the whole industry calibrates against: 15 °C and 1013.25 hPa at mean sea level, with temperature falling at about 1.98 °C per 1000 ft. That lapse continues up to the tropopause at roughly 36,090 ft, where the temperature reaches about −56.5 °C and then stays constant (isothermal) into the lower stratosphere.

Real air is rarely exactly standard. ISA deviation is simply how far today's temperature sits above or below that reference at your level.

How to compute the standard temperature and the deviation

Standard temperature at a level = 15 − 1.98 × (pressure altitude ÷ 1000) °C, which most pilots round to 2 °C per 1000 ft for mental maths. ISA deviation = actual OAT − standard temperature. A result of +10 is written ISA+10 (10 °C warmer than standard); −10 is ISA−10.

Above the tropopause the standard temperature stops decreasing and holds at −56.5 °C, so keep that in mind for high cruise levels.

Worked example

Cruising at FL300 (30,000 ft) with an outside air temperature of −30 °C. Standard temperature = 15 − (1.98 × 30) = 15 − 59.4 = −44.4 °C. ISA deviation = −30 − (−44.4) = +14.4 °C, so the air is ISA+14, noticeably warmer than standard.

That warm deviation means thinner air than the chart assumes: a higher true airspeed for a given Mach, a higher true altitude than indicated, and reduced engine performance.

Where it feeds in

ISA deviation is a building block, not an end in itself. It drives the temperature error correction for true altitude (about 4 ft per °C of deviation per 1000 ft), it is the temperature input to the density altitude calculator, and it sits behind the true airspeed and Mach relationships. The same lapse-rate maths runs right through ATPL Meteorology, General Navigation and Performance.

Good to know

Background and tips for getting the most out of this calculator.

Read the deviation

ISA+10 means the air is 10 °C warmer than standard at that level. Positive deviations mean thinner air and reduced performance.

Where it feeds in

Use the ISA deviation here as the input to the density altitude and true airspeed calculators for a full performance picture.

Exam-ready

The same lapse-rate maths appears throughout ATPL Meteorology and Performance, practise the mental model, then check it here.

Frequently asked questions

Tap to read the full background and answers.

What is ISA deviation?: ISA deviation is the difference between the actual outside air temperature and the International Standard Atmosphere temperature for that pressure altitude. ISA+ means warmer than standard, ISA- means colder.
How is the standard temperature calculated?: SkyStudy uses the ISA lapse rate of about 1.98 °C per 1000 ft from 15 °C at sea level, held at -56.5 °C at and above the tropopause (around 36,090 ft).
How do I work out ISA deviation in my head?: Round the lapse rate to 2 °C per 1000 ft. Standard temperature ≈ 15 − 2 × (altitude in thousands of feet). Subtract that from the actual OAT: a positive answer is ISA+, a negative answer is ISA-.
Why does ISA deviation matter?: ISA deviation feeds true altitude corrections, density altitude, and true airspeed. Warmer-than-standard air is less dense, so it degrades performance and makes the aircraft true-altitude higher than indicated.
What is the ISA temperature at the tropopause?: About −56.5 °C, reached at roughly 36,090 ft. Above the tropopause the standard temperature no longer falls with altitude: it stays at −56.5 °C through the lower stratosphere, which is why deviation questions at very high flight levels use a fixed reference instead of the lapse rate.

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