Pressure altitude calculator.
Convert your field or indicated altitude and QNH into pressure altitude, the figure performance charts and density altitude both start from.
Pressure altitude
Enter the field or indicated altitude and the QNH. SkyStudy applies the standard altimetry rule (30 ft per hPa, or 1000 ft per inHg) to give the pressure altitude you would read with 1013 / 29.92 set.
Enter an altitude and QNH to see the pressure altitude.
In the EASA ATPL exam
Altimetry is one of the most reliable question sources in ATPL Instrumentation (subject 022), and the same conversion opens most Performance (032) problems, because take-off and landing tables are indexed by pressure altitude, not by field elevation. A typical question gives you an aerodrome elevation and a QNH and asks either for the pressure altitude itself or for a performance figure that silently depends on it, so the conversion has to be automatic before you can even start the real question.
Exam-style example
An aerodrome sits at an elevation of 1,200 ft and the QNH is 983 hPa. What is the pressure altitude?
- Find the difference from standard: 1013 − 983 = 30 hPa.
- Convert at the exam rate of 30 ft per hPa: 30 × 30 = 900 ft.
- The QNH is below standard, so the correction is added to the elevation.
Pressure altitude = 1,200 + 900 = 2,100 ft.
Common trap: Getting the sign backwards. When the QNH is below 1013 the pressure altitude is above the elevation; when the QNH is above 1013 it is below. The distractor built from the wrong sign (here 300 ft) is almost always among the answer options.
How to calculate pressure altitude
The definition, the formula, a worked example and how it shows up in the ATPL exams.
How to calculate pressure altitude
The definition, the formula, a worked example and how it shows up in the ATPL exams.What pressure altitude actually is
Pressure altitude is the altitude in the International Standard Atmosphere (ISA) at which the current air pressure would normally be found. In practical terms it is what your altimeter reads when you set the standard subscale of 1013.25 hPa (29.92 inHg) instead of the local QNH. It is a height above the standard datum plane, not a height above the ground or above mean sea level.
Because performance and air-density behaviour depend on pressure rather than on your geographic elevation, pressure altitude is the number engineers use to index almost every chart in the aircraft flight manual. That is why you convert to it before doing anything else.
The formula and the 30 ft per hPa rule
The standard approximation taught for the ATPL is: pressure altitude ≈ field elevation + (1013 − QNH) × 30 ft. Each hectopascal below 1013 adds about 30 ft; each hectopascal above it subtracts about 30 ft. If you are working in inches of mercury, the equivalent rule is about 1000 ft per inHg, using 29.92 as the standard.
The 30 ft per hPa figure is a near-surface approximation. The true value is closer to 27 ft per hPa at sea level and increases with altitude, but 30 ft per hPa is the accepted ATPL exam convention and is what this calculator uses.
Worked example
Field elevation 1,500 ft, QNH 993 hPa. The difference from standard is 1013 − 993 = 20 hPa. Multiply by 30 ft: 20 × 30 = 600 ft. Pressure altitude = 1,500 + 600 = 2,100 ft. The low pressure has pushed the pressure altitude 600 ft above the physical field elevation.
Now a high-pressure day at the same field: QNH 1023 hPa. The difference is 1013 − 1023 = −10 hPa, so −10 × 30 = −300 ft, and pressure altitude = 1,500 − 300 = 1,200 ft, below the field elevation. The saying 'high to low, look out below' captures the terrain-clearance risk of flying from high pressure into low pressure on a fixed altimeter setting.
Where it shows up in the ATPL exams
Pressure altitude is the input to take-off, landing and climb performance tables (ATPL Performance), the starting point for the density altitude that the aircraft really feels, and a recurring theme in altimetry questions across Meteorology and General Navigation. Getting fluent with the 30 ft per hPa conversion removes a whole class of avoidable mistakes.
Once you have the pressure altitude here, carry it into the density altitude calculator with the outside air temperature to see the altitude your aircraft and engine actually perform at on the day.
Good to know
Background and tips for getting the most out of this calculator.
Good to know
Background and tips for getting the most out of this calculator.The rule of thumb
About 30 ft of altitude per hPa, or 1000 ft per inHg. Below 1013 / 29.92 the pressure altitude sits above your field elevation.
Next step
Feed the pressure altitude into the density altitude calculator with the OAT to see the altitude the aircraft actually performs at.
Why approximate
The 30 ft/hPa figure is the standard near-surface approximation taught for the ATPL. Use the aircraft altimetry system for primary reference.
Frequently asked questions
Tap to read the full background and answers.
Frequently asked questions
Tap to read the full background and answers.- How do I calculate pressure altitude?
- Pressure altitude is your field or indicated altitude corrected for the difference between QNH and 1013.25 hPa. SkyStudy uses the standard 30 ft per hPa (1000 ft per inHg) altimetry rule.
- What is the formula for pressure altitude?
- Pressure altitude ≈ field elevation + (1013 − QNH) × 30 ft when QNH is in hectopascals. In inches of mercury it is field elevation + (29.92 − QNH) × 1000 ft. A low QNH gives a pressure altitude above your elevation. SkyStudy uses the 1013 subscale, the same figure as the worked examples, so a sea-level field on a standard-pressure day reads exactly zero. The two rules are approximations of the same thing and do not agree exactly: 1,000 ft per inHg comes out about 1.6 percent smaller than 30 ft per hPa for the same change in pressure.
- Why is pressure altitude higher when QNH is low?
- Setting the standard 1013 / 29.92 in place of a lower QNH makes the altimeter read higher. Low pressure days give a pressure altitude above your field elevation.
- What is the difference between pressure altitude and density altitude?
- Pressure altitude corrects only for pressure. Density altitude takes that pressure altitude and corrects it again for temperature. On a standard (ISA) day the two are equal; on a hot day the density altitude is higher.
- What is pressure altitude at sea level?
- At a sea-level airfield on a standard-pressure day (QNH 1013), the pressure altitude is zero. If the QNH is lower than 1013, even a sea-level field has a positive pressure altitude.
- What do I use pressure altitude for?
- It is the entry argument for performance charts and for density altitude. Almost every take-off, climb, and cruise performance table is indexed by pressure altitude.
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