What is the Standard Datum Plane (SDP) in aviation?

The Standard Datum Plane is a theoretical barometric reference level where atmospheric pressure equals exactly 29.9213 inHg (1013.25 hPa) under standard atmospheric conditions. It is the baseline from which all pressure altitudes and en-route flight levels (FLs) are measured.

What is the difference between Indicated Altitude and Pressure Altitude?

Indicated altitude is the direct reading shown on the cockpit altimeter when the Kollsman subscale is set to the current local barometric pressure (QNH). Pressure altitude is the reading when the Kollsman subscale is set to standard 29.92 inHg (QNE), indicating height above the standard datum plane.

What does "High to Low, Look Out Below" mean for pilots?

When flying from an area of high barometric pressure into an area of low barometric pressure without resetting the altimeter, the instrument reads higher than your actual true altitude above sea level. This poses a severe terrain clearance hazard because the aircraft is flying closer to obstacles than the cockpit instrument indicates.

What is the difference between QNH, QNE, and QFE?

QNH is the local barometric pressure setting that causes the altimeter to read field elevation above mean sea level when on the ground. QNE is standard pressure (29.92 inHg / 1013.25 hPa) used at and above the transition altitude to assign Flight Levels. QFE is the local surface pressure that causes the altimeter to read exactly zero on the runway surface (height Above Ground Level / AGL).

When do pilots set their altimeter to 29.92 inHg (1013.25 hPa)?

In the United States and Canada, pilots set their altimeter to standard 29.92 inHg when climbing through 18,000 ft MSL (Flight Level 180 / FL180), entering Class A airspace under 14 CFR § 91.121. In other ICAO regions, the transition altitude varies regionally (often between 3,000 ft and 10,000 ft MSL) and is published on aerodrome departure and arrival charts.

How accurate is the 1,000-ft rule of thumb compared to the exact ICAO equation?

The FAA cockpit rule of thumb (1 inHg = 1,000 ft) is highly accurate near sea level, differing from the exact ICAO Doc 7488 geopotential equation by less than 20 to 50 ft up to 5,000 ft MSL. Above 10,000 ft, the non-linear density lapse rate causes the rule to diverge, which is why flight computer and performance calculations rely on exact geopotential models.

Does pressure altitude alone determine aircraft takeoff and climb performance?

No. While pressure altitude corrects for barometric pressure deviations, aircraft aerodynamic performance is primarily governed by Density Altitude, which also factors in ambient outside air temperature (OAT) and air density. High temperatures significantly degrade performance beyond what pressure altitude alone indicates.

Atmosphere & Airspeed Hub →ICAO DOC 7488 • FAA-H-8083-25C • AIM SECTION 7-2

Pressure Altitude Calculator & Standard Datum Plane

Calculate altitude above the standard 29.92 inHg (1013.25 hPa) datum plane from field elevation and current altimeter setting (QNH). Features dual computation models comparing the standard FAA 1,000-ft rule of thumb with exact ICAO Doc 7488 geopotential barometric equations.

Units:

Presets:

📋Paste Live METAR / ATIS String (Auto-Extracts QNH):e.g. KDEN 251853Z ... A2980 or EHAM ... Q1013

Barometric Altimetry Controls

Specify aerodrome field elevation and current altimeter setting (QNH) from ATIS/METAR.

Field Elevation / Indicated Altitude2,500 ft

Altimeter Setting (QNH / Kollsman Window)

29.72 inHg(1006.43 hPa)

inHg

Exact Pressure Altitude (ICAO Doc 7488)

2,682ft MSL

FAA Cockpit Rule2,700 ftDelta: -18 ft

Lowest Usable Flight Level:FL190

14 CFR § 91.121

⚠️ Low Pressure:Barometric pressure is below standard (29.92 inHg). The Standard Datum Plane lies 200 ft below sea level, increasing pressure altitude above field elevation by +182 ft. FL190 is the lowest usable Flight Level.

Calculate Density Altitude with this PA→

Standard Atmosphere Model

Atmospheric Datum Slice

Low Barometer (PA > Elev)

QNH Setting29.72 inHg(1006.43 hPa)

Standard Datum Plane200 ft BelowRef: 29.92 inHg

Pressure Altitude2,682 ftRule: 2,700 ft

Lowest Usable FLFL19014 CFR § 91.121

Ambient Pressure27.13 inHg(918.7 hPa)

Aeronautical Altimetry Methodology & Governing Standards

Physics of the aneroid altimeter, barometric lapse rates, and international Q-codes.

QNH (Local Altimeter)

Height Above Sea Level

The barometric pressure setting broadcast by ATIS/tower that causes the cockpit altimeter to indicate elevation above Mean Sea Level (MSL) when the aircraft is on the runway.

QNE (Standard 29.92)

Pressure Altitude (En-Route)

Standard sea-level pressure (29.92 inHg / 1013.25 hPa) dialed into the Kollsman window at and above the transition altitude (FL180 in North America) to define standardized Flight Levels.

QFE (Field Pressure)

Height Above Aerodrome (AGL)

The actual station barometric pressure measured at the airfield datum. When dialed into the altimeter subscale, the instrument reads zero feet on the runway threshold.

Real-World Altimetry Step-by-Step Worked Scenarios

Sample calculations illustrating how barometric pressure swings alter cockpit indications.

Scenario 1: Denver Summer Low PressureLow QNH (29.62 inHg)

Conditions: Field elevation 5,431 ft MSL, local altimeter setting reported as 29.62 inHg.

1. Pressure departure: 29.92 − 29.62 = +0.30 inHg

2. Standard datum offset: 0.30 × 1,000 ft = +300 ft

3. Rule Pressure Altitude: 5,431 + 300 = 5,731 ft

4. Exact ICAO Doc 7488: 5,698 ft (Delta: −33 ft vs rule)

Takeaway: Low barometric pressure inflates pressure altitude by +267 ft above field elevation, increasing takeoff distance.

Scenario 2: Winter Arctic High Pressure RidgeHigh QNH (30.52 inHg)

Conditions: Field elevation 1,500 ft MSL, dense cold anticyclone altimeter 30.52 inHg.

1. Pressure departure: 29.92 − 30.52 = −0.60 inHg

2. Standard datum offset: −0.60 × 1,000 ft = −600 ft

3. Rule Pressure Altitude: 1,500 − 600 = 900 ft

4. Exact ICAO Doc 7488: 956 ft (Delta: +56 ft vs rule)

Takeaway: High barometric pressure depresses pressure altitude 544 ft below field elevation, improving engine thrust and climb.

Pressure Altitude & Barometric Governing Equations

MATHEMATICAL SPECIFICATIONICAO Doc 7488/3 & FAA-H-8083-25C

Standard FAA Pilot Cockpit Rule of Thumb (FAA-H-8083-25C)

PA_rule=h + (29.92 − QNH) × 1,000[ft]

Δh_SDP=(29.92 − QNH) × 1,000[Datum Plane Offset relative to MSL, ft]

Exact Geopotential Atmosphere Model (ICAO Doc 7488/3)

P_ambient=QNH × (1 − 6.875586 × 10⁻⁶ × h)^5.25588[Ambient Static Pressure, inHg]

PA_exact=145,366.45 × (1 − (P_ambient / 29.9213)^0.190284)[ft]

Physical Variables & Aviation Unit Definitions

Symbol	Parameter	Physical Meaning	Unit
h	Field Elevation / Indicated Altitude	Actual aerodrome elevation or indicated reading on the cockpit instrument	ft
QNH	Altimeter Setting	Current barometric pressure setting broadcast by ATIS/tower	inHg
PA_rule	Rule of Thumb Pressure Altitude	Standard linear 1,000-ft approximation used in FAA ground schools	ft
PA_exact	Exact ICAO Pressure Altitude	True thermodynamic geopotential height in the standard atmosphere	ft
P_ambient	Ambient Static Pressure	Calculated static pressure of the atmospheric column at field elevation	inHg
Δh_SDP	Standard Datum Plane Offset	Vertical distance of the 29.92 inHg plane above or below sea level	ft

Exact ICAO Doc 7488 Pressure Altitude Reference Matrix

Cross-tabulated exact geopotential pressure altitude values (ft MSL) across elevations (-500 to 10,000 ft) and barometric altimeter settings (28.50 to 30.50 inHg).

Elevation	28.50 inHg	29.00 inHg	29.50 inHg	29.92 inHg (STD)	30.20 inHg	30.50 inHg
-500 ft	845 ft	366 ft	-107 ft	-500 ft	-757 ft	-1,032 ft
0 ft	1,340 ft	862 ft	392 ft	0 ft	-257 ft	-531 ft
1,000 ft	2,330 ft	1,856 ft	1,389 ft	1,000 ft	745 ft	472 ft
2,500 ft	3,816 ft	3,347 ft	2,884 ft	2,500 ft	2,247 ft	1,977 ft
5,000 ft	6,292 ft	5,831 ft	5,376 ft	5,000 ft	4,750 ft	4,485 ft
7,500 ft	8,768 ft	8,315 ft	7,868 ft	7,500 ft	7,253 ft	6,993 ft
10,000 ft	11,244 ft	10,799 ft	10,361 ft	10,000 ft	9,757 ft	9,501 ft

Aeronautical Knowledge: "High to Low, Look Out Below" & FL180 Transition

Critical FAA instrument rating and commercial pilot checkride oral defense topics.

⚠️The High-to-Low Altimeter Trap

When flying from an area of high pressure into low pressure without resetting the Kollsman window, the aneroid wafers in the altimeter expand, falsely reading higher than the aircraft's true altitude.

Rule: For every 0.10 inHg decrease in uncorrected pressure, the aircraft is flying approximately 100 ft lower than the cockpit instrument indicates.

🛫FL180 Transition Altitude (14 CFR § 91.121)

In the United States and Canada, 18,000 ft MSL marks the floor of Class A airspace. Above this transition altitude, all aircraft dial standard 29.92 inHg (1013.25 hPa) into their altimeters.

En-Route Safety: This guarantees that two aircraft flying at FL350 have identical vertical references regardless of whether they are over a low-pressure hurricane or high-pressure anticyclone.

Legal Disclaimer & AFM / POH Precedence

Pressure altitude alone corrects for barometric pressure variations; it does not account for ambient outside air temperature. In hot conditions, Density Altitude will be substantially higher than Pressure Altitude.

Pilots MUST verify actual takeoff ground roll, climb gradients, and landing performance directly from the official Aircraft Flight Manual (AFM) or Pilot's Operating Handbook (POH).

Next Step: How temperature converts Pressure Altitude to Density Altitude:Read Density Altitude & Atmosphere Technical Guide →

Frequently Asked Questions

When barometric pressure is lower than standard (29.92 inHg / 1013.25 hPa), the standard pressure datum plane drops below sea level. Since pressure altitude is measured from this lower datum plane up to the aircraft, your vertical distance above that plane is greater than your actual height above mean sea level.

✓

Aeronautical E-E-A-T Quality Verification

Validated against ICAO Doc 7488 & FAA-H-8083-25C Standards

Technical Review: CFII / ATP Standard

Last Mathematical Verification: 2026-09-25

Aviation Workflow Handoffs

Downstream StepProceed →

Use in Density Altitude Calculation

Feed this pressure altitude and local temperature into the density altitude engine.

Downstream StepProceed →

Use in True Airspeed Solver

Apply pressure altitude to resolve IAS into TAS.

Technical Basis & Governing Sources

View full source registry →

official handbookFAA-H-8083-25C

Pilot's Handbook of Aeronautical Knowledge

Issuing Authority: Federal Aviation Administration (FAA)

Citations:

Chapter 4: Principles of Flight
Chapter 8: Flight Instruments
Chapter 11: Aircraft Performance
Chapter 16: Navigation

Official FAA/ICAO Document Reference

official handbookFAA-H-8083-28B

Aviation Weather Handbook

Issuing Authority: Federal Aviation Administration (FAA)

Citations:

Chapter 2: Earth Atmosphere & Heat
Chapter 4: Moisture, Clouds & Precipitation
Chapter 19: Standard Atmosphere & Altimetry

Official FAA/ICAO Document Reference

technical standardDoc 7488/3

Manual of the ICAO Standard Atmosphere (extended to 80 kilometres / 262,500 feet)

Issuing Authority: International Civil Aviation Organization (ICAO)

Citations:

Part 1: Standard Atmosphere to 32 km

Official FAA/ICAO Document Reference