Use this calculator for flight-planning study or a first-pass cross-check when all inputs describe the same aircraft, atmospheric condition, and phase of flight.

QNH (hPa)

QNH altimeter setting.

Compare a QNH setting against the ISA standard pressure of 1013.25 hPa. The tool evaluates the supplied inputs together and returns the named outputs below; it does not infer omitted operating conditions or change the units shown.^[1]

Pressure difference (hPa)

Signed difference from standard pressure; positive above standard, negative below.

• Do not use a generic calculator in place of the current aircraft flight manual, approved performance data, operating limitations, NOTAMs, weather, or regulations. Real performance changes with configuration, runway condition, altitude, temperature, wind, loading, and technique.
• Use finite inputs in the displayed units and preserve more precision than the final presentation requires. Independently verify safety-critical, financial, compliance, or production decisions.

Cross-check the result against the aircraft flight manual or approved planning system, then apply the required safety margins. Keep signed wind, heading, altitude, speed, and fuel conventions explicit.

• ISA Deviation Calculator

  Compare the outside air temperature with the ISA standard temperature at a pressure altitude.

• Pressure Ratio Calculator

  Compute the ISA static pressure ratio against sea-level standard pressure.

• Density at Altitude Calculator

  Compute the ISA air density ratio against sea-level standard density in the troposphere.