qnh calculator

What is QNH?

QNH is the altimeter setting that makes your altimeter read elevation above mean sea level when you are on the ground at a given location. In practical flying, it is the pressure value you dial into your altimeter so indicated altitude corresponds to your height relative to sea level.

This is important for terrain clearance, traffic separation, and instrument procedures. If your pressure setting is off, your altitude indication is off. Even small pressure errors can create meaningful altitude errors.

QNH vs QFE vs QNE (Quick Refresher)

• QNH: Altimeter reads airfield elevation on the ground (MSL reference).
• QFE: Altimeter reads zero on the runway (field reference).
• QNE: Standard pressure setting (1013.25 hPa / 29.92 inHg), used for flight levels.

Different countries and procedures emphasize different settings, but for most enroute and approach operations, QNH is the familiar local altimeter setting.

How this QNH calculator works

1) Convert pressure and elevation units

The calculator accepts pressure in either hPa or inHg, and elevation in feet or meters. Internally it converts pressure to hPa and elevation to meters for consistent physics-based computation.

2) Apply ISA barometric reduction

The main result uses a standard-atmosphere reduction from station pressure to sea-level pressure:

QNH = QFE × (1 - 0.0065 × h / 288.15)^(-5.2558797)

where h is field elevation in meters. This gives a practical estimate suitable for learning, planning, and cross-checking.

3) Show quick mental estimate too

A second value uses the common cockpit approximation:

QNH ≈ QFE + (Elevation in feet / 27)

This is fast and often close enough for sanity checks, but less precise than the barometric method.

How to use the calculator

1. Enter the observed station pressure (QFE).
2. Choose the pressure unit (hPa or inHg).
3. Enter your airfield elevation.
4. Select elevation unit (ft or m).
5. Click Calculate QNH.

You’ll receive:

• QNH in hPa and inHg
• Nearest whole-hPa value (common operational format)
• Quick approximation and difference from the ISA estimate

Worked example

Suppose station pressure is 1002.0 hPa at an aerodrome elevation of 900 ft. The calculator returns a QNH near 1035 hPa (about 30.56 inHg, depending on rounding and method).

This demonstrates why pressure corrections matter: field elevation can materially change the sea-level equivalent pressure.

Operational notes and limitations

• This is an educational and planning tool, not a replacement for official ATIS/METAR/QNH broadcasts.
• Real-world QNH issued by meteorological services may include local modeling and non-ISA effects.
• Temperature structure, non-standard lapse rates, and local pressure gradients can shift true values.
• Always use published/cleared altimeter settings in actual flight operations.

FAQ

Can I use inHg input directly?

Yes. Select inHg, and the calculator converts automatically.

What if the airfield is below sea level?

Enter a negative elevation. The formula supports it as long as values remain physically realistic.

Why do I get two QNH values?

You get a physics-based ISA estimate and a quick mental approximation. Comparing both helps with cockpit reasonableness checks.

Bottom line

A reliable QNH estimate is essential for accurate altitude awareness. Use this QNH calculator to convert station pressure and elevation quickly, understand the underlying method, and build stronger pressure-setting intuition for aviation operations.